terms in metal detector

[Bayu]

hello all geotech masters and members, in understanding metal detectors there may still be many who don't know the terms in the metal detector section including me, maybe our teachers can explain the term and its uses or how it works such as; gain, SAT, sens, dipht width, integrator, threshold, pream, oscillator, and many more, maybe there are masters who can and are willing to explain the meaning and use, I am personally very grateful. and maybe someone wants to add other than the terms I mentioned earlier

[Qiaozhi]

hello all geotech masters and members, in understanding metal detectors there may still be many who don't know the terms in the metal detector section including me, maybe our teachers can explain the term and its uses or how it works such as; gain, SAT, sens, dipht width, integrator, threshold, pream, oscillator, and many more, maybe there are masters who can and are willing to explain the meaning and use, I am personally very grateful. and maybe someone wants to add other than the terms I mentioned earlier

Or you could simply buy a copy of Inside the METAL DETECTOR - Second Edition - Published 2015
and learn the answers to all your questions, including many you haven't even thought of.

[Carl-NC]

hello all geotech masters and members, in understanding metal detectors there may still be many who don't know the terms in the metal detector section including me, maybe our teachers can explain the term and its uses or how it works such as; gain, SAT, sens, dipht width, integrator, threshold, pream, oscillator, and many more, maybe there are masters who can and are willing to explain the meaning and use, I am personally very grateful. and maybe someone wants to add other than the terms I mentioned earlier

Most of these terms -- gain, sensitivity, integrator, threshold, preamp, oscillator -- are the same for metal detectors as for any other electronics. If you don't know what they mean then I suggest some more study in electronics. I certainly don't mind answering questions but if I need to explain what "gain" means then you probably are not ready for these forums.

I don't know what you mean by "dipht width."

SAT is Self-Adjusting Threshold. It is usually a derivative/high-pass filter stage that removes offsets and slow-moving responses. Here a thread on it:

https://www.geotech1.com/forums/showthread.php?25100-what-is-Self-Adjusting-Threshold&highlight=sat+stage

[kt315]

How Metal Detectors Work

By Mark Rowan & William Lahr





Introduction


Metal detectors are fascination machines. Many of the people who use them are just as enthusiastic about extolling the virtues of their favorite metal detector as they are about setting off in search of buried treasure. Those of us who design and build these instruments for a living listen carefully when one of our customers talks about his or her experience in the field, because this is the primary means by which we determine how well we are doing our jobs, and what sort of things we need to do better. Sometimes though, communication is difficult. Almost as though we and our customers speak different languages. Which in a sense, we do. The purpose of this page is to try to narrow that communication gap a little. And, to resolve some of that "typical curiosity" metal detector operators have regarding what is going on inside their instruments.

Is it necessary to know how a metal detector works in order to use it effectively? Absolutely not. Will knowing how it works help someone to use it more effectively in the future? Quite possibly yes, but only with persistence and practice. The best metal detector available is still only as good as the person using it.



VLF (Very Low Frequency) Transmitter & Receiver
Transmitter
Inside the metal detector's loop (sometimes called a search head, coil, antenna, etc.) is a coil of wire called the transmit coil. Electronic current is driven through the coil to create an electromagnetic field. The direction of the current flow is reversed several thousand times every second; the transmit frequency "operating frequency" refers to the number of times per second that the current flow goes from clockwise to counterclockwise and back to clockwise again.

When the current flows in a given direction, a magnetic field is produced whose polarity (like the north and south poles of a magnet) points into the ground; when the current flow is reversed, the field's polarity points out of the ground. Any metallic (or other electrically conductive) object which happens to be nearby will have a flow of current induced inside of it by the influence of the changing magnetic field, in much the same way that an electric generator produces electricity by moving a coil of wire inside a fixed magnetic field. This current flow inside a metal object in turn produces its own magnetic field, with a polarity that tends to be pointed opposite to the transmit field.
Receiver
A second coil of wire inside the loop, the receive coil, is arranged (by a variety of methods) so that nearly all of the current that would ordinarily flow in it due to the influence of the transmitted field is cancelled out. Therefore, the field produced by the currents flowing in the nearby metal object will cause currents to flow in the receive coil which may be amplified and processed by the metal detector's electronics without being swamped by currents resulting from the much stronger transmitted field.

The resulting received signal will usually appear delayed when compared to the transmitted signal. This delay is due to the tendency of conductors to impede the flow of current (resistance) and to impede changes in the flow of current (inductance). We call this apparent delay "phase shift". The largest phase shift will occur for metal objects which are primarily inductive; large, thick objects made from excellent conductors like gold, silver, and copper. Smaller phase shifts are typical for objects which are primarily resistive; smaller, thinner objects, or those composed of less conductive materials.

Some materials which conduct poorly or not at all can also cause a strong signal to be picked up by the receiver. We call these materials "ferromagnetic". Ferromagnetic substances tend to become magnetized when placed in a field like a paper clip which becomes temporarily magnetized when picked up with a bar magnet. The received signal shows little if any phase shift. Most soils and sands contain small grains of iron-bearing minerals which causes them to appear largely ferromagnetic to the metal detector. Cast iron (square nails) and steel objects (bottle caps) exhibit both electrical and ferromagnetic properties.

It should be pointed out that this discussion describes an "Induction Balance" metal detector, sometimes referred to as "VLF" Very Low Frequency (below 30kHz). This is the most popular technology at the present time, and includes the "LF" Low Frequency (30 to 300kHz) instruments made for prospecting.
Discrimination
Since the signal received from any given metal object exhibits its own characteristic phase shift, it is possible to classify different types of objects and distinguish between them. For example, a silver dime causes a much larger phase shift than an aluminum pull-tab does, so a metal detector can be set to sound off on a dime yet remain quiet on the pull-tab, and/or show the identification of the target on a display or meter. This process of distinguishing between metal targets is called "discrimination". The simplest form of discrimination allows a metal detector to respond with an audio output when passed over a target whose phase shift exceeds a certain (usually adjustable) amount. Unfortunately, with this type of discriminator the instrument will not respond to some coins and most jewelry if the discrimination is adjusted high enough to reject common aluminum trash for example pull-tabs and screw-caps.

A more useful scheme is what is called "Notch Discrimination". With this type of system, a notch in the discriminate response allows the metal detector to respond to targets within a certain range (such as the nickel/ring range) while still rejecting targets above that range (pull-tabs, screw-caps) as well as below it (iron, foil). The more sophisticated notch metal detectors allow for each of several ranges to be set for either accept or reject responses. White's Spectrum XLT for example, provides 191 individually programmable notches.

A metal detector may provide a numeric readout, meter indication, or other display mechanism which shows the target's likely identity. We refer to this feature as a Visual Discrimination Indicator, or V.D.I. Metal Detectors with this capability have the advantage of allowing the operator to make informed decisions about which targets they choose to dig rather than relying solely on the instruments audio discriminator to do all the work. Most, if not all, V.D.I. metal detectors are also equipped with audio discriminators.

Metal detectors can distinguish metal objects from each other based on the ratio of their inductance to their resistivity. This ratio gives rise to a predictable delay in the receive signal at a given frequency. An electronic circuit called a phase demodulator can measure this delay. In order to separate two signals, such as the ground component and the target component of the receive signal, as well as to determine the likely identity of the target, we use two such phase demodulators whose peak response is separated from each other by one fourth of the transmitter period, or ninety degrees. We call these two channels "X" and "Y". A third demodulated signal, we call "G", can be adjusted so that its response to any signal with a fixed phase relationship to the transmitter (such as the ground) can be reduced to zero regardless of the strength of the signal.

Some metal detectors use a microprocessor to monitor these three channels, determine the targets's likely identity, and assigning it a number based on the ratio of the "X" and "Y" readings, whenever the "G" reading exceeds a predetermined value. We can find this ratio with a resolution of better than 500 to 1 over the full range from ferrite to pure silver. Iron targets are orientation sensitive; therefore as the loop is moved above them, the calculated numerical value may change dramatically. A graphic display showing this numerical value on the horizontal axis and the strength of the signal on the vertical axis is extremely useful in distinguishing trash from more valuable objects. We call this display the "SignaGraph" (TM).
Phase Chart GIF Image
Ground Balance

As previously mentioned, most sands and soils contain some amount of iron. They may also have conductive properties due to the presence of salts dissolved in the ground water. The result is that a signal is received by the metal detector due to the ground itself which may be thousands of times stronger than the signal resulting from small metal objects buried at modest depths. Fortunately, the phase shift caused by the ground tends to remain fairly constant over a limited area. It is possible to arrange things inside the metal detector so that even if the strength of the ground signal changes dramatically--such as when the loop is raised and lowered, or when it passes over a mound or hole--the metal detector's output remains constant. Such a metal detector is said to be "ground balanced". Accurate ground balance makes it possible to "pinpoint" the location of the targets with a good deal of precision as well as to estimate the depth of the targets in the ground. If you choose to search in a non-discriminate, or "all-metal" mode, accurate ground balance is essential.

The simplest form of ground balance consists of a control knob which the operator adjusts while raising and lowering the loop until good balance is achieved. Although this method can be quite effective, it can also be tedious, and some people find it to be difficult or confusing. More advanced metal detectors will perform ground balance automatically, typically by a two-step sequence in which the metal detector is balanced with the loop raised, then balanced once more with the loop lowered to the ground. The most sophisticated ground balance metal detectors will gradually adjust themselves as changes in the composition of the ground occur. We refer to this as "Tracking Ground Balance". A good tracking metal detector allows you to balance once, then hunt for the rest of the day without having to balance again. A word to the wise - many metal detectors which are advertised as having "automatic" or "Tracking" ground balance are actually factory preset to a fixed balance point. Its a little like welding your car's accelerator halfway to the floor and calling it "cruise control".
Motion/Non-Motion Modes
Athough the ground signal may be much stronger than the target signal, the ground signal tends to remain the same, or change very slowly, as the loop is moved. The signal from the target, on the other hand, will rise quickly to a peak and then subside when the loop is swept over it. This opens up the possibility of using techniques to separate ground from target signals by looking at the rate of change of the receive signal rather than looking at the receive signal itself. Metal detector modes of operation which rely on this principle are called, not surprisingly, "Motion" modes. The most important example is a mode called "Motion Discrimination". If we wish to isolate the target signal well enough to determine the target's identity, the ground balance alone is not enough. We need to look at the target from a couple of different perspectives, sort of like the way distances can by measured by triangulation if you have more than one viewpoint. We can only be ground balanced from one particular "viewpoint"; the other will contain some combination of target and ground signal. Fortunately, we can use the motion technique to minimize the effect of the remaining ground signal. At the present time, all discriminating and V.D.I. metal detectors require loop motion to be effective. This turns out not to be much of a penalty in practice since you have to move the loop anyway in order to cover any ground.

Once you have located a target in the motion discrimination mode, you will probably want to more precisely locate it for easy recovery. If your metal detector is equipped with a depth meter, you will also want to measure the target's depth. "Pinpoint" locating and depth measurement are done in what is called the "All Metal" mode. Since discrimination is not required to perform these functions, loop motion is not usually required -- except for that motion required to get the loop over the center of the target. More precisely, the speed at which you move the loop is not important. The All Metal mode (also sometimes called the "Normal" mode, or "D.C." mode) is therefore called a "Non Motion" mode.

There are a few potential points of confusion here. Some metal detectors are equipped with a feature called "Self Adjusting Threshold", or S.A.T., which gradually increases or decreases the audio output in an attempt to maintain a quiet but audible "threshold" sound. This helps to smooth out audio changes caused by the ground or inadequate ground balance. S.A.T. may be very rapid or very slow depending on the metal detector and how it's adjusted, but strictly speaking, S.A.T. implies a motion mode of operation. This is why you will hear certain metal detectors referred to as having a "True Non Motion" mode; meaning, of course, an All Metal mode without S.A.T. Another sometimes confusing thing is that some discriminators allow for adjustment down to the point that the discriminator responds to all metals -- in other words, it's a discriminator that doesn't discriminate. This is something very different, however, than the All Metal mode previously described. For this reason we often refer to it as a "Zero Disc" mode.
Microprocessor Control
The microprocessor is a complex electronic circuit which can perform all of the logic, arithmetic, and control functions necessary to build a computer. A sequence of stored instructions called a "Program" is performed by the microprocessor, one at a time, at a speed which can be as high as several million times every second.

The use of microprocessors in modern metal detectors has opened up possibilities which were undreamed of just a few years ago. In the past, adding new and useful features to a metal detector meant additional control knobs and switches. There were obvious limits to this approach; at some point size, cost, and operator confusion got out of hand. With a microprocessor, a liquid crystal display, and a simple keypad the problem is solved. A virtually unlimited number of features can be added without adding any additional hardware. These features can be arranged by a system of "Menus", so that anybody who can follow the prompts on the display can easily find the control they're after and adjust it to their liking. In this way, a single metal detector can be set up for just about any application, or to suit anyone's personal preference.

You might think that this sounds a little complicated -- what if you don't want to be bothered with making all of those adjustments? Here's the real beauty of microprocessor control; you don't have to. Each control can be set to a typically useful position by the microprocessor each time you turn the machine on so the beginner or casual user never has to know that all those advanced features are there. Or better yet, you can select your preference from the menu -- coin hunting, prospecting, relic hunting, etc. -- and the microprocessor will make all of the adjustments for you choosing settings that have been proven in actual use by seasoned veterans.

In addition to these advantages, powerful software routines can be used to enhance the metal detector's audio discrimination capabilities and to display information in a variety of formats on an L.C.D. making the operator's job of interpreting target responses faster and easier.
VLF Summary
Although the modern high performance VLF metal detector has been several decades in the making, new advances will continue to be made. Better, smarter, easier-to-use machines will eventually be introduced. Today's very best metal detectors will not be easy to improve on but as long as there is treasure to be found, you can be sure that research is underway to take metal detecting technology to the next level.



P.I. (Pulse Induction)

Transmitter

The search coil or loop of a Pulse Induction metal detector is very simple when compared to a VLF instrument. A single coil of wire is commonly used for both the transmit and receive functions.

The transmitter circuitry consists of a simple electronic switch which briefly connects this coil across the battery in the metal detector. The resistance of the coil is very low, which allows a current of several amperes to flow in the coil. Even though the current is high, the actual time it flows is very brief. Pulse Induction metal detectors switch on a pulse of transmit current, then shut off, then switch on another transmit pulse. The duty cycle, the time the transmit current is on with reference to the time it is off, is typically about 4%. This prevents the transmitter and coil from overheating and reduces the drain on the battery.

The pulse repetition rate (transmit frequency) of a typical PI is about 100 pulses per second. Models have been produced from a low of 22 pulses per second to a high of several thousand pulses per second. Lower frequencies usually mean greater transmit power. The transmit current flows for a much longer time per pulse however, there are fewer pulses per second. Higher frequencies usually mean a shorter transmit pulse and less power however, there are more transmit pulses per second.

Lower frequencies tend to achieve greater depth and greater sensitivity to items made from silver however, less sensitive to nickel, and gold alloys. They typically have a very slow target response which requires a very slow coil sweep speed.

Higher frequencies are more sensitive to nickel and gold alloys however, less sensitive to silver. They may not penetrate quite as deep as the lower frequency models regarding silver however, can be used with a faster coil sweep speed. Higher frequency models are generally more productive for treasure hunting because the faster sweep speed allows more area to be searched in a given time, and they are more sensitive to the ultimate beach find, gold jewelry.

As previously mentioned a typical PI search loop contains a single coil of wire which serves as both the transmit and receive coil. The transmitter operates in a manner similar to an automobile ignition system. Each time a pulse of current is switched into the transmit coil it generates a magnetic field. As the current pulse shuts off, the magnetic field around the coil suddenly collapses. When this happens, a voltage spike of a high intensity and opposite polarity appears across the coil. This voltage spike is called a counter electromotive force, or counter emf. In an automobile it is the high voltage that fires the spark plug. The spike is much lower in intensity in a PI metal detector, usually about 100 to 130 volts in peak amplitude. It is very narrow in duration, usually less than 30 millionths of a second. In a PI metal detector it is called the reflected pulse.
Receiver
Resistance is placed across the search coil to control the time it takes the reflected pulse todecay to zero. If no resistance, or very high resistance is used, it will cause the reflected pulse to "ring". The result is similar to dropping a rubber ball onto a hard surface, it will bounce several times before returning to rest. If a low resistance is used the decay time will increase and cause the reflected pulse to widen. It is similar to dropping a rubber ball onto a pillow. Since we are interested in having it bounce once critical damping for a rubber ball might be like dropping it onto carpet. A PI coil is said to be critically damped when the reflected pulse decays quickly to zero without ringing. An over or under dampened coil will cause instability and or mask the fast conducting metals such as gold as well as reduce detection depth.

When a metal object nears the loop it will store some of the energy from the reflected pulse and will increase the time it takes for the pulse to decay to zero. The change in the width of the reflected pulse is measured to signal the presents of a metal target.

In order to detect a metal object we need to concern ourselves with the portion of the reflected pulse where it decays to zero. The transmit coil is coupled to the receiver through a resister and a diode clipping circuit. The diodes limit the amount of transmit coil voltage reaching the receiver to less than one volt so as not to overload it. The signal from the receiver contains both the transmit pulse and the reflected pulse. The receiver has a typical gain of 60 decibels. This means the area where the reflected pulse reaches zero is amplified 1,000 times.
Sampling Circuit
The amplified signal coming from the receiver is connected to a switching circuit which samples the reflected portion of the pulse as it reaches zero. The reflected pulse up to this point references in actuality a series of pulses at the transmit frequency. When a metal object nears the coil the transmit portion of the signal will remain unchanged while the reflected portion of the pulse will become wider. The metal object stores some of the electrical energy from the transmit pulse and increases the time it takes for the reflected pulse to reach zero. An increase in duration of a few millionths of a second is enough to allow the detection of a metal target. The reflected pulse is sampled with an electronic switch controlled by a series of pulses which are synchronized with the transmitter.

The most sensitive sampling point on the reflected pulse is as near as possible to the point where it reaches zero. This is typically about 20 millionths of a second after the transmitter shuts off and the reflected pulse begins. Unfortunately, this is also the area where a PI can become unstable. For this reason most PI models sample the reflected pulse at a decay of 30 or 40 millionths of a second, well after it decays to zero.
Integrator
In order for an object to be detected the sample signals must be converted to a DC voltage. This task is performed by a circuit called an integrator. It averages the sampled pulses over time to provide a reference voltage. This DC reference voltage increases when metal nears the coil, then decreases as the object moves away. The DC voltage is amplified and controls the audio output circuitry which increases in pitch and/or volume to signal the presents of metal.

The time constant of the integrator determines how quickly the metal detector will respond to a metal object. A long time constant (in the range of seconds) has the advantage of reducing noise and making the metal detector easier to tune. Long time constants require a very slow sweep of the coil because a target might be missed if it passes quickly by the search coil. Short time constants (in the range of tenths of a second) respond more quickly to targets. This allows a quicker sweep of the loop however, it also allows more noise and instability.
Discrimination
PI metal detectors are not capable of the same degree of discrimination as VLF metal detectors.

By increasing the time period between transmitter shut-off and the sampling point (pulse delay), certain metal items can be rejected. Aluminum foil will be the first to be rejected followed by nickel, pull tabs and gold. Some coins can be rejected at very long sample delays however, iron cannot be rejected.

There have been many attempts to design a PI that can reject iron however these attempts have had limited results. Iron is detectable at very long time delays however, silver and copper have similar characteristics. Such long time delays also have a negative affect on detection depth. Ground mineralization will cause some widening of the reflected pulse as well, changing the point at which a target responds or rejects. If the time delay is adjusted so that a gold ring doesn't respond in an air test, that same ring may respond in mineralized ground. Mineralized ground thus changes everything regarding the time delays and discrimination of PI metal detectors.
Ground Balance
Ground balancing, while very critical on VLF metal detectors, is not necessary with PI circuits. Average ground mineralization will not store any appreciable amount of energy from the search coil and will not usually produce a signal. Such ground will not mask the signal from a buried object. On the contrary, ground mineralization will add slightly to the duration of the reflected pulse increasing the depth of detection. The term "automatic ground balance" is often applied to PI instruments because it will normally not react to mineralization and there are no external adjustments for any specific ground conditions.

Heavy black sand is an exception. It will cause a VLF coil to overload, making metal detector penetration poor at best. A PI detector will work in black sand however, some false signals may result if the coil is held very close to the ground. Ground responses can be minimized by using a longer time delay between the shut-off and sample point (pulse delay). Advancing the time delay slightly will help to smooth out the noises caused by most mineralization.
Automatic vs. Manual Tuning
Most PI detectors are manually tuned. This means the operator has to adjust a control until a clicking or buzzing sound is heard in the headphones. If the search conditions change, such as when moving from black sand to neutral sand or from dry land to salt water, the tuning must be re-adjusted. Failure to do so can result in reduced detection depth and missed targets. Manual tuning is very difficult with short integration time constants, so most manually tuned models use long time constants and the search coil must be swept slowly.

This is not a problem when a PI is used for scuba diving because the coil cannot be swept quickly underwater. When used at the surf line, where the coil will be in and out of salt water, a manually tuned metal detector can be very frustrating to use. The tuner must be adjusted continually to maintain a threshold. Some operators elect to set it slightly below the threshold however, that can result in a reduction in depth as the ground conditions change.

Automatic tuning, or S.A.T. (Self Adjusting Threshold) offers a significant advantage when searching in and out of salt water or over mineralized ground. S.A.T. helps keep the metal detector operating at maximum sensitivity without requiring constant adjustments by the operator. It improves the stability, reduces noise, and allows higher gain settings to be used. PI metal detectors do not emit strong, negative signals like a VLF. As such they do not "overshoot" on pockets of mineralization. With S.A.T. the coil must be kept in motion while detecting a target. Stopping over a target will cause the S.A.T. to tune it out or cease responding.
Audio Circuits
PI audio circuits generally fall into two categories: variable pitch and variable volume. Variable pitch or V.C.O. (Voltage Controlled Oscillator) audio has the advantage for faint targets because the change in pitch is easier to hear than a change in volume at lower aud io levels. This is primarily true for manually tuned models. The "fire siren" sounds can become annoying and many have trouble hearing the higher tones. A variant of this is the mechanical vibrator device primarily used for deep water. It emits a slow clicking sound and vibration that increases to a buzz to signal a find. The mechanical device is easier to hear and feel over the sound of an underwater air supply.

Many people prefer a more conventional audio tone that increases in volume rather than pitch to signal a find. This audio system works best with a PI metal detector that has a fast target response and automatic tuning (S.A.T.). Automatic tuning makes the PI sound and respond similar to a typical VLF metal detector.
PI Summary Pulse Induction metal detectors are specialized instruments. They are generally not suitable for coin hunting urban areas because they do not have the ability to identify or reject ferrous (iron) trash. They can be used for relic hunting in rural areas where iron trash is not present in large quantities, or is desired. They are intended for maximum depth under extreme search conditions such as salt water beaches and highly mineralized ground. In such conditions PI type metal detectors produce superior results when compared to VLF models, particularly in the ability to ignore such extreme ground and penetrate it for maximum depth.


White's instruments are protected under one or more of the following patents.
Other patents pending.

USA 4030026
USA 4128803
USA 4249128
USA 4293816
USA 4783630
USA 4862316
USA 4868910
USA 5414411
UNITED KINGDOM 1548239
MEXICO 147016
CANADA 1038036
CANADA 1118867
CANADA 1165817
CANADA 1266107
AUSTRALIA 53196


Return to Jesse's Metal Detector page

[kt315]

What Does It All Mean?
The metal detection industry is full of jargon that may not mean much to someone new to the hobby (or even veterans). That's why at Garrett, we strive to keep you informed of the latest technology, trends and features in the industry. Why do we want to keep you updated? Because we feel strongly that supplying you with good information will empower you with the ability to recognize that Garrett detectors shine where all others fall short. We want you to be successful. We want you to be a better treasure hunter. We want you to be satisfied with your detector. So, whether you are buying your first detector or upgrading to a new one, you can rely on Garrett to give you straightforward information that will lead you to the right detector for your treasure hunting needs.



TERM EXPLANATION
All-Metal Mode A metal detector setting that detects all metal objects, no discrimination
Audio Threshold The background audio level produced when no target is being detected - it is best to adjust the audio threshold to the lowest audible level, and recommended the operator use headphones when treasure hunting
Audio Tone The pitch or frequency of the sound made by a detector. The tone on the GTI 2500, 1500 and GTP 1350 detectors can be adjusted on a treble to bass scale.
Cache Larger deposits of treasure that generally consist of money and valuable objects
Classifier: A filtering device, typically found at the head of a sluice, which helps prevent rocks and other large debris from falling into a gold pan.
Coin Shooting Hunting for coins regardless of location or era of coins targeted
Composite Digger Trowel made of durable plastic that helps prevent coin damage during recovery. Ideal for soft terrain
Control Box Contains the detector's main circuitry, controls, speaker, batteries and microprocessor chip

DD Searchcoil A special configuration of the transmit and receive coils to minimize the effects of ground minerals
DSP (Digital Signal Processor) A highly advanced computer chip used in Garrett detectors and other sophisticated electronic equipment
Discrimination The ability of a metal detector to reject a target, such as a pull tab and foil or accept a target such as a coin or jewelry based on its metallic composition
FastTrack (See Ground Balance) Garrett's exclusive technology that analyzes ground mineralization and adjusts to "cancel" its effects in a matter of seconds
Frequency The number of times per second the energy transmitted from a detector's coil changes direction (e.g. 7.0 kHz = 7000 times per second) - higher frequencies are typically used to find targets such as gold nuggets, while lower frequencies are best for general purpose hunting
Gold Pan A bowl-shaped, shallow container that traps gold flakes
Gravity Trap? Gold Pan A patented gold pan made by Garrett which has 90 degree riffles to trap small gold
Ground Balance (See GroundTracking) An adjustment made to "cancel" or ignore ground mineralization; may be done manually or automatically
Ground Tracking The ability of a metal detector to continuously measure the ground's mineralization and automatically adjust the detector's ground balance setting for optimum performance
GTA (Graphic Target Analyzer) Exclusive Garrett technology that visually identifies a target's conductivity or ID and also shows the discrimination pattern
GTI (Graphic Target Imaging) Exclusive Garrett technology that measures and displays a target's true size and depth

LCD (Liquid Crystal Display) A graphical display that indicates target information, detector settings, etc...
Multiple Frequency See Pulse Induction and Multiple Frequency article
Microprocessor Computer chip that performs digital functions that make many features such as Target ID and Discrimination possible on today's Garrett detectors
Mono Searchcoil Refers to searchcoils with one ring where both transmitter and receiver antennae are located
Motion Mode Refers to the setting where coil motion is needed to detect targets
Notch Discrimination Targets above and below these discrimination settings
Pinpoint A mode of operation that allows the operator to detemine the precise location of a target still in the ground
Pulse Induction Used primarily for heavily mineralized environments such as the beach or the gold fields of Australia and is found in many of today's specialty detectors (See also Multiple Frequency)
PowerMaster Exclusive Garrett feature that increases the detector's ability to detect deeper and wider - up to 20 percent
Probe A long screwdriver-like device made of brass used to penetrate the ground and physically locate a detected target before digging it up
Prospecting Hunting for valuable metals such as gold

Relic Hunting Hunting for targets with historical value, such as old battlefield items or family heirlooms
Salt Elimination A detector's ability to eliminate the interference of salt mineralization, which adversely affects detection depth and target ID capabilities
ScanTrack A unique Garrett feature that automatically adjusts to the operator's scan speed to achieve optimum performance
Searchcoil Also referred to as the "coil", the searchcoil is the flat, typically circular disk swept over the ground to sense the presence of metal
Sensitivity Synonymous with Depth, the adjustment that determines how deep or small a target can be detected - the higher the sensitivity, the greater the detection depth
Shaft The adjustable stem that connects the control box and the searchcoil

Single Frequency Offers greater potential depth capabilities, better discrimination and enhanced target ID under most common soil conditions where most treasure hunting occurs (See Multiple Frequency article for more information).
Super Sluice? Large 15" gold pan with 1/2" deep riffles. Traps small gold nuggets up to one ounce and larger in size
Surface Elimination A detectors ability to ignore all targets located on or near the ground's surface, which is useful in heavy trash areas
Surface Mount PC Board Technology The latest trend in constructing electronic circuit boards
Target Any metallic item sensed by a detector
Target ID Cursor A graphical indication of the target's probable identity (e.g. coin, gold, pull tab) based on its electrical properties
TreasureTalk Garrett's exclusive voice function found only on the GTI 2500 that audibly announces various settings, adjustments and target information continuously or on demand
Volume Control The ability to adjust the loudness of the audible response produced by the detection of a target

[kt315]

UNDERSTANDING THE PI METAL DETECTOR
BY REG SNIFF


One of the more popular metal detectors used for nugget hunting today is a type of detector commonly called the Pulse Induction or PI for short. A lot has been written on the general principles of operation but many questions are still unanswered or not answered completely about this strange machine. Also, there is a lot of misinterpretations of information that has been written about PI's and how they work.

As an example, in some books there is a statement that a PI does not "see mineralization" so it is therefore a great detector to use in mineralized areas. Is this really a true statement? The answer is both yes and no.

PI's basically do not respond to the typical iron mineralization such as magnetite or black sand. However, other minerals of the same family can and many times do cause a response. Iron oxides such as maghemite, clays, and other things such as salts commonly found in the ground can cause a PI to produce a rather strong signal. So, generally a very sensitive PI, normally used for gold hunting will respond to ground signals, especially if it does not have some form of ground balancing circuitry built in.

One question that is often asked is what is the operating frequency of a PI. This question is often asked by someone who is trying to relate their knowledge of VLF's to the PI. Unfortunately, because of the nature or differences between types of detectors, comparing a PI to a VLF is sort of like comparing an apple to a potato, so trying to relate the operating frequency of a PI to a VLF or sensitivity to small gold is of little value. The differences between the two types of detectors or the affects of their operating frequencies are quite dramatic so it is best to not try to use the same standards when trying to determine certain things about a PI.

As for a PI, the pulse rate or pulses per second (pps) refers to the number of high current pulses that occur over the time specified. Rates vary from a few hundred to several thousand per second: Generally, more pulses allow for a little better averaging and thus a little better signal to noise ratio. However, a detector will have a tendency to consume more current with a higher pulse rate. A faster pulse rate doesn't mean a detector will detect small gold better. In fact, it is quite easy to build a PI that has a very low pulse repetition rate (PPS) that is very sensitive to very small gold while designing a PI with a high PPS that is not sensitive to small nuggets.

Now, both PI's and VLF's will detect metals, respond to different ground conditions, and even respond to salt water. Both use a coil, specialized circuitry and usually generate a similar output to indicate some object has been detected However, the techniques, circuitry and in many cases, the coils are dramatically different.

VLF's generally produce a relatively low power continuous sinewave into the transmit coil and, analyze a signal received with a separate receive coil winding. A signal from an object will increase the amplitude of the receive signal level but will also shift the receive signal with respect to the transmit signal. Thus, an object can be analyzed by not only the intensity or amplitude increase of the signal but by just how much the signal has shifted.

VLF's generally operate at a single frequency but can be produced to operate at different frequencies. However, each frequency has to be analyzed as if it is the primary frequency and as such, both the signal strength and the shift are used to determine the presence of an object as well as type of metal.

PI's are a different beast all together. Instead of transmitting a low power continuous signal, the PI generates a brief high current pulse to energize the coil and this pulse is repeated at some nominal repetition rate, which can vary from a few hundred pulses per second to thousands per second.

The technique to determine whether an object is present is to analyze the signal coming from the receive coil shortly after the high current pulse is turned off. This is done by sampling the signal coming from the coil some time after each high current pulse. This time after the pulse is often referred to as the delay time Remember, on a PI, the transmit coil may become the receive coil once the transmit signal is turned off so there is no need for a separate receive coil winding. This type of coil is often referred to as a Mono coil.


PI History


There has been considerable work on PI type detectors since the early 1960's. One of the main reasons for their design was so they could be used for archaeological purposes. Most of the work in the evolution of the PI occurred in Europe during those early years, and much of this work was done in England by a young engineer by the name of Eric Foster.

As a result of his involvement with PI's during their early years, Eric Foster began his own business building PI's for industry as well as the consumer market. Many of his initial designs are the cornerstones of some of the PI's used today. Sometime in the early 1980'5, Eric Foster built a PI with ground balancing capability and a rudimentary form of discrimination. He also built a much better discriminating PI around the same time frame.

Minelab was the first to introduce a PI specifically designed for gold hunting in the US some time the 1990'5. The introduction of the SO 2000 really started the serious use of PI's to search for gold even though people began using Eric Foster's detectors for nugget hunting sooner in Australia. What made this ML PI detector excel was the introduction of the use of a DD coil on a PI. The DD coil had the ability to eliminate much of the ground problems making it a quieter choice. One other major advantage of the ML was it operated and sounded much more like a VLF. A PI equipped with either a mono or a DD coil will ignore many hotrocks and have additional depth of detection. However, this depth advantage is greatly reduced in very quiet ground.

Eric Foster's ground canceling detector had a putt-putt type audio, required the operator to retune the detector frequently, and had several different modes, some of which made the ground balance mode seem much less sensitive. Also, since only a mono type coil was available, some of the more severe areas still caused problems even when the ground balance was used. As a result, Eric Foster's ground canceling PI, the Goldscan, never really caught on.

Strange as it may seem, one of the first US patented designs using a high current pulse to detect metals that also had ferrous/non-ferrus discriminating capabilities was designed by George Payne in about 1978 or so. This design not only would distinguish iron objects but also had a basic form of ground balance. This strange design used a bi-polar form of pulsing, which was also unique. Unfortunately, because of the high current necessary for operation and thus, the need for a very large battery, the design was never produced and sold. Instead, American manufacturers focused on developing VLF's for both coin and gold hunting.


How do PI's really work and what makes them sensitive to small gold?


As stated earlier, PI's operate on the principle of generating a large current pulse in the coil and then analyzing the signal in the coil a short time after the pulse is turned off. This cycle is repeated on a continual basis.
As simple as this sounds, the design is quite critical. The key to increasing the sensitivity of a PI is to turn off the current pulse as soon as possible, and then stopping the resulting high voltage spike as soon as possible.

By nature, a PI coil is an inductor and as such, any immediate disruption in current will cause the inductor to produce a very large voltage spike in its attempt to keep the current flowing. This high voltage spike is a side affect of the current disruption that has to be dealt with as quickly as possible so any signal from a metallic object can be distinguished.

When the current is flowing in the coil, a magnetic field is generated that expands from the coil. When this field encounters a metallic object such as a gold nugget, current begins to flow in the nugget as the result of this magnetic field. When the current suddenly stops in the coil, the coil field collapses which in turn causes the current in the object to collapse. This secondary collapse of current in the nugget causes it to produce its own field that now generates back to the coil. This target signal ultimately adds to the collapsing coil signal, thus making the coil signal change very slightly.

The signal strength, and just as important the duration or time of the signal produced by a detected object is a function of the size, shape, and actual composition, among other things. Gold and other low conductive materials may produce a strong signal but the duration of the signal is much shorter than a signal from something like a piece of iron, copper or silver. Very small nuggets, in the few grain range, not only generate a very small signal, but also a very short signal.

Small iron objects, on the other hand, will produce a much larger signal as well as a much longer signal than a piece of gold of similar size. Thus, it is much easier to detect a very small piece of iron than it is to detect a very small piece of gold.


Finding the small stuff in Greater Detail


The key to the success or sensitivity of a PI to small conductive objects such a small gold nuggets is the ability of the PI circuitry to turn the coil pulse current off very rapidly, and then be able to analyze a signal very shortly after the pulse of current has ended. This sudden stop of current in the coil will cause a very large voltage spike that rises almost instantaneously to some voltage generally between 50 to 400 Volts (V). Generally, the voltage level is a function of the FET (field effect transistor) used to deliver the high current. Once this voltage peaks, it will then quickly decay to very near 0 Volts ( 0V) in just a few microseconds (usecs). The rate of the decay is extremely important, just as is the characteristics or shape of the decay of this large voltage spike.

One important factor to remember is a large current normally requires more time for the spike to decay. This becomes important when determining the best design for small gold. Another critical factor is the inductance of the PI coil itself. The larger the inductance, the longer the decay time to OV.

It is also critical that this high voltage spike doesn't result in oscillations, which can easily happen. Generally, the coil, for a PI, is made by first determining the desired inductance. Then the coil size or diameter selected. Once the two characteristics are determined, calculations are made to determine the required number of turns of wire to produce the calculated value of inductance.

Once built, the coil of wire is basically an inductor that has some internal resistance. However, because the coil consists of multiple windings, generally a number between 10 and 35, the windings produce a certain amount of capacitance between windings. This capacitance when combined with the inductance of the coil will create a "tuned circuit that will oscillate if additional circuitry isn't added to dampen or stop the oscillation. The basic damping device normally used is a resistor, generally called the damping resistor.

So, by carefully selecting the right resistor, a coil will produce a rapidly decaying voltage spike that doesn't ring or oscillate. If the resistor has too high a value, there will be some very minor oscillation, and if the resistor is too low in value, the spike voltage will take too much time dropping to the OV range.

One other critical part of a search coil that is seldom talked about is the shielding of the coil. Generally, coils have some form of a shield called a Faraday shield. The purpose of this shield is to minimize the capacitive effect between the coil and the ground, reduce static, and to absorb or reduce external noise. Like other factors, the shielding and the technique used, is quite critical. Too much or the wrong type of shielding can reduce sensitivity, especially to small objects such as gold nuggets. Too little shielding will allow other factors such as noise, signal variations due to the ground capacitance, etc to affect the signal. The shielding can also affect the decay time so it can affect the ability to detect small nuggets.

It should be noted that some manufacturers do not use any shielding at all. However, these detectors normally are designed for the detection of very large iron objects so any minor variations in noise or ground capacitance that normally affect very small non-ferrous objects such as small gold nuggets is not a problem. Such detectors normally operate with a very long delay before sampling. This long delay will cause most of the ground signal to be eliminated since it will decay much faster than a signal from a large iron object.

The technical information mentioned above is of little value to the average user of a PI. However, it can be important to anybody who wants to try to build a coil for their detector. The first rule of thumb when trying to build a different coil is to try to duplicate the electrical characteristics of the factory coil. By this I mean, one should try to keep the resistance the same as well as the inductance the same.


How is detection really done?


Both PI's and VLF's take a sample of the receive signal for analysis. In the case of the VLF, the receive signal sample is analyzed with respect to the transmit signal. By doing this, any signal "shift", commonly called phase shift, can be "seen". In other words, the sample is taken by syncing the sample to the transmit signal so the sample is always synchronized to the transmitter. The circuitry used to sample the received signal is normally called the synchronized demodulator.

On a PI, the signal from the coil is initially amplified and some time after the large current pulse is stopped, a sample of the amplified coil signal is taken. Since there is no transmitting going on at the time of the sample on a PI, timing is generally done by waiting a finite time after the termination of the large current pulse and then taking a sample. In this way, there is a form of synchronization also. The time between when the pulse quits and the sample is taken is often referred to the delay time. The delay time on most Gold Hunting PI detectors is 15 usec or less. A delay of 10 usec will show a distinct improvement, especially to very small gold in the few grain range over a detector having a delay of 15 usec.

This delay time is quite critical and is sometimes changed to create a crude form of discrimination, or rather reverse discrimination in the case of gold. As I mentioned before, the signal from gold can decay very quickly. In fact, the signal from most gold nuggets smaller than a 1/4 oz can decay in less than 50 usecs. If the delay is adjusted to 50 usec, then most small nuggets will be ignored, or phrased another way, will not produce any audio response. However, signals from objects made of iron, copper, silver or other highly conductive metal will normally still produce a strong signal. So, if a detector samples the signal at a time later than 50 usec or so, and this sample does not "see" a target, there is a good possibility the object is gold or some other type of low conductive material.

Since the analysis or sampling of this decaying signal is normally only done when the signal gets very near OV, any additional time to drop to the OV level will cause very small gold nuggets to be missed. The reason is because the reflected signal caused by the nugget is very brief and it combines with the normal signal from the coil.

If the nugget signal dissipates before the main signal decays to OV, then, when the sample is taken to determine whether an object is present, the signal from the small nugget will have already subsided and the nugget will be ignored.

Once a sample is taken, this sample voltage is held in suspension, for a better choice of words, until the next sample occurs, which adds to or subtracts from the previous sample. Because of the suspension, normally called sample and hold, and the filtering process built in to reduce noise, multiple samples are required before a true average signal is developed.

Once this average has leveled out, which normally takes a very brief time (in the thousandths or hundredth's of a second), any object that produces a change sufficient to be seen, will cause an additional signal that alters the receive sample average, which then causes the output to change or increase. This subsequent change is further amplified and ultimately is heard as an audio response, normally in a set of headphones.




The Up Side of PI's


Probably the biggest claim to fame of a PI is the additional depth that can be obtained. The key to this is the increased amount of power into the coil that can cause a stronger return signal from a buried object. However, even though there is a significant increase in current, the depth difference between a PI and a VLF isn't as dramatic as one might expect.

Many people question whether this depth advantage between a VLF and a PI is really that great in areas having almost no mineralization but overall, the PI appears to be superior simply because such places are few and far between. Where the PI really excels is in places having a much higher ground mineralization as will as locations where magnetite type hotrocks are common.

Next comes the big debate of just how a PI only using AA batteries can even come close to obtaining the depths of a different PI using a very large heavy duty battery .Obviously, the PI using the AA batteries cannot be pulsing with the same amount of current as a PI using a much bigger battery.

The fact is, the PI that uses AA batteries can approach the depth of other more powerful PI's, especially on gold less than an ounce in weight. There are multiple reasons this can be true. One reason is the fact that there because of a law of diminishing returns, which simply means it takes a whole lot of current to produce a very small depth increase because of shear power alone. As an example, it may take something like 4 amps of current to increase the depth 1 inch over a PI only pulsing with 1 amp, and this is only true if all other factors are equal.

One important factor that determines the sensitivity of the detector is sampling delay time. The sooner a sample can be taken, the stronger the signal that will be seen. In other words, it is quite possible to take a sample sooner and produce a stronger signal on a PI operating with less current than might be seen on a more powerful PI using much more current and having a longer delay. One simple way to allow earlier sampling is to reduce the coil current.

In other words, there are a whole lot of other factors that need to be taken into account to determine what is the best combination. Ah, but someone who just read the previous information might simply say, pulse with a strong signal and then simply sample sooner to make the best detector. Well, unfortunately, the stronger the pulse, the more difficult it is to sample sooner because of the reasons mentioned before. A longer pulse or lower coil resistance will result in more coil current, which will affect how long it takes for the spike to decay. A larger inductance will also result in a longer decay time. In fact, it becomes almost impossible to obtain the very short delay times when using a very strong pulse of long duration.

One way to help shorten the delay time of the decaying pulse is to reduce the number of turns of wire in the search coil. However, the field strength of the coil produced when current flows in the coil is a function of both the current and the number of turns, so reducing the number of turns also reduces the field strength produced. So any reduction in number of turns directly relates to potential depth loss.

If all this seems confusing, it is. Not only does the actual current have an effect, but the actual pulse length or time the current flows has an effect as mentioned before. Therefore, it is possible to pulse fewer times and use a shorter pulse and obtain very satisfactory results.

Pulse lengths of 50 usec or less will still produce a very decent signal from most of the gold nuggets that are found with a detector. Increasing the pulse length to 200 usec will really only have an impact on the signal coming from very large gold objects. The reason why the large increase in pulse time doesn't help on most smaller gold is simply because most of the smaller gold is fully saturated by the shorter pulse. Any additional pulse really does nothing to the potential signal that will come back form that gold object.

As a general rule though, a more powerful PI having a very long pulse will generally go deeper on very large gold, meaning nuggets weighing several ounces or more will be more readily detected to greater depths.

A detector using a high current short pulse will have a tendency to be more sensitive than a detector using less current. However, this difference normally is not dramatic, if at all. The key lies in early sampling and noise reduction.

One other major advantage of a Pl over a VLF is the fact that many of the hotrocks or black sand that make a VLF Scream will cause little or no signal on a PI. Normally, these intense hotrocks create a response on a VLF because of the magnetite within the rock.

A PI will seldom create a response to a magnetite rock or black sand due to the fact a magnetite hotrock or black sand signal will normally dissipate well before a sample is taken. However, an unbalanced earth field effect elimination can cause a hotrock to create some response, as can a very quick sample. In the case of some of the PI kits people build, where there is no earth field effect elimination, a long transmit pulse will cause a magnetite hotrock to produce a very strong signal, much like the signal from a metal object.

When a PI is pushed to the limits, even a PI will begin to produce a much louder signal on more and more magnetite type rocks just due to all the factors involved.

Where a PI really excels is in situations where a nugget is buried under or along side a magnetite hotrock. This combination spells disaster for most VLF's simply because the rock will generate a much stronger negative signal than the slight positive response from a piece of gold. So, it is very possible a VLF will miss a piece of gold in such a combination.

A PI, on the other hand, will look though the magnetite as if it isn't there, so a magnetite hotrock and gold combination will produce a desired signal. In some cases, the rock may just add a little positive signal thus causing the evasive gold target to be detected.

I do want to mention again that if the delay is extremely short, the earth field effect cancellation circuitry isn't perfect, or the rock or black sand contains other types of certain materials, a typical magnetite rock or black sand can generate a small but noticeable signal. One should also remember that it is quite likely that there are other members of the iron oxide family or even other metals in very small quantities that will cause a response and these other oxides may be present in the black sand or in a rock that appears to have large quantities of magnetite. So, any response a person gets from a rock my be caused by a combination of things.

Other hotrocks such as basalt or other similar hotrocks may cause a weak but noticeable signal much like a deep target. Fortunately, the signal from such rocks quickly subsides as the coil is raised a little. Thus, deeper hotrocks will seldom produce a signal.


PAGE TWO
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By Reg Sniff
Copyright 2003 The author reserves all distribution rights.



NUGGETSHOOTER HOME

[kt315]

UNDERSTANDING THE PI METAL DETECTOR
BY REG SNIFF
PART TWO



The down side of PI's


All detectors are subject to external noise, but PI's are extremely temperamental in this regard. The reason PI's are more sensitive to noise is due to the design of the preamp or first amplifier stage. For a PI to work at optimum this amplifier has to be built to amplify a very wide range of frequencies to assure the decay signal isn't altered. This type of amplifier is commonly called a broadband amplifier.

On a VLF there normally is one specific operating frequency and the preamp or first amplifier is somewhat tuned to that frequency. Thus, other signals such as noise will not be amplified as much.

On a PI, all signals are basically amplified the same, thus all noise, especially electrical noise is amplified just like the signal from a nugget. This problem is compounded by the fact that the coil itself will generate a very small voltage as it is moved or swept above the ground.

This very small voltage is the result of the coil moving through the earth's magnetic field and this extremely small voltage is often referred to as the earth field effect (EFE) signal.

For a PI to be very sensitive, there has to be a lot of amplification of the sampled signal. As such, even the small voltage produced by the coil movement will create a signal large enough to be heard. To eliminate this EFE problem, a second sample is normally taken much later in time and this second sample is effectively subtracted from the first or main sample. Since the earth field effect is a very slow signal, taking the later sample will eliminate most of the earth field effect signal, thus it is normally not heard.

However, this subtraction process is seldom perfect and because the sample is taken at a different point in time, there will always be a very slight response, that just might cause a very slight increase or decrease in a target response. Most likely this will occur or be noticed on the targets just on the threshold of detection. However, if for some reason the subtraction process is not near perfect, then it is quite possible that there will be a noticeable increase in signal strength when passing over a target from one direction than when passed over on the opposite direction of the coil swing.

The earth field effect is one of the reasons there may be a slight response at the end of a coil swing. The sudden stop of the coil before moving the opposite direction produces a much stronger EFE signal. One should also remember that it is almost impossible to totally eliminate the EFE so some of the minor responses mentioned should be considered normal.

One final noise problem is the noise generated within the detector itself. Just due to the nature of the circuitry used, a lot of noise is generated within the electronic circuitry .Much of this noise is "filtered" by the battery and aided by large capacitors and other filtering devices such as ferrite cores. However, no battery or capacitor is perfect so some noise always gets through.

The result of all the combined noises commonly creates a form of chatter or warble that can significantly reduce the sensitivity, especially to very small or deep objects producing very weak signals. In many cases, the noise may not even be really noticeable but be of sufficient amplitude to cause a reasonable depth loss.

Ground signals and Ground Balance



PI's are susceptible to many types of ground conditions, and, depending upon the type of ground, the sensitivity, and the delay, may generate a very strong signal due to the ground.

Terms like magnetic viscosity are used to explain just why certain types of ground can cause a strong response. Ground conditions having concentrations of maghemite will create very strong ground signals.

Areas having a clay base seem to produce strong ground responses also indicating that clay itself is part of the problem. Articles such as those written about geophysical research indicate that the clay problem can vary dramatically because of the type of clay as well as the moisture within the clay.

An article written by the Army Corps of Engineers indicates that clay will actually create a field that opposes the transmitted field of the PI and moisture enhances the ability for the clay to oppose the field due to the ionic behavior within the clay.

Coils, Coils, and more Coils



VLF's always have a transmit coil and a separate receive coil of wire in the search head. PI coils, however, can be produced in several variations. If the same coil is used for both the transmit and the receive signal, the coil is normally called a "MONO" coil.

If two sets of coil windings are used, and those coils are basically the same size and shape with one coil used as a transmit and the other the receive, and they overlap a small amount on one side, the coil is generally called a DD coil. The name DD generally refers to the design of the coils where they are sort of like D's with one D reversed and the backs of the D's overlapping slightly. This overlap area is the main detection zone and is the area where an object is under at least part of both coils at the same time. This detection zone is most noticeable on deeper objects.

By nature, DD coils are somewhat less sensitive when compared to a mono coil of the same size. One reason for the reduction in sensitivity is the fact that the DD electrical coil windings are smaller in size than the coil windings of a mono coil even they may have the same size coil housing.

One other key factor that is important is the fact to remember about a DD coil is the main detection zone is quite narrow. This narrow detection zone, normally at or near the overlap will create a very brief or narrow signal when compared to the signal on a mono coil. This situation makes the sweep speed of the search coil much more critical. Swinging the coil too fast can easily cause a very weak object to be missed simply because the signal is so short and the circuitry filtering used to eliminate the noise will also almost eliminate such a signal.

The fact that DD coils have smaller diameter windings for the transmit and receive coil, when compared to a mono coil using the same size housing, has some advantages. Generally, the smaller receive coil is not as good of an antenna as a larger mono coil, thus less noise is detected and amplified. As a result, the detector can be much quieter when using a DD coil. In many cases the reduction in noise can outweigh the depth loss due to the size difference.

One other major asset of a DD coil is the fact the receive coil is isolated from the transmit coil. This helps in the fact that any low level noise that is generated by the transmit circuitry during the sampling time is isolated from the receive circuitry. This isolation therefore reduces the combined noise that can negatively affect a target response.

One final advantage of a DD coil is, by nature, a DD coil partially cancels the ground signal. If the coils are properly aligned or positioned, most ground signal in the receive coil is eliminated. This results in a detector that has very little ground response, yet still responds with a strong signal from a buried object.

Another type of coil that is made for PI's is called the figure 8 or "Salt" coil. In this design, there is a large transmitting coil and two receive coils that are wired such that the receive coils are opposite of each other, meaning that one coil will produce a positive signal and the other a negative signal. On this type of coil, it is quite common to build a larger receive coil, elongate it, pinch the center of the elongation and then twist one half of the receive coil one half turn to create two coils, much like a figure 8. As mentioned before, this type of receive coil is also called a "figure 8 coil" just due to how it is constructed.

One advantage of a "salt" coil having a large transmit coil and two smaller receive coils is the design is both ground canceling and noise canceling. The ground canceling relies on the principle that both receive coils are equally spaced from the ground for maximum ground signal elimination.

The disadvantage of the Salt or figure 8 coil is there is a depth loss that occurs when compared to a similar sized mono coil or even a similar sized DD coil. Part of the reason for the depth loss is the fact the two receive coil signals basically oppose each other since one will produce a positive receive signal and the other will produce a negative signal. This opposition will cause some receive signal to be eliminated.

Because the signals from two receive coils have a tendency to cancel each other, any noise detected by the two coils basically is also canceled. This cancellation process has one other advantage and that is, it will eliminate the earth field effect.

A little different figure 8 coil can be built where there is only one coil used as both the transmit coil and the receive coil. This coil is again, elongated, pinched, and one half of the coil is twisted over so half of the coil is transmitting up when the other coil is transmitting down. This type of coil eliminates or cancels external noise extremely well but does not ground cancel. Since the two xmit coils are much smaller, there is also some depth loss on this type of coil because of the size of the coils as well as the signal from the two halves of the coil have a tendency to cancel each other. As such, the signal from a buried object will be the greatest when it is centered or near centered under either of the two coils and the weakest when the object is right at the crossover point of the two coils.

One of the most common coils found on a VLF is something called a concentric coil. In this case there generally is a large transmit coil and a smaller receive coil basically centered in the large coil. For this type of coil to really work correctly on a VLF, there will be an additional transmit coil wound directly on the smaller receive coil, but will wound opposite to the main transmit coil. The purpose of the smaller transmit coil is to cancel any signal in the receive coil caused by the larger transmit coil A concentric coil design can be used for a PI, but it is rare to find one..

Of course, there can be variations of the above coils, meaning they can be rectangular, round, oval, or any other shape a person should desire. Also, the windings can be changed or possibly additional windings can be incorporated to produce the desired results. So, the ultimate design of a search coil is left to the imagination of the designer.

Finally, some mention has to be made regarding coil size. The coil size of most PI's normally ranges from an 8" diameter coil to greater than 3 feet in diameter. It is quite common to hear of a person using an 18" diameter coil, but the most popular sizes range from 11" to about 14".

Recently, Eric Foster posted some interesting findings regarding the general detection ranges of different sized coils versus target or object size. This information can be viewed on the PI forum and displayed on September 16, 2002. Several discussions occurred during that time pertaining to the depth verses coil diameter, versus target size.

As one might expect, the larger the coil, the deeper one may find objects. However, it is quite possible a smaller coil will find an object deeper than a large coil, especially if the object is small. Contrary to the some of the discussion that resulted on the above mentioned forum, there is a more direct relationship between the size of the coil, size of the object and the ideal maximum depth such an object can be detected. An error in calculations led to some information being incorrectly noted.

One should realize that information such as what Eric Foster posted is generally theoretical and as such is subject to some distortion in the real world. However, as a rule, the general principle is quite accurate.

When searching for information about depth or size of objects that can be found with different size coils, extreme cases always show up. For example, many people have found extremely small nuggets ranging in the few grain range with an 18" coil. Normally such a large coil will not be able to see such a small target at any depth, or even in the middle of the coil if the nugget is small enough. However, this small nugget can produce a signal if it is very near the coil windings themselves.

One final point that I am sure will cause controversy and that is a smaller coil will not show as dramatic increase to sensitivity to small gold on a PI like it does on a VLF. The reason, again, lies in the fact that the sensitivity to small gold on a PI, is much more dependent upon the delay before sampling than it is on the coil itself.

Ground Balance Differences There is a world of difference in ground balancing techniques between a VLF and a PI. On a VLF, a sample can be taken such that the signal from the ground appears to be eliminated. Actually, it is still there, but by the sampling at the right time half of the signal is positive and the other half negative, so the net effect is 0.

On a PI, no such condition can exist because of the fact the transmit time is separate from the receive time. So, another method has to be used. One common method is to take advantage of the fact the ground signal lasts for a long time. So, if the initial sample is taken to look for a target and then a later sample is taken that still contains ground signals and this later sample is amplified, and then subtracted from the first or main sample, the ground signal can be minimized, thus leaving the target signal.

Unfortunately, any subtraction process also reduces the signals of targets also having a long decay. As it turns out, some gold signals will be very similar to the ground signal, so this subtraction process can effectively reduce the response from some gold objects.

In the case of larger gold objects, the subtraction process can actually cause the signal to change from an increasing response to a decreasing response, meaning a piece of gold would create a negative signal. This negative signal can easily be "rectified" much like the rectifier in any other circuit. The rectification process will then make the large gold also respond with a positive signal, rather than a negative signal.

However, as mentioned before, some gold will respond much like the ground so there will be some gold objects that will be cancelled much like the ground signal To overcome this problem, different length pulses can be used and multiple subtraction processes incorporated.

If a longer pulse is also used, the longer pulse alters the ground signal characteristics. This alteration is sufficient that it requires a different ratio of subtract signal to cancel the ground response. This change in subtraction level then changes which gold might be eliminated. As such, any gold that might be eliminated by only using a short pulse will produce a strong signal when using a long pulse, and visa versa. The result is most gold will be detected quite strongly if pulses of different duration are used. However, in all cases, many of the larger nuggets that have a decay lasting longer than the time when the ground signal sample is taken will also be reduced in signal strength.

Another technique can be used for ground balance but it is generally used with a DD coil. In this case, a sample is done during the pulse on time as well as the pulse off time. The two different samples produce different signals, which then can be combined to minimized the ground response. This type of sampling can also be used to produce a better form of discrimination which would be much more accurate.

One technique that can be used is a variation of the first ground balance technique. This method just minimizes the ground response using the subtract method. By doing this, the ground signal is minimized significantly but gold responses are not eliminated. Some nugget responses are, however, reduced in signal strength and as such, there is some depth loss. This normally occurs with nuggets greater than 2 gram or so.

Finally, Discrimination on a PI



Due to the nature of the signals caused by different objects, it is extremely difficult, or put another way, almost impossible to build a good discriminating PI.

Since the time it takes for a target signal to decay can vary because of the size, shape, and chemical makeup of the object, then any type of later sampling will not produce a reliable form of discrimination.

Many PI's rely on the ability of an adjustable delay whereby the operator can simply adjust the delay longer to see if an object is a piece of gold or not. If the delay is increased and the signal from an object disappears, then the operator can assume the object is made of a lower conductive material such as gold. This is acceptable for those hunting something like gold rings, but does not work well on gold nuggets. larger gold nuggets can produce a much longer delay, so any attempt to use this delay technique will result in one thinking a large gold nugget to be junk.

Another concept used on a PI for discrimination is to sample during the "on" time of the pulse. Any target will produce a slight change in the signal seen at that time as well as a change when the normal target sample is taken.

If the analysis is done correctly, then one can use both the "pulse on" and "pulse off" signals and get a better analysis of a target. This type of design can lead to a better form of discrimination. However, few if any PI's are actually using this technique.

Regardless of the technique used, no form of discrimination is perfect, and, most likely, never will be. Some techniques are better than others, but all can be fooled, and this is true of both PI's and VLF's.


By Reg Sniff
Copyright 2003 The author reserves all distribution rights.



NUGGETSHOOTER HOME

[Bayu]

Or you could simply buy a copy of Inside the METAL DETECTOR - Second Edition - Published 2015
and learn the answers to all your questions, including many you haven't even thought of.

I've read version 1, and haven't read the second

[Bayu]

Most of these terms -- gain, sensitivity, integrator, threshold, preamp, oscillator -- are the same for metal detectors as for any other electronics. If you don't know what they mean then I suggest some more study in electronics. I certainly don't mind answering questions but if I need to explain what "gain" means then you probably are not ready for these forums.

I don't know what you mean by "dipht width."

SAT is Self-Adjusting Threshold. It is usually a derivative/high-pass filter stage that removes offsets and slow-moving responses. Here a thread on it:

https://www.geotech1.com/forums/showthread.php?25100-what-is-Self-Adjusting-Threshold&highlight=sat+stage

some of me have understood when building several metal detectors, but when I studied other MD schemes, sometimes I found new terms that I had never encountered in previous schemes, for basic electronics on metal detectors I already understood, thanks master carl for answering my question.

I understand most of the terms when assembling and using metal detectors in the field, but it's hard to explain in words

[Bayu]

Thanks kt315 you always help me, I hope you are always healthy,,, thanks also to other masters,

[dbanner]

hello all geotech masters and members, in understanding metal detectors there may still be many who don't know the terms in the metal detector section including me, maybe our teachers can explain the term and its uses or how it works such as; gain, SAT, sens, dipht width, integrator, threshold, pream, oscillator, and many more, maybe there are masters who can and are willing to explain the meaning and use, I am personally very grateful. and maybe someone wants to add other than the terms I mentioned earlier

James c. Maxwell
Hi guys! I'm back, just a shadow of my former self.

Look.... Stop asking such silly questions, cause if you can spell the question, you can certainly spell the answers.

[Kemper]

Or you could simply buy a copy of Inside the METAL DETECTOR - Second Edition - Published 2015
and learn the answers to all your questions, including many you haven't even thought of.

Isn't what the Op inquired about what this forum is for ? Instead of pushing a book maybe you should read one yourself.

[Qiaozhi]

Isn't what the Op inquired about what this forum is for ? Instead of pushing a book maybe you should read one yourself.

Thanks for sharing. We’re all refreshed and challenged by your unique point of view.

[Kemper]

Thanks for sharing. We’re all refreshed and challenged by your unique point of view.

You shouldn't speak for everyone . Some may have a different point of view whether they share it or not .