Oscillator power!!!!

[mikebg]

POWERFUL SEARCH OSCILLATOR for BFO

(R)EMI group renamed BFO metal detector "REGEN MD" because its search oscillator operates with regeneration (positive feedback). In a specific case (induction balanced regenerator), the amplifier starts to oscillate when there is target. The opposite is also possible (amplifier stops to oscilate when there is target).

Oscillating circuits invented by Armstrong and Meißner have two windings, ie TX coil and RX coil with mutual inductance. If we place both coils in induction balance, the oscillation stops and TX current is only noise. For sensitive metal detecting with search oscillator, it should use an amplifier having very high gain (A>60dB) which can not be achieved with only one transistor. The amplifier should excite TX coil with large current (i >10mA), which can not be achieved with single CMOS gate.

Oscillating circuits invented by Colpitts and Hartley are specific case of mutual inductance because is used self-inductance (monocoil). In this case coefficient of coupling is allways maximal k=1 and amplifier always oscillates despite amplifier use only one transistor. The target can cause frequency shift and amplitude change (absorbing metal detector).

I will use block diagram of Pebe to explain the general block diagram of REGEN metal detector. 'General' means that diagram is valid for all kinds BFO MD. Their block diagrams are variants or specific case of shown below diagram. Note that here block 2 is more complicated than oscillating block, however in project HOUNDOG the block 2 is simply a buzzer.

The term BFO is used in amateur radio for reference oscillator as shown below.
Most sensitive REGEN MD is motion type. It uses PLL circuit (for example 4046) for BFO (reference oscillator). The PLL slowly makes beat frequency zero without target (or without motion).
I stop here with explanation because all is described comprehensive in other my postings.

[WM6]

The AM radio plays no part in the actual detection of the target.

.

Probably statement of the year. Why you then use it in BFO detector?

And again: we start discussion about existence of antenna in metal detector not about "actual detection of the target".

[Qiaozhi]

Probably statement of the year. Why you then use it in BFO detector?

And again: we start discussion about existence of antenna in metal detector not about "actual detection of the target".

In fact this discussion started when mikebg stated that a metal detector coil does not act like an antenna. I agreed with his statement, and pointed out that some metal detector manufacturers erroneously call the coil an antenna in their literature.

I think you are getting a little confused with the specific case of a simple BFO that uses an external AM radio to provide the reference oscillator and audio stage. This is just a simple shortcut for hobbyists who are electronically-challenged. With this setup they only need to build a one transistor transmit circuit, and using the radio removes a lot of other circuitry that they might not understand anyway. The ferrite antenna in the radio does indeed pick up the signal from the transmitter, but this is incidental to the metal detection process. In a conventional BFO there may only be one coil (the search coil) and the reference oscillator may not have a coil at all. There is no antenna in this setup. Likewise for IB and PI detectors.

The bottom line is: "The search coil in a metal detector is not an antenna".

[WM6]

In fact this discussion started when mikebg stated that a metal detector coil does not act like an antenna. I agreed with his statement, and pointed out that some metal detector manufacturers erroneously call the coil an antenna in their literature.

I think you are getting a little confused with the specific case of a simple BFO that uses an external AM radio to provide the reference oscillator and audio stage. This is just a simple shortcut for hobbyists who are electronically-challenged. With this setup they only need to build a one transistor transmit circuit, and using the radio removes a lot of other circuitry that they might not understand anyway. The ferrite antenna in the radio does indeed pick up the signal from the transmitter, but this is incidental to the metal detection process. In a conventional BFO there may only be one coil (the search coil) and the reference oscillator may not have a coil at all. There is no antenna in this setup. Likewise for IB and PI detectors.

The bottom line is: "The search coil in a metal detector is not an antenna".

Maybe you are right, but according analogy we can put in question MD terms (or shortcuts) TX (transmitter) and RX (receiver) too. "Coil" is only general term and we need prefix RX (or TX) to determine its functionality. Then what is "RX coil" or "Receive(r) coil" actually? Sensor? Antenna? Probe? It cannot be "search coil" as this is term for search module in common. Simple RX coil? Ok, mean "receive coil", but if coil something receive what is then so wrong with term antenna?

I never use term antenna for search coil or its parts but can easily accept it and cannot set it as wrong or right.

[maikl]

Some basics for you (who is speaking about two TX coil?):

BFO (Beat Frequency Oscillation) - Metal Detectors using BFO technology have two coils of wire, one large coil is located in the search coil of the detector, the other small coil (act as RX antenna) of wire is located within the System Control Pack. Each coil of wire is connected to an oscillator that produces pulses of current. These pulses of current pass through the coils generating radio waves. A receiver housed within the System Control Pack receives the radio waves and makes a series of tones based upon the frequencies of the radio waves. When the detector search coil passes over a coin or other metal item a magnetic field is created around the coin or metal item, this magnetic field causes interference with the frequency of the radio waves generated by the search coil. And changes the tone produced by the reciever (of radio waves, and radio waves can be received only by antenna, not by finger, and antenna is coil - always - in this case RX coil).

PS: Telescope antenna act electrically as coil (inductor) too.

According to your philosophy on this figure has three coils (L1,L2 and L3). Perhaps it is a hybrid of "BFO-TR"?

[PERTINAX]

Pertinax

Maybe we can discuss about antenna in metal detector like this http://www.youtube.com/watch?v=tNos9...layer_embedded .I am shure in that there is no coil and the ribbon is acting at this circuit almost as real antenna !

[Qiaozhi]

Maybe we can discuss about antenna in metal detector like this http://www.youtube.com/watch?v=tNos9...layer_embedded .I am shure in that there is no coil and the ribbon is acting at this circuit almost as real antenna !

I thought this was going to be an LRL video, but it was much better than that!

[mikebg]

According to your philosophy on this figure has three coils (L1,L2 and L3). Perhaps it is a hybrid of "BFO-TR"?

The "phylosophy" is shown above as two blocks. In other my postings is explained why the circuit of search oscillator operates with reduced TX power with DC loaded output because of incompetent design.

[WM6]

According term antenna in metal detecting.

If for someone is hard to understand that MW/LW radio receive signals with an antenna and that such radio is used in some BFO solution as pure reciver (so antenna built in stay antenna and act as receive coil), then we can point (except BFO) on other metal detectors that undoubt use antenna too - like VHF/UHF metal detector:

http://www.freepatentsonline.com/5936586.html

citation from linked patent:

"A VHF-UHF waveguide non-metal and metal detector is disclosed having a waveguide antenna system ....."

or this:

http://www.freepatentsonline.com/5696490.html

Citation:
"In accordance with another aspect of the present invention further objects as mentioned above are accomplished basically by a 121/4 by 12 inch modified loop and full circle non-directional antenna system, with a full curve antenna on top of full circle and loop antennas at crossover points, mounted on a wood frame for search/pass 1 to 5 inches above the ground. "

If it is so hard to understand how telescope antenna can be inductor, then probably "full circle" or "loop antenna" cah be accepted from those as coil.

Nor VLF detector are excluded. If you wish here some word from one of detector authority:

"Here is some information from the book written by Dave Johnson Chief Designer Fisher Research Labs which is available at this link; How VLF Metal Detector works.

How VLF Metal Detectors Work

....citation from book ......
One coil is a transmitting antenna which is surrounded by a rapidly alternating magnetic field. If metal is present, the alternating magnetic field will induce current to flow in the metal.

The other coil in the searchcoil assembly is a receiving antenna which detects changes in the magnetic field caused by the electric current circulating in the piece of metal. Electronic circuits amplify this weak signal, analyze it to determine the changes which occur as the searchcoil sweeps over the target and then convey the information to the user in the form of a visual display or audio tones. ........"

With this I conclude my contribution in the debate on the term antenna regarding metal detectors.

[Qiaozhi]

Nor VLF detector are excluded. If you wish here some word from one of detector authority:

"Here is some information from the book written by Dave Johnson Chief Designer Fisher Research Labs which is available at this link; How VLF Metal Detector works.

How VLF Metal Detectors Work

....citation from book ......
One coil is a transmitting antenna which is surrounded by a rapidly alternating magnetic field. If metal is present, the alternating magnetic field will induce current to flow in the metal.

The other coil in the searchcoil assembly is a receiving antenna which detects changes in the magnetic field caused by the electric current circulating in the piece of metal. Electronic circuits amplify this weak signal, analyze it to determine the changes which occur as the searchcoil sweeps over the target and then convey the information to the user in the form of a visual display or audio tones. ........"

With this I conclude my contribution in the debate on the term antenna regarding metal detectors.

This is one of the erroneous manufacturer quotations I was referring to.

[pebe]

WM6,
In spite of the loose descriptions given in dubious patent applications and in salesmen's blurb, a transmitting antenna is a length of wire or metal tubing that resonates at the transmitted frequency in order to transmit EM energy. Typically its length is equal to a quarter or a half wavelength.

This is the transmitting antenna (aerial) at Droitwich, UK.
http://tx.mb21.co.uk/gallery/droitwich/droitwich-ab.php
It operates at 200kHz (a wavelength of 1500metres) in the long wave band. Do you think they could have used a coil instead?

Now imagine an antenna 10 times that size for 20kHz...!!
Mindboggling, isn't it?

[Bill512]

OK WM6 ,a VLF coil from a metal detector radiates EM energy for sure and also is a extreme low efficiency (magnetic) antenna.
In practice,at these frequencies and specially in sort range TX/RX applications, is not considered as a antenna but as a induction coil.
If you use a coil to receive DX VLF signals, then the term antenna is somehow suitable (working mostly at far field).

[mikebg]

WHAT IS THE OPTIMAL TX POWER?

As I read disputes and argues over in this thread, I feel that participants are irascible retired elders who do not understand that the subject of the dispute is minor and not important.
Instead of identifying a serious problem in which we all to think, experiment and design collective, you are wasting time and energy for some old radio terminology.
As the title of this thread is "Oscillator power", let's dispute and consider what should be optimum TX power for a metal detector. The problem is important because the correct answer will show how should operate the ultimate metal detector.

[WM6]

As I read disputes and argues over in this thread, I feel that participants are irascible retired elders who do not understand that the subject of the dispute is minor and not important.
Instead of identifying a serious problem in which we all to think, experiment and design collective, you are wasting time and energy for some old radio terminology.
As the title of this thread is "Oscillator power", let's dispute and consider what should be optimum TX power for a metal detector. The problem is important because the correct answer will show how should operate the ultimate metal detector.

World is more interesting if it is coloured (in opinion too) than mono, dear mikebg, but you are right, this is not proper topic for such "off topic" debate. I take this just as a little exercise in (my bad) English

Back to the topic, I think that optimum TX power not exist isolated byself, but optimum TX power need to be discussed in connection to specific terrain/soil and specific target.

[maikl]

I agree that we have strayed from the topic. Just another to add: VLF coils on the system of ordinary transformer. VLF has a TX (transmitter) and RX (receiver). The transformer has a primary coil (transmitter) and the secondary coil (receiver). The best example of transformer welding machines with him by moving the metal core increases and decreases the current in the secondary coil (RX). And now to say something about the topic: should work to reinforce TX. If we increase the sensitivity of the RX will lose the stability of the detector.