Oscillator power!!!!

[pebe]

Or you are way off the beam.

OK, then. Please enlighten me.

[WM6]

OK, then. Please enlighten me.

Read again, look at picture and study (plenty BFO articles on web) how pure basic BFO (there are many BFO mod too) work.

[Qiaozhi]

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.

I don't know who dreamed up that definition of a BFO detector, but he was way off the beam.

Pebe is correct. The description you have quoted is utter nonsense. It was obviously written by someone with very little electronics knowledge.

[pebe]

Read again, look at picture and study (plenty BFO articles on web) how pure basic BFO (there are many BFO mod too) work.

WM6,
I’ve read again and came to the same conclusions. Allow me to justify myself when I said the writer was ‘off beam’.

BFOs were used as far back as the 1930s in good communication receivers in order to make CW transmissions audible, and their use is well understood. The technology used in metal detectors is basically the same.

A BFO type metal detector utilises the basic fact that the inductance of a coil changes when a metal is introduced into its core and the magnetic lines of force are affected. In order to measure the change in inductance the coil is made part of an LC oscillator – usually running at a low radio frequency. The aim is to be able to measure the frequency – not to ‘transmit’ that frequency – so the coil is not a transmitter in the true sense of the word.

The frequency of the oscillator is then compared with a locally generated, fixed, reference frequency with an offset to produce an audible beat when the two are mixed together in a non-linear device.

So let us look at what was said.

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 Metal.

Not necessarily. The ‘small coil’ may not be present. If present it would form part of the reference oscillator, but the reference may be an RC oscillator, a PLL, or a synthesiser. And it is certainly is not an antenna

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.

The current through the coil(s) is sinusoidal – not pulsing. It certainly emits EM waves but that is not its primary function.

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.

The receiver generates a beat frequency whose pitch varies with the changing inductance of the detector coil.

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

Taken literally, this is nonsense! Depending on its length and the frequency involved, it may look capacitive, resistive, or inductive. And it has no bearing on the matter.

[Qiaozhi]

Having read the description for a 2nd/3rd time, I think I see what the author is trying to say (very badly though). He is referring to the simplest BFO setup with an external oscillator and an AM radio. In this case it is true that the AM radio has a ferrite antenna, but this plays no part in the metal detection process. It is the TX coil that does the detecting. As a metallic object enters the electromagnetic field of the coil it causes the coil inductance to change. This in turn alters the frequency of the TX oscillator. The part played by the AM radio is simply to replace the local oscillator, mixer and audio stage in a conventional BFO.

[PERTINAX]

Pertinax

BFO detector works as two oscilators at same frequency in balance. First oscilator is made (usualy) with ferite core inductance inside box and the other with air coil inductance at stick as search head .When metal object pass near search head, balance of this two oscilators disturbs and as result of this balance disturbance we have audio signal in output stage (after processing in mixer stage ). Altitude of audio signal in speaker depends from sort of metal under the search head and this is the main advanage of this kind of detectors - discrimination. According to this BFO has no antenas instead inductance.
By the way, in radio theory ideal antena is shown as capacity with two plates moved apart for 180 degrees.
Hope somebody has same opinion .

[WM6]

The receiver generates a beat frequency whose pitch varies with the changing inductance of the detector coil.

Sure there is no pulse oscillator but sinewave (it was wrong simplification). If you search for nonsence you can find biggest in your answer. Receiver does not generate nothing. Receiver can only receive beat signal and forward it to (beat) mixer.

And receiver cannot receive nothing without antenna at least not in practice (in theory all is possible). And antenna is coil (with or without ferrite).

There is no coil (in practice) without (C) capacitive and (R) resistive component, even telescope antenna, however coil remain coil.

On block scheme presented BFO construction are very clasical, published in different articles back in 60, like:

Calvert, Frederick H., Simple Metal Locator, "Electronics World," July, 1961,

and patented too (not only once):

http://www.freepatentsonline.com/3769575.pdf

[mikebg]

Qiaozhi is right. BFO has only one TX. CCO (double BFO) has two TX coil. No transmitter TX and receiver RX.

Monocoil (self-inductance) is specific case of mutual inductance (separate TX and RX coils).
IMNHO, the BFO is "noise induction metal detector". It is very sensitive when oprates with induction balance between TX and RX coil. Rev Thomas Scarborough reinvented this method and renamed it "CCO metal detector".
http://www.geotech1.com/forums/showt...eferrerid=2910
http://www.geotech1.com/forums/showt...eferrerid=2910
http://www.geotech1.com/forums/showt...eferrerid=2910
http://www.geotech1.com/forums/showt...eferrerid=2910
http://www.geotech1.com/forums/showt...eferrerid=2910
Because of worst modulation index (maximal AIR signal) the Monocoil is the worst sensor for metal detecting. For example a PI metal detector operates better with induction balanced coil.

[WM6]

http://www.geotech1.com/forums/showt...eferrerid=2910
Because of worst modulation index (maximal AIR signal) the Monocoil is the worst sensor for metal detecting. For example a PI metal detector operates better with induction balanced coil.

Hi mikebg

did you follow Aziz's threads? He invented solution for high efective use of (PI) monocoil.

[Aziz]

Hi mikebg

did you follow Aziz's threads? He invented solution for high efective use of (PI) monocoil.

I did not invent anything (yet). It was a cooperation work with Tinkerer! Indeed, he has done some very valuable work in the field.

The principle is: Get almost the whole energy stored in the coil back and do not generate heat (no damping resistor).

The best configuration is an induction balance (IB) coil (seperate TX/RX). For mono coils, one have to switch off the energy holding capacitor from the coil and interrupt the process between charging and discharging the capacitor. But this is quite a non-trivial task as the switch itself acts as a parasitic capacitor in switch-off mode. And a damping resistor needs to be switched in during the sampling process.

The IB coil configuration can achieve a very high power efficiency (>90 %) so the so-called AIR response can be extended by a large period of time. This is what I like very very much.
Aziz

[pebe]

Sure there is no pulse oscillator but sinewave (it was wrong simplification). If you search for nonsence you can find biggest in your answer. Receiver does not generate nothing. Receiver can only receive beat signal and forward it to (beat) mixer.

And receiver cannot receive nothing without antenna at least not in practice (in theory all is possible). And antenna is coil (with or without ferrite).

My ‘receiver’ was the one in your quote. But if you want to be pedantic, then brush up on your theory because the ‘receiver’ does not receive a beat signal – the beat (heterodyne) is generated in the mixer. Look at the real block diagram below.

But you have missed the whole point. The primary aim is to measure the inductance of the coil by the beat frequency method. Only the voltages of the two sources are required to get the beat. The fact that the coil radiates EM waves is purely incidental.

So all this talk of Tx coils and Rx coils and radio waves and antennas is totally irrelevant.

[Qiaozhi]

My ‘receiver’ was the one in your quote. But if you want to be pedantic, then brush up on your theory because the ‘receiver’ does not receive a beat signal – the beat (heterodyne) is generated in the mixer. Look at the real block diagram below.

But you have missed the whole point. The primary aim is to measure the inductance of the coil by the beat frequency method. Only the voltages of the two sources are required to get the beat. The fact that the coil radiates EM waves is purely incidental.

So all this talk of Tx coils and Rx coils and radio waves and antennas is totally irrelevant.

Exactly.

[WM6]

The fact that the coil radiates EM waves is purely incidental.

You described and pictured another valid and known BFO solution, which I am not talking about.

I am not talking about coil that radiates EM waves (which is fact regarding target), but abuot RF wave or radio waves which is not the same. And radiating of radio waves in my BFO solution is not "purely incidental", but intended, because in my solution there is not wire connection between TX tank and Mixer.

All changes of signal (mean changes in frequency) in my BFO sulution are transmitted by radio waves from TX coil (acting here as TX antena) to RX coil (acting here as RX antenna) or Receiver.

Pure RF connection between TX and RX stage nothing else. No EM influence between RX and TX stage, only target has EM influence to TX oscillator and hence to the transmitted RF frequency.

And RF connection can bi done (in practice) only by RF antenna wich is coil somethimes on RX side field streghtened by ferrite core.

Remember, we start discussion with question if we can speak about "antenna" in field of metal detectors. My claim is that in some cases we can and give you argumet that such claim is correct.

At the end, if you use at the Receiver/Mixer/NF side usual AM (MW/LW) portable radio, what is inside those radio to help recive signal instead of antenna? Or radio antenna is no more antenna when it is used in field of MD. What is then? RX coil?

[Tinkerer]

I did not invent anything (yet). It was a cooperation work with Tinkerer! Indeed, he has done some very valuable work in the field.

The principle is: Get almost the whole energy stored in the coil back and do not generate heat (no damping resistor).

The best configuration is an induction balance (IB) coil (seperate TX/RX). For mono coils, one have to switch off the energy holding capacitor from the coil and interrupt the process between charging and discharging the capacitor. But this is quite a non-trivial task as the switch itself acts as a parasitic capacitor in switch-off mode. And a damping resistor needs to be switched in during the sampling process.

The IB coil configuration can achieve a very high power efficiency (>90 %) so the so-called AIR response can be extended by a large period of time. This is what I like very very much.
Aziz

Aziz's help in analyzing and perfecting the energy recovery has been the key to using this method.
I have continued the development and have accomplished very good results. Detecting a 1 Euro coin (in air) at 70cm from the coil, inside the house with staggering 60Hz noise and steel re-bars in floor, ceiling and walls is a benchmark for me.
I am now getting a simplified lower power model ready for field testing. This will be a general purpose detector, with a reasonable (not perfect) FE discrimination of 50 cm for a steel crown cork and a detection depth of 50cm for a 1 Euro coin or 40 cm for a dime.

I am still working on the XXXdeep detector that shall find a 1 Euro coin at 1 meter depth. I am quite confident that this is achievable.

I really could use help for the PIC programming for my projects.

Tinkerer

[Qiaozhi]

At the end, if you use at the Receiver/Mixer/NF side usual AM (MW/LW) portable radio, what is inside those radio to help recive signal instead of antenna? Or radio antenna is no more antenna when it is used in field of MD. What is then? RX coil?

In that case you have just confirmed what we are saying. The AM radio plays no part in the actual detection of the target. The ferrite antenna is simply a means of getting the TX signal into the mixer stage of the radio. You could have done the same thing with a piece of wire that bypasses the RF amplifier stage. In other words, the ferrite coil in the radio is not the receive coil.