BFO ground balance - has this been done?

[Teleno]

There's been a flood of BFO circuits being posted lately and I couldn't help but wonder how this old tech could be improved.

My idea for cancelling ground in a BFO is as follows:

- Use two identical search coils (ScA and ScB) in DD configuration (no overlap)
- Use two oscillators (OscA and OscB), one per search coil.
- A slow-responding PLL tunes OscB to the frequency of OscA (the reference oscillator).
- Output is the beating frequency of the oscillators.

The effect of the PLL is that, long term, OscA and OscB work at the same frequency. When the coils approach mineralized ground their frequencies change at the same time and by the same amount, remaining tuned albeit at a lower freq. The Beat frequency remains zero and ground is cancelled. Any target breaks the balance and the BFO beeps.

It's pretty simple, so I figure it's been done before. Anyone?

[homefire]

This is why you shield the Coil. No Ground Bal Needed . It may weep and Peep but will not ignore a Target. The Last Point of Discrimination Demodulation is Between your Ears.

[Teleno]

This is why you shield the Coil. No Ground Bal Needed

Wrong. Shielding avoids ground capacitance, but it does not prevent magnetic ground from increasing the search coil's capacitance, which lowers its oscillating freq. and causes the detector to miss targets.

One solution is to lower the ref. oscillator by the same amount, as I described, by using another search coil as the ref. oscillator.

[homefire]

Your Proposal is to have two detector in the coil. Motion would be needed to Respond. Differential would be needed. That would just give you a Blip. I like to hear the Sounds. I can hear the Ground as I walk. Tuned ears can hear the subtle changes better then a BLIP.

[Teleno]

Your Proposal is to have two detector in the coil. Motion would be needed to Respond. Differential would be needed. That would just give you a Blip. I like to hear the Sounds. I can hear the Ground as I walk. Tuned ears can hear the subtle changes better then a BLIP.

Motion will not be required at all to respond if the PLL autotuning is slow enough. Better than missing targets anyway. I'm about to build a prototype, it's so simple.

But my main concern is not the merits of the approach but whether it had been tried before.

[homefire]

Will be watching with Hope. Thank You.

[Gwil]

Not quite the same thing, but this reminds me of Thomas Scarborough's "Beat Balance" designs. In that scheme there are two oscillators running at slightly different frequencies, and two overlapping search coils arranged so that the oscillators are loosely coupled, pulling their freq's closer together than they would otherwise be. The audio output is the beat frequency. A target tends to (a) change oscillator frequencies and (b) change the coupling between the oscillators, both making the beat freq vary. A feature of this scheme is that with luck the ground affects both oscillators in the same way, which is a good thing.

[Teleno]

Not quite the same thing, but this reminds me of Thomas Scarborough's "Beat Balance" designs. In that scheme there are two oscillators running at slightly different frequencies, and two overlapping search coils arranged so that the oscillators are loosely coupled, pulling their freq's closer together than they would otherwise be. The audio output is the beat frequency. A target tends to (a) change oscillator frequencies and (b) change the coupling between the oscillators, both making the beat freq vary. A feature of this scheme is that with luck the ground affects both oscillators in the same way, which is a good thing.

Thank you! This design is even simpler.

I'ill investigate whether the coupling between search coils is truely an advantage or not.

Here are two versions of the circuit, one with a single, affordable TLC556 chip!

http://www.zen22142.zen.co.uk/Circui...tgear/bbmd.htm

http://thomasscarborough.blogspot.nl...-detector.html

[kt315]

new matter? http://www.geotech1.com/forums/showt...detector-genre

[Skippy]

When I read that you were planning on having your two coils NOT balanced, I immediately thought that the coupling between them would cause them to lock together, and that would cause some odd behaviour. Like a failure to detect anything, or hysteresis.

[Gwil]

I haven't experimented with the Beat Balance scheme, but I suspect there are two factors that matter - the degree of coupling, and the free running frequencies of the two oscillators.
Incidentally, in versions in which both oscillators are in the same chip, there will probably some internal coupling between them. This would mean that the phase of the coil coupling may be significant because the two kinds of coupling will add, or subtract, or whatever.

[WM6]

It's pretty simple, so I figure it's been done before. Anyone?

Probably is, search for an old Bounty Hunter Outlaw.

[Carl-NC]

There's been a flood of BFO circuits being posted lately and I couldn't help but wonder how this old tech could be improved.

My idea for cancelling ground in a BFO is as follows:

- Use two identical search coils (ScA and ScB) in DD configuration (no overlap)
- Use two oscillators (OscA and OscB), one per search coil.
- A slow-responding PLL tunes OscB to the frequency of OscA (the reference oscillator).
- Output is the beating frequency of the oscillators.

The effect of the PLL is that, long term, OscA and OscB work at the same frequency. When the coils approach mineralized ground their frequencies change at the same time and by the same amount, remaining tuned albeit at a lower freq. The Beat frequency remains zero and ground is cancelled. Any target breaks the balance and the BFO beeps.

It's pretty simple, so I figure it's been done before. Anyone?

I drew this up about 15 years ago, except that I had both loops on PLLs with the loop frequencies running VLF and the PFDs running 100kHz or so. I assumed induction balanced, and I called it a multifrequency induction-balanced VLF BFO. I never built it, though.

[Chet]

I drew this up about 15 years ago, except that I had both loops on PLLs with the loop frequencies running VLF and the PFDs running 100kHz or so. I assumed induction balanced, and I called it a multifrequency induction-balanced VLF BFO. I never built it, though.

Hi Carl
Did you intend to attach the circuit?
Chet

[Davor]

It could work well provided the frequencies of the oscillators are not in harmonic relationship. Otherwise they simply lock to the same frequency, and you lose sensitivity. It could work with frequencies in 2:3 ratio. PLL loop filter can be made slow enough to provide for motion type detection.