The big question

[dbanner]

Referring to a tesoro tgsl type concentric coil (tx-14.5kHz, Rx-16.1kHz), what waveform on scope would one expect to see for the Rx if you were to remove the bucking (feedback) coil altogether? What would be the phase displacement relative to the Tx waveform? Anybody know? Any guesses?
Next, with the Rx tank capacitor removed, what would be the phase?

[waltr]

Referring to a tesoro tgsl type concentric coil (tx-14.5kHz, Rx-16.1kHz), what waveform on scope would one expect to see for the Rx if you were to remove the bucking (feedback) coil altogether? What would be the phase displacement relative to the Tx waveform? Anybody know? Any guesses?
Next, with the Rx tank capacitor removed, what would be the phase?

Without the bucking coil the RX would have a huge peak-peak signal, same as if you extremely over lapped DD coils.

Never tried removing RX resonance cap but that would probably also result in a huge AC signal and no phase shift since the RX coil is simply a transformer secondary instead of being resonant at a frequency different from the TX signal.

[waltr]

Do now remember looking at the RX coil only, no cap, since Ivconic suggested this to null coils.

You can null with adjusting the over-lap of DD or the buck winding on a concentric.
What I don't remember is the phase.

Ivconic was not using an O'scope so he didn't know phase.

[dbanner]

I am interested in the phase. Don't care about the nulling at this point.
I am having an idea of phase shift of the "off resonance" coil (where the Rx is tuned to resonate either above or below the resonance of the Tx.

If one were to examine the phase displacement encountered at the output of low pass/high pass filters for example, You put a sine in, and the sine out is phase shifted, in the low pass case, the output lags the input (negative phase shift), in the high pass case the output leads the input (positive phase shift). More importantly, the degree of phase response is quantifiable. Its value can be calculated.

Now in the case of an "off resonance" coil topology, I am speculating that perhaps the detuned Rx has some attributes of a pass filter in that it invokes a phase response. ( I am not speaking of targets here, I am simply speaking of a coil whose bucking coil has been removed altogether, and the Rx output is scoped relative to the Tx, no targets!)

Now I am hoping somebody can employ the math used around pass filters to determine the phase response, and adapt it to Rx coil off resonance topology.

If I am talking rubbish, then someone please enlighten me.

[waltr]

Just put the scope on the RX only coil without the resonance cap.
NO phase shift relative to the TX.

However, moving a target within the coil does produce a phase shift. The the left for ferrous and to the right for non-ferrous.

As far as removing the Bucking coil---This would cause the RX pre-amp to completely saturate, square output.
The coil's much be Nulled to reduce the peak to peak amplitude to obtain a linear sine output from the pre-amp.
Then the following circuits can process small changes in the amplitude and phase of the signal.

From building and playing with the TGSL it seems that a wide range of coil inductance and coil resonate frequencies would work.

Another part is the circuits that produce the synchronous sampling pulses. These circuits are setup to sample the input waveform at the times needed to obtain GB and Discrimination.

I did buy two TGSL and two IDX PCBs from Silverdog. Reason is to have one for experimenting.

I have tried reading the patents of the ideas used in the TGS and White's detectors but they don't make it easy to understand the how and why.

Your idea of filter math may help. Building circuit simulations and comparing to real circuits is needed.

[dbanner]

These tests were meant to be conducted without the Rx preamp connected.
Anyways, I wanted to try the Nautilus feeback circuit to balance a concentric coil to perfection, so as to get rid of fiddling around with that extra loop that run off the bucking coil. Better if the thing could be fine tuned with just a small trimmer pot.

[Carl-NC]

The no-cap scenario is easy: the RX coil will be in-phase with the TX, except for a slight phase error (maybe 1 deg) due to the TX coil resistance.

If the RX coil & cap are tuned to exactly the TX frequency, then you should see a 90 degree shift in the RX voltage. You will also see the maximum RX signal amplitude. As you go higher than f0 (less capacitance) the phase will move back towards 0 and the amplitude decreases. If you increase the cap and move lower than f0, the phase will increase towards 180 and the amplitude decreases.

[dbanner]

The no-cap scenario is easy: the RX coil will be in-phase with the TX, except for a slight phase error (maybe 1 deg) due to the TX coil resistance.

If the RX coil & cap are tuned to exactly the TX frequency, then you should see a 90 degree shift in the RX voltage. You will also see the maximum RX signal amplitude. As you go higher than f0 (less capacitance) the phase will move back towards 0 and the amplitude decreases. If you increase the cap and move lower than f0, the phase will increase towards 180 and the amplitude decreases.

Thanks Carl, that's exactly what I was trying to figure out. Funny, it's very similar to the phase response experienced with single pole low pass/high pass filters either side of the centre (cutoff) frequency.

A good idea would be to take the Tx frequency of the whites coil at 6.59kHz(f0), and plot the Rx phase response and amplitude either side of f0 up and down to say between 4.5kHz and 7.5kHz. Or something along those lines. Same with tgsl coil.

What is curious is that Dbowers self made concentric IDX coil has a phase shift of about 90 degrees, even though the Tx/Rx are detuned to 6.59kHz and 4.6kHz. Whereas the tgsl coil is approx. 20 degrees for DD or 200 degrees for concentric. (these figures relate to final balancing with buck coil in the case of the concentric)

But what is clear is that the amplitude decreases either side of f0, so one might think that this is not good, but as it turns out, to make a functional, useable coil, it is better when the coils are detuned from one another.

[dbanner]

Perhaps Whites have tuned Rx to some harmonic of Tx, likewise tesoro.