Induction Balance Stuff - Single/Multi Frequency Response, GB, Disc, Measurements, Ideas, Fun, etc.

[Aziz]

Hi all,

instead of the projection method for ground balance, there is a more elegant and efficient way to do it.
In the I/Q vector space, you have the unity ground vector G, obtained by ground balance procedure. |G| = 1 (unity vector)
Your signal S is also a vector in the I/Q space.

The vector cross product P of the vectors G and S is:
P = G x S (vector operation, P is also a vector)
A = |P| is the surface area (scalar) of the vectors G and S (forming a parallelogram). |P| is the lenght of the vector P.


Reactive response will show always towards the direction of ground vector G. And hence, A is becoming zero.
Anything different than zero, A is your target signal.
A threshold for A is for manual GB adjustment.

Any objections?

[Sergey_P]

It can be made as much sensitive as a DD coil configuration now.

No
Noise(mono coil) = Noise(TX)+Noise(Vcomp), / Noise(TX)<->Noise(Vcomp) - are not correlated /
Noise(DD) = Noise(TX)-Noise(TXcomp), / Noise(TX)<->Noise(TXcomp) - correlated /
mono coil VLF - It works, but very poorly

[Aziz]

Hi Sergey,

It can be made as much sensitive as a DD coil configuration now.

No
Noise(mono coil) = Noise(TX)+Noise(Vcomp), / Noise(TX)<->Noise(Vcomp) - are not correlated /
Noise(DD) = Noise(TX)-Noise(TXcomp), / Noise(TX)<->Noise(TXcomp) - correlated /
mono coil VLF - It works, but very poorly

I haven't had time yet.
To increase the sensitivity of the mono coil VLF, I have to wind a good low loss signal transformer yet ( < 50 mW, it's still too much, < 20 mW would be better).

The idea is using a simple high impedance voltage mixer, buffering / amplifying the difference (j-fet input) and then processing the residual target response. Effectively subtracting the high TX voltage by the high reference voltage. The high TX voltage (let's say up to 60 Vpeak) is being capacitive mixed with a high voltage reference signal (let's say up to 60 Vpeak or even more if it is possible). The high voltage reference signal is being generated on the second line output channel and transformed up with the good signal transformer.

The mixer is causing some target signal loss however (Ohm's law). The higher the reference voltage compared to the TX voltage, the lower the TX coupling mixer capacitor and hence the lower the target signal loss.

I have a reference signal up to 6 Vpeak on the second line output channel. And this will be transformed up as much as possible. The more, the better.
But I don't know, what is really possible. The signal transformer is quite critical. I would be really happy to get 60 Vpeak out of 6 Vpeak voltage.

The high-Q TX should not be degraded. So the signal path is a high impedance path till the j-fet buffer/amplifier.

The AI gives me good tips to wind the signal transformer however.
Aziz

[Sergey_P]

Hi Aziz
Even the introduction of residual imbalance compensation in DD (-54...60 dB) leads to an increase in noise and a decrease in sensitivity.
<The high-Q TX should not be degraded>
... and the reference signal.
what SNR? (spectrum analyzer, THD... ) desired noise -120dB in the passband

[Aziz]

Hi Sergey,

Hi Aziz
Even the introduction of residual imbalance compensation in DD (-54...60 dB) leads to an increase in noise and a decrease in sensitivity.
<The high-Q TX should not be degraded>
... and the reference signal.
what SNR? (spectrum analyzer, THD... ) desired noise -120dB in the passband

we don't need such a perfect imbalance compensation. Just enough, that the amplified/buffered signal is in the input voltage level (max 1 V peak).
The reference signal should have the same SNR as the stimulation TX signal (approx 120 dB I think).

If we have 30 Vp TX voltage and 30 Vp reference voltage, we would have 50 % target signal loss in the mixer.
If we have 30 Vp TX voltage and 60 Vp reference voltage, we would have approx. 33 % target signal loss in the mixer.
If we have 10 Vp TX voltage and 60 Vp reference voltage, we would have approx. 14 % target signal loss in the mixer.

But we want a high TX voltage (=high TX current), to increase the SNR too. So we need a high reference voltage. As much as possible. So we can operate the TX at its max. voltage.
Aziz

PS:
The target signal isn't really lost. It is only attenuated. The amplifier could compensate the overall mixer attenuation.

[Sergey_P]

Hi Aziz
Prototype->
Power supply: 5 V
Half-bridge driver (minimal noise level,+/-2.5V ), series resonant circuit
Coil: diameter: 20 cm; 415 turns; resistance 190 Ω
At a resonant frequency of 8 kHz: Quality factor = 20, peak voltage = 63 V, field = 7 Amp-turns, current = 17 mA, power drawn from source = 26 mW
Signal-to-noise ratio: ~ 150 dB (limited by the Nyquist limit within a 40 Hz bandwidth)
Is the compensating signal identical?

[Aziz]

Hi Sergey,

I was referring to my basic & simple solution. Your configuration differs too much from my.

Below is the schematics I am referring to:

Even the source coupling capactor C1 (1n) is quite high, it is degrading the Q down to approx. 30. Still enough to work with. I can push up to almost +/- 500 mA to the TX coil (with only 17 mA source current).
The compensation mixer (C2 + C3) would attenuate the target response 50% and with input impedance R1 (1Meg) and base current of J1 would attenuate it slightly further.
So the j-fet amplifier amplifies the difference signal and does the high input -> low output impedance conversion (Lin impedance approx. 10 kOhm). So the target signal isn't further attenuated.
The sound card Sound BlasterX G6 is operated in the high-gain mode (delivering up to +/-6 V peak voltage).
The signal transformer is 1:10 in this example.

This shows only the basic concept. Fine tuning must be done yet.

Aziz

[Aziz]

here is the LTspice file for your convenience, playing, tweaking, adapting & arranging ..

[Aziz]

BTW,

the Sound BlasterX G6 has an output current protection circuit built-in. That's the reason, why I have a high inductance primary coil for the signal transformer. On initial condition, the high current through the primary coil could trigger the current protection circuit. And the sound card switches totally off.

But there is a software work around. Start with 0 V output voltage and increase it steadily to the desired output voltage level during 5 - 10 seconds maybe. This should avoid the current protection triggering. And we could reduce the primary coil inductance at the end.
A series resistor R3 (10 Ohm) helps too. But it has also a damping function to the signal transformer transfer characteristic.
Aziz

[Aziz]

Look,

there is a good solution for the AC current limiter on start-up procedure without triggering the output current protection circuit.
Found here:
https://www.linkedin.com/posts/linea...050729473-CbPC


Nice.

[Aziz]

Hi all,

I have found a ferrite ring core (Do=26 mm, Di=14.5 mm, H=20 mm) to try the step up signal transformer.
I don't know the core material anymore. But the AL is approx. 12000 nH/N². Could be a bad core material with high losses. I don't know.

I have to build a nice tool to wind the secondary windings on the ring core. It is difficult to wind up to 100 or more turns on the ring core. The secondary windings will be partitioned into several sections to lower the parasitic coil capacitance.
I will try to keep well below the resonant frequency (load capacitance C2 + inductance of the secondary coil winding of the signal transformer forms an LC resonant circuit).

At resonant frequency, there would be much much more compensation voltage generated. Ring core losses at resonant frequency could cause heavy temperature drifts too.
Let's try the save region first and look at the temperature drift. Could be a knock-out issue.

Aziz

[Sergey_P]

Hi Aziz
Sound BlasterX G6 -> https://reference-audio-analyzer.pro....php#gsc.tab=0
OK
What is high Q-factor used for? Is it preferable to focus directly on the signal from the target, rather than on changes in coil parameters caused by ground effects?

[Aziz]

Hi Sergey,

Hi Aziz
Sound BlasterX G6 -> https://reference-audio-analyzer.pro....php#gsc.tab=0
OK
What is high Q-factor used for? Is it preferable to focus directly on the signal from the target, rather than on changes in coil parameters caused by ground effects?

There are several reasons for using high Q resonant tank.
1. Limitted power source:
Sound cards don't give enough power to drive the TX coil. High Q factor will amplifiy the source current by a factor Q.

2. Narrow bandwidth: ( < 100 - 200 Hz)
The TX coil will pickup less noise.

3. High sensitivity:
The TX coil will respond to targets and ground with high sensitivity. Very small changes will lead to big changes to the TX coil voltage (amplitude, phase shift) .

4. Acting like an Off-Resonance principle:
Targets will "steal" energy from the TX coil. Any tiny energy loss will lead to big changes (amplitude decrease, phase shift).

These basic facts will lead to a good detection sensitivity and depth on a mono coil at the end.
Aziz

[Sergey_P]

Hi Aziz
<1. Limited power supply>
A technically solvable problem
<2. Narrow bandwidth (< 100–200 Hz)>
Insignificant; the detector bandwidth is 20...40 Hz
<3. High sensitivity>
of the high-Q circuit to coil parameters
<4. Operating principle based on detuning relative to resonance>
Discussed below...

A high-Q circuit is characterized by an impedance mismatch relative to the ground. Ground material located in the immediate vicinity of the sensor coil induces phase noise within it, which masks the useful signal.
To reduce phase noise caused by ground effects, there are three options:
1. Low quality factor (Q) of the sensor's resonant circuit: Q ~ 5
2. Non-resonant sensor
(based on materials from md4u.ru):
<<Грунт обладает электрическими и магнитными свойствами и воздействуя на приемную катушку изменяет её параметры. Эти изменения выражаются как внесение дополнительной индуктивности и сопротивление потерь в параметры катушки. В случае безрезонансной RX катушка представляет собой LR-цепь фазовый сдвиг в которой определяется выражением ф=arctg(w*L/R). Если в присутствии грунта w*(Lкат+Lгр)/(Rкат+Rгр) будет равна w* Lкат/ Rкат, то фаза останется неизменной.
w* Lкат/ Rкат - это добротность катушки, Qкат,
w* Lгр/ Rгр - это добротность грунта, Qгр. Фазовый сдвиг грунта (АКА Сигнум) разместил порядка 86гр , что соответствует Qгр ~14
Удельное сопротивление Грунта (чернозем, суглинок...) в зависимости от влажности меняется в очень широких пределах(10...150ом/м). Примем что-то среднее =75ом/м в размере нашего поискового поля =33см имеем Rгр=25ом
Для согласования сопротивлений для максимума передачи энергии примем: Rкат=Rгр=25ом
на частоте 8кГц ->
L=Q*R/w=14*25/(2*pi*8000) =7000мкГ
На катушечном калькуляторе с размером катушки 150мм имеем примерно
137витков, провода 0,25мм, Rкат=24ом>>
3. Operation with detuning relative to resonance
(based on materials from md4u.ru):
<<В МД приемный резонансный контур изображается в виде параллельного контура расстроенного по частоте, но на самом деле источником ЭДС является сама катушка!, т.е. источник эдс включен _последовательно _ с последовательным колебательным контуром RLC.
Критерии те же самые: добротность (Q) и фазовый сдвиг (ф), калькулятор вам в помощь ->
https://www.translatorscafe.com/unit...rlc-impedance/
например:
L=12мгн, С=0,1мкф, R=25ом, F=8кгц получаем -> Q~14, ф~86гр, Fконтура резонансное=4,5кгц, что соответствует катушке -> диаметром 150мм, проводом 0,28мм, имеющей 185 витков...
(калькулятор катушек -> https://esm62.ru/calculator/raschet-...uctql546374216 ) >>