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

[Aziz]

Hi all,

I am still thinking of to improve the SNR on mono coil VLF (single TX-coil solution). Particularly to reduce the line-output noise of the sound card.

As the high Q-factor is very sensitive to line-output noise of the sound card, there is still a way to reduce the source noise contribution.
A second Lock-in demodulator from the line-out to the other line-input channel. So the pure source reference signal. The noise contribution can be detected and removed from the TXref signal.
There is a SNR improvement of 40 dB possible (100 times lower source noise).


But the picked up EMI noise is still there. Only the TX-current increase will improve the SNR.
Aziz

[Aziz]

Hi all,

now the fitfall with High-Q: The SNR.
If your source has some noise, the noise will be amplified by the Q-factor. No free lunch. No gain.
The line-output of the sound card is quite noisy.
How to overcome this source issue and improve the SNR on this side?
Pulse the source with a 50% duty-cycle pure clean and digital clock source (rectangular pulse), where it's frequency is controlled by a crystal oscillator and its phase and amplitude noise is low.
This is a good job for an embedded microcontroller PWM-output.

The TX-coil will pick-up EMI noise. This noise will be amplified by the Q-factor. Again no free lunch, no gain.
The figure-8 TX coil will help a bit, but will effectively reduce the coil size and detection depth. No gain.

We have to realise a compromise: Lower Q-factor (4 - 10), pushing more TX-current to get more from the RX-coil side.
You see, that the design concept is moving towards the standard the IB-coil configuration.
The pure mono coil VLF design has reached it's limitation now.
No matter what I do, there is no free lunch with my sound card solution. No gain.

Cheers,
Aziz

[Aziz]

Hi all,

good for me, not to order the missing parts yet.
Ask the AI for pitfalls, power efficiency and so on before doing this. So I did.

I am working on the high power High-Q transmitter design with the choke.

The appropriate ring core material for the choke for minimum losses (first attempt):
N27 or N87 (Mangan-Zinc ferrite core material, Al approx. 500 - 2000 nH/N²)
Size approx. 20 - 30 mm outer diameter. Better 30+ mm.
Careful design should minimize the choke losses (saturation current, operating voltage = step-up voltage conversion, expected losses)...

I will study the AI proposals for best power efficiency further. It can calculate all your design parameters. What a nice feature.
We can achieve even a Q-factor up to 200. Or more.
The High-Q transmitter is damn powerful. But requires careful design to tame its extreme power.

But we are dealing with more issues: Temperature drift, shielding, etc.
Did you know, that even the LC-tank capacitors need a proper shielding? We would require a shielded housing for the transmitter. All cables need a shielding too.

[Aziz]

Hi Marchel,

I have analyzed the coil of the XP Deus 2 metal detector and the TX circuit draws approximately 30-40mA and the range to a small coin is approximately 30-45cm. I think that the current of 500-1000mA is quite excessive.

yep, it looks like quite excessive.

But we have the magic High-Q feature.
Let's say, we have Q=50 and a TX-coil current I(Tx) of +/-500 mA.
I(in) = I(Tx) / Q = 500 mA / 50 = 10 mA.
We are feeding the LC-circuit with 10 mA only. This is the High-Q-trick. No magic here.

On the otherside, we need as much TX-current as possible to have enough target stimulation on the RX-side. We don't want to use an amplifier at all.

We want to upgrade the High-Q VLF into high-quality VLF.

Cheers,
Aziz

[Marchel]

Hi all,

I didn't try the high-power version for the true IB-coil system yet.
I have to order some appropriate ring cores for the choke and low voltage FKP1 capacitors, which are relative small compared to the high voltage ratings. Note, that High-Q requires best foil capacitors with very low losses. Metalized film/foil capacitors won't work well.

The high power version makes use of the High-Q LC-tank. Feeding low current in, to make big current flow in the TX-coil. A Q of 20 - 80 is quite realistic.
A TX-coil current of +/- 500 mA - 1 A should be enough, to get reasonable RX-signal in the RX-coil. We would not require an amplifier yet.

The IB-system would deliver the best sensitivity and depth. If we process both TX and RX channels, we would improve it further.
Aziz

I have analyzed the coil of the XP Deus 2 metal detector and the TX circuit draws approximately 30-40mA and the range to a small coin is approximately 30-45cm. I think that the current of 500-1000mA is quite excessive.

[Aziz]

Hi all,

I didn't try the high-power version for the true IB-coil system yet.
I have to order some appropriate ring cores for the choke and low voltage FKP1 capacitors, which are relative small compared to the high voltage ratings. Note, that High-Q requires best foil capacitors with very low losses. Metalized film/foil capacitors won't work well.

The high power version makes use of the High-Q LC-tank. Feeding low current in, to make big current flow in the TX-coil. A Q of 20 - 80 is quite realistic.
A TX-coil current of +/- 500 mA - 1 A should be enough, to get reasonable RX-signal in the RX-coil. We would not require an amplifier yet.

The IB-system would deliver the best sensitivity and depth. If we process both TX and RX channels, we would improve it further.
Aziz

[Aziz]

Hi pito,

should be 90 deg

Referred to what?
Remember, this isn't a standard IB-coil system, where we can refer to the TX-coil's phase. It's a mono coil VLF and we have only the TX-coil itself. We have to make our own reference system for I/Q demodulation. And we have to process the I and Q changes only.

To make a reference system for the IQ-demodulation, we are internally generating a reference I and Q decoding signal using a lockin-amplifier. By just adjusting the phase of the reference of the I/Q demodulator, we can do the ground balance at once (doing the rotation to the I-axis). The phase angle can be obtained by the angle of the IQ-vector at ground balance procedure. So we are rotating back the demodulated IQ output to the I-axis. But we don't use the IQ output itself. We are using the tiny changes referred to a reference IQ-vector. The change is the dQI vector.
The Q-part (quadrature-phase, imaginary part) of the dIQ-vector is our ground balanced target response (R-Response).
The I-part (in-phase, real part) of the dIQ-vector is our X-Response.
The phase angle of dIQ-vector (0..180 °) can be used for VDI calculation.

I have shown the proof of concept. It is working similar to standard VLF. But it's not the same.
Cheers,
Aziz

[pito]

Let's look at the pure X-response by placing a ferrite .

should be 90 deg

[pito]

So lets look at different target responses,

In your program, can generators be replaced by Ux and Uy applied to microcontroller inputs ?

[Aziz]

Next time, I will specify a query for Claude Code AI,
- with the fully specified measurement environment
- describing the Gaussian and Flicker noise behaviour
- to ask it, to remove the unwanted noise
- and combining the cleaned signal with lock-in amplifier output

This is so nice.

[Aziz]

Google AI for noise prediction and elimination solution.
I haven't tried Claude Code AI yet.

[Aziz]

How about a figure-8 mono coil?
It gets rid of the EMI-noise by design.

[Aziz]

Hi all,

I didn't have much time yet. But I'm thinking of to increase the SNR on mono coil VLF yet.
The mono coil is picking up at least 20 dB more EMI noise above my noise floor level.

There is an elegant way to get rid some EMI noise. The Hilbert-Transform. I have to ask the AI.
The AI says yes.

It can isolate the in-band noise. Amplitude and phase noise isolation is possible as the stimulus frequency and amplitude is fixed by design.
The target response is usually changing slowly and can be considered to be constant in a short period of time. The high frequency in-band EMI noise isn't.
The phase noise can be isolated much more accurate. The unrolled phase(t) demodulation should be a straight line. As the stimulus frequency is fixed, it forms an ideal straight line. Every unrolled phase(t), which is displaced from the ideal straight line, must be EMI caused phase noise.

I have to think of it further..

[Aziz]

Hi all,

now I have an another interesting idea to test. The goal is to improve SNR and enabling both mono and IB-coil operations. Even at the same time (both TX/RX channels will be processed).
With a new free running high-Q LC oscillator, Hilbert-Transform, digital filters, ..

[Aziz]

Hi all,

my next steps will be:
- Degrading the high Q of the LC-tank (pushing Q down to 50 or lower) and looking at the noise and sensitivity.

- After that, changing the transmitter from mono coil system into an IB-coil system (TX/RX) with more TX current. This is the standard VLF operation. Comparing the results against the mono coil system.
Aziz