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

[Aziz]

BTW,

the Franklin LC Oscillator is taken, adapted and arranged from here:
https://hackaday.io/project/19315-ye...lin-oscillator
It's the last circuit.


Cheers

[Aziz]

Hi all,

I have tested several resonant frequencies by replacing the resonant capacitor Ct1.
Now the big good news:
You don't need a fancy & fast sound card anymore. Sample rate (SR) of 44.1/48 kHz is by far enough. ADC bit resolution should still be 24 bit and good SNR on its input channels.
You don't need a 192 kHz SR gizmo. A relative cheap USB-Dongle sound card should be enough.

Why?
On higher frequencies, the magnetic susceptible materials under the coil (hot ground, hot rocks, etc.) are causing more resistive response to the LC oscillator. So the magnetic field applied to the materials can not follow the speed of excited magnetic field. They cause more and more resistive losses (energy absorption). So you are losing detection depth to cancel out the X-response.
On lower frequencies, these resistive losses go to zero (buried in the noise and not detectable).

Therefore, it doesn't make any sense to go up with the operating frequency.
Frequency region between 3 kHz to 10 kHz is ok. 3 kHz is well above the low frequency EMI noise sources.

Cheers,
Aziz

[Aziz]

Now I am happy to use my Creative G6 sound card. Even on 192 kHz SR. It improves still ENOB due to having more signal samples. 48 kHz SR -> 192 kHz SR, 4 times more samples, which is effectively 1 bit more ADC resolution (ENOB).

But the good specs are in the audio band (0 - 20 kHz). It perfectly fits into this application now.

I have to look for a higher inductance TX coil now. Even with enamelled copper wire.

[Aziz]

More surprizes.

At quite low frequencies (below 8 or 7 kHz around), hot ground feeds the LC oscillator with some energy. It's amplitude grows slightly. There must be a sweet spot in the operating frequency, where the X-reponse is fully cancelled.

On the other side, small targets suffer from sensitivy.

[moodz]

Hey Aziz my AI mate read your whole thread ( in about 5 seconds ) and this is what he reckons ...

Ah, now I see the full context. This is Aziz's thread, and he's been on quite a journey.

What he's actually doing:

He built a "TEM2" transmitter with a double-tuned tank, then discovered that by adjusting the operating frequency and using a high-impedance sensing point ("TXref") with series resistor R2 and choke L1 for decoupling, he can find a sweet spot where targets produce amplitude changes with minimal phase shift. He's calling this "X-free" mode.

The Franklin oscillator in post #86 is a new tangent - he wants to explore the "off-resonance principle" with a local LC oscillator where the TX coil is the L. Ultra-low power, single cell operation.

My reading of what's happening:

His gut feeling in post #83 is probably correct: "There is no free lunch." What he's likely observing is that his particular circuit topology - high-impedance source, R2/L1 decoupling, operating slightly off the natural resonance - creates a situation where the reactive and resistive components combine in a way that appears to cancel X at one specific measurement point.

It's not that X doesn't exist; it's that he's found a linear combination where X contributions from different circuit elements happen to sum to zero at his observation point. As he says: "It must be the linear combination subtractive GB method."

The Franklin oscillator experiment:

Using the TX coil as the tank L in a Franklin oscillator will show him exactly what you predicted - frequency pulling from ground coupling. Whether he interprets that as a feature (detection mechanism) or a bug (instability) remains to be seen.

He's a good experimenter - he'll figure it out. But I don't see novel physics here, just an interesting circuit exploration.

[Aziz]

Hi Paul,

your AI mate is short of I.
Nevertheless, it gives funny answers.

I have to exclude circuit related issues first (leaving optimal operating point, non-linearity, and so on). These can quickly change the math and observations behind the physics.
We will see, what happens. Step-by-step.
But the Off-Resonance principle is very interesting and promissing.
Cheers,
Aziz

[Aziz]

Hi all,

the local Off-Resonance oscillator version is destined for mono coils (only TX-Coil). If I connect an IB RX-coil, I won't detect much target signals. We would require more TX-current (upto 100 times more). But then the Off-Resonance won't work. It's a dilemma. We have still the TEM2 design for this purpose.

The local oscillator however has much better SNR. Approx. 30 - 40 dB better than TEM2. No switching noises. Easy design.
I'll finish the mono coil version because it is a nice KISS project. Just for fun.

We can switch back to the TEM2 later. This will work with both coil versions (mono, IB).
Cheers

[Aziz]

Hi all,

I have really bad news:
It didn't work anymore after I have tested several other TX coils. No more X-cancelling effects observed.
This is very mysterious. A phantom effect.

I couldn't reproduce it anymore with other coils. Could be coil related, circuit related, what ever related.
Back to the point, where it seemed to work. And let's look, what went wrong.

So physics don't change. Only the measurement environment. Or the observer.
Cheers,
Aziz

[Aziz]

Yep,

it is obviously coil related phantom effect. Or combination of coil and circuit.
So what is different to the other coils?
I have no idea yet. This is really very mysterious.

Cheers,
Aziz

[moodz]

...its not a phantom effect .. but achieving it using lumped components like LCR will only work for very specific setups on the bench. It will never keep working in the field waving the coil around and temperature, different gronds, etc and definitely not for the small and deep targets that might only be moving the rx voltage in the nanovolt range.
It can be done mathematically though .. but you are concentrating on the wrong part of the solution.
I could tell you of course but that would be like telling you the last page of a very interesting book >>

[Aziz]

Hi Paul,

...its not a phantom effect .. but achieving it using lumped components like LCR will only work for very specific setups on the bench. It will never keep working in the field waving the coil around and temperature, different gronds, etc and definitely not for the small and deep targets that might only be moving the rx voltage in the nanovolt range.
It can be done mathematically though .. but you are concentrating on the wrong part of the solution.
I could tell you of course but that would be like telling you the last page of a very interesting book >>

yep, you are probably right.

It seems, that the Miller Effect in combination of the specific TX-coil could be responsible for X-cancellation. And the LC oscillator is operated at it's critical starving operating point. This operating point will be changed on the presense of targets (metal or magnetic materials). Parasitic effects dominate. I wonder, how this could affect the X-cancellation.

The specific TX-coil doesn't really see any X-response at specific operating frequency. Frequency shift yes, but no amplitude change.
I haven't finished the tests however to be sure.
Cheers,
Aziz

[Aziz]

Hi all,

I could reproduce the X-cancel effect on an another TX-coil now.

Yes, parasitic effects makes it possible. But to get this point is very critical and not worth to exploit it. It is nice, that mono coils can be operated to ignore X-response. The depth detection is not very much however (maybe 30-50% of the standard methods). At the critical point, the LC oscillator is also more noisy.

So this little off-resonance local oscillator project does not make sense to follow it further.

What's next?
TEM2 with more bang puff smoke with the IB RX-coil connected. But I need new parts.
Cheers,
Aziz

[Aziz]

Hi all,

there is still a way of using the off-resonance principle with mono coils. It's the advanced & complex solution however.
Even the local LC oscillator is set to a stable (not critical) operating point with less noise and having X-response, all the information is there for DSP processing to cancel the X-response with software coding.

The frequency shift around the resonant frequency is there. The amplitude change is there. And even the changing of emitter base control voltage of the transistors (node Vtap).
A tricky software coding will solve the GB issue. No doubt.
The Vtap node information could also help to increase the SNR of the amplitude signal. I am wondering, whether this could significantly improve the SNR.

I have to look for stable and low noise operating points. Then check its sensitivy (SNR). If it is ok, it could still be worth to try it out.
Let's think of it further.

I like the mono coil idea.
Cheers,
Aziz

[Carl-NC]

Proximity methods -- frequency shift, loading, and off-resonance -- are inherently less sensitive than IB methods or PI. In the mid-late 70s A.H. Electronics and Gardiner tried to stay competitive with TR and TR-Disc but could barely keep up, and when VLF motion detectors arrived it was game over for them. However, there is another mono coil method that gives IB-like results:


Something similar was used in the Minelab Go-Find models.

[Qiaozhi]

Have a look at this thread from 2015 regarding Minelab's Go-Find -> https://www.geotech1.com/forums/foru...-go-find/page3
You can skip a lot of the preamble by jumping to post #38.