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

[Aziz]

BTW,

the amplifier can be seen as a Transimpedance Amplifier (TIA) as the input impedance is very very low (Rin < 1 Ohm).
The pre-amp is amplifying the induced current in the RX coil. Not voltage! There is a big difference.

Therefore the input capacitor C1 must be matched to the operating frequency and RX coil inductance so the reactance X gets 0 and maximum current can flow to the amplifier.
IB-Nulling is made via current injection to the input of the amplifier through resistor R1.

PS: You don't need the low-pass filter capacitor C4 (Cf) with the RX coil of course. We have a much better built-in resonance filter. Leave C4 completely.

https://en.wikipedia.org/wiki/Transimpedance_amplifier

[Aziz]

Hi all,

I am very keen to see the practical use of the Richard Lee's Pre-amp. Once again. This time with impedance matching.
This is a super-duper KISS project.
And to cope with physical limitation issues (temperature drift, gain drift, battery voltage diminishing drift, IB-nulling drift, etc....) and whether they can be solved well in the software part.

If this is going to be very critical, well we have the another op-amp based voltage amplifier alternative shown earlier.
Aziz

[Aziz]

Hi all,

there are a few questions left over for the KISS variant:

The RX-coil with its residual IB-mismatch and coupling coefficient to the TX-coil at the operating frequency is almost shorted (Rin preamp < 1 Ohm) and is acting like a "big target".
So what will happen? Will it degrade the high-Q of the TX-coil very much?
Will there be an interaction (modulation) between TX and RX coil?

I don't know the answer yet until I test it with real circuit.
Aziz

[Sergey_P]

Will there be an interaction (modulation) between TX and RX coil?

No, the transmitting coil (TX) "knows" nothing about the receiving coil (RX). From the perspective of the transmitting coil regarding the receiving coil: EMF (TX) → 0, current (TX) → 0, R (TX) → infinity.

[Aziz]

Hi Sergey,

No, the transmitting coil (TX) "knows" nothing about the receiving coil (RX). From the perspective of the transmitting coil regarding the receiving coil: EMF (TX) → 0, current (TX) → 0, R (TX) → infinity.

I think we will face huge problems due to very very low input impedance of the preamp.
Double tuned parts: TX & RX
RX has also very high-Q. Any change in the RX inductance will affect the gain too.
This could make fail or make it ultra-ultra sensitive (also to hot rocks).

The "interaction" might come from sweeping when targets or hot rocks will be presented. So out of steady state condition.

In case of bad news, we need an amplifier with higher input impedance (>1 kOhm) and untuned RX coil (amplifying the induced voltage only).

Well, I don't know the answer yet. I have to try it out.
Aziz

[Sergey_P]

Hi Aziz,
Traditional radio reception methods are inapplicable to metal detectors because they do not account for the influence of the surrounding soil. In the past, high-Q transmit (TX) and receive (RX) coils were manufactured, but their performance in the ground proved unsatisfactory.