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.

[Aziz]

Hi all,

pity, but we can't use low input impedance (Rin) amplifiers. Additional tuning of the RX path will give us huge problems.
The AI tells me, that it is a bad idea.
-> Bye bye Richard Lee's pre-amp. Bye bye en=0.2 nV/rt Hz.

We have to follow the op-amp based pre-amp. This will still enable us en=0.3 nV/rt Hz.
And I have noticed, that all the ".ic" commands in ".ac" and ".noise" analysis will lead LTspice in wrong calculations.
We have to switch-off all the ".ic" commands in the circuit simulations.

The op-amp based pre-amp has a dynamic Rin of approx. 6.2 kOhm @ 45 kHz. Bootstrapping the transistor stage to increase the Rin does not give any benetifs (it is even worse).
Rin is still high enough for our purpose and the signal in the RX coil won't be loaded (diminished) much.

Let's give the op-amp based pre-amp the last optimization until I fire-up my soldering iron.
Aziz

[Aziz]

Hi all,

I have prepared a blank bread-board for the new pre-amp. Note, that the whole detector controller consists of the pre-amp only. ;-)
The transmitter part is located in the search coil (C0G/NP0 capacitors).

But I have to buy some new parts soon. Also the upgrade version from NE5532 to OPA1612 (WTF!, 8.13 bucks each!).
And many NPN/PNP Zetex transistors.
Cheers

[Aziz]

Hi Sergey,

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.

yep. In the past, it was a nice built-in passive "amplifier" with the tuned RX-path (for single frequency VLF).
We don't need it today. With multi-frequency RX, we can't use it.

This is what the AI says (see attachment).
Yes, it will have some "interaction" with the very low input impedance amplifier.
Aziz

[Carl-NC]

pity, but we can't use low input impedance (Rin) amplifiers.

I don't see why not. You can "read" the RX coil signal in current mode, as I did with the MagFieldProbe in ITMD3:



Here is a version of Lee's preamp I was playing with a few years ago:



You can replace the the Vin signal source with an RX coil and it will work just fine. There is a caveat: you lose the input derivative of the RX signal that you normally get with a voltage-mode coil.

[Aziz]

Hi Carl,

I don't see why not. You can "read" the RX coil signal in current mode, as I did with the MagFieldProbe in ITMD3:
..
You can replace the the Vin signal source with an RX coil and it will work just fine. There is a caveat: you lose the input derivative of the RX signal that you normally get with a voltage-mode coil.

no, it won't work.
Things dramatically change, if the Vin signal source comes from a coil.
I will post a direct circuit comparison later, which shows this.
Aziz

[pito]

perfect ferrite balance, the first it is a ferrite, the second any metal, the blue it is a Uy, orangre Ux

[Aziz]

Hi all,

I have made a comparison circuit simulation using the data in the recent AI paper.
4 independent configurations with ideal op-amps (Level 1). The low input impedance amplifier is made in inverting ideal op-amp amplifier configuration.
The high input impedance amplifier in standard non-inverting configuration with a load resistance at the input simulating the input impedance.

As the operating frequency of 45 kHz is very high, the reactance of the RX coil dominates heavy. It is acting like a choke in very low input impedance amplifiers.
This is really a revealing evidence.

Look at the spice simulation file in the attachment.
Cheers

[Aziz]

Hi pito,

perfect ferrite balance, the first it is a ferrite, the second any metal, the blue it is a Uy, orangre Ux

nice. very nice.
I should look at the ferrite induction balancing. Maybe I can leave the IB-nulling part and drive the TX with differential input (more TX power).
Aziz

[Aziz]

Hi all,

where to get the OPA1612?

Ali Baba and the Forty Thieves?

https://www.alibaba.com/countrysearch/CN/opa1612.html

This is for sure a fake op-amp.
;-)

Aliexpress?
https://de.aliexpress.com/w/wholesale-opa1612.html
They cost much more. I'm sure, they are all still fake.
;-)

They all will probably underperform NE5532.
;-)

Be careful. NE5532 is still a good choice.
Aziz