lobo improved

[simonbaker]

Why a coil is a low pass filter, easy, it produces voltage from the applied flux, and does that in perfect order up to a corner frequency where series inductance becomes significant factor.

Ok, I thought we were talking about RLC tank for hi-Q RX front end.

Instrumentation amplifier is an option, however, those are seldom optimised for low input impedance operation. If you happen to find some instrumentation amplifier boasting with low noise, it automatically means it is optimised for low input impedance as well. Point is that low voltage noise comes together with high current noise, and such solution is not optimal for high impedance operation. Otherwise it is a perfect choice for differential inputs.

For a perfect example, look at Lobo's frontend. All the gain setting resistors are in the inverting branch, while coil signal is introduced to the non-inverting input. The gain setting resistors are of low resistance value, hence optimised for very low noise. Such low resistor values are not seen in instrumentation amplifiers. In fact, those Lobo resistors are set for a much better op amp than you can find in a commercial Lobo.

Yes, the Lobo front end looked sort of like half of an instrumentation amp -- That's why I asked why not use actual instrumentation amp with low resistors for full differential design.

Regarding bandwidth, remember that your system noise can get as close to the thermal noise floor only in a case your frontend does not introduce too much of its own noise.

Do you mean thermal noise floor of the coil only?

Out of that sum of natural noise and the introduced noise, your Rx will sort out the narrow bandwidth signal/noise, and ... this is important ... the noise floor there in the narrow band path will be ruined by the same number of dB as your frontend is responsible for.

So, in case your frontend introduces 20dB of noise on top of the thermal noise floor, your narrow band gain block will also have noise risen by the very 20dB on top of the narrow band noise floor.

Maybe that is what I was saying too, I'm trying to understand your way of describing. I see it this way: any resistors from your "front end" (amplification stage) that couple to the passive RLC "sensor" (coil and resonant cap) will have their noise resonate in the RLC circuit within the band. But some some out-of-band noise is truly reduced, including from those resistors. Whether this is useful or not depends on how you demodulate the resulting signal. Since the Synchronous Detector suppresses everything outside an even narrower band, the RLC filter has no use, it would seem.

There is a small exception. Some front-end resistor noise and op amp noise is probably not coupled to the coil RLC tank, and does not get "boosted". In that case, a high-Q RLC tank does improve the S/N over that noise. I'm not saying it is significant -- I agree not worth dealing with the steep phase gradient.

If we're still not saying the same thing, please continue.

Regards,

-SB

[Davor]

I think we are, but in somewhat different fashion. To sort out apples and pears, yes, I'm referring to the thermal noise. When all other annoyances are sorted out it is your ultimate frontier. The way you are referring to the resonant circuit fits much better to the way of combating the interference, but there are enough similarities with approaches to reducing noise as well.

I'm moving my attention towards a better frontend, and I think I have one up my sleeve that everyone can build. It will be posted on a FKK coils topic because I believe they together will make some serious advancement.

[simonbaker]

I think we are, but in somewhat different fashion. To sort out apples and pears, yes, I'm referring to the thermal noise. When all other annoyances are sorted out it is your ultimate frontier. The way you are referring to the resonant circuit fits much better to the way of combating the interference, but there are enough similarities with approaches to reducing noise as well.

I'm moving my attention towards a better frontend, and I think I have one up my sleeve that everyone can build. It will be posted on a FKK coils topic because I believe they together will make some serious advancement.

Ok. Probably equations would be the only way to be really specific (maybe or maybe not more clear ).

-SB

[Derx]

It goes like this...
Your opamp is noise limited to 1kohm equivalent, or 4nV/sqrt(Hz), and your working frequency is 10kHz. You want your phase to remain under 18° shift, and your input is configured as Lobo's with noninverting input shunted by some arbitrary but not too small resistor to keep self resonance and impulse phenomena at bay, say 10k (ten times 1k, rule of thumb thing).
18° shift is found at 1/3 frequency of -3dB point (12° is at 1/5), so it must be at 33kHz. At -3dB point the shunt resistance and coil reactance are the same, and a calculator gives 48mH. Upon checking the noise - in simulation of course, I find a tad below 4nV/sqrt(Hz).
So I get everything WITHOUT resonance. Largest possible input voltage at desired noise level. No resonance troubles.

Please don't get me wrong, but in case of VLF metal detectors resonance is so overrated.

Here go a few examples non-resonant and resonant, AC and noise

Hi, Davor!

How to improve lobo preamp & sheet for current coil ?
RX R=18.2 ohm L=9.17 mH
TX R=3.7 ohm L=1.44 mH

F TX=17500 Hz

TS952IN TS971IDT compatible in this circuit?

[Davor]

Unfortunately not. Lobo has a very nice design of the Rx front end that deserves a much better op amp, but unfortunately there is a small problem: too high power supply. Most of the nowadays low noise op amps work at below 10V and it makes your choice somewhat limited. The existing op amp LS204 has noise at 10nV/sqrt(Hz), and it is a dual amp.

You could replace it directly with LT1124. It has noise below 4nV/sqrt(Hz), and works happily with larger supply voltage.

LM833 and NE5532 are not much of improvement noise-wise, so don't bother using them.

MAX412 and TS972IDT have good noise, but would give up smoke if put directly into Lobo

[Derx]

Thanks!


How to calculate capacitors RX TX for coil parameter up my post?

What is type of mosfet in original pcb lobo?

[kt315]

there may be a mistake in schematic. jude me it is principally or not.