lobo improved

[kt315]

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

[Derx]

Thanks!


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

What is type of mosfet in original pcb lobo?

[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]

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?

[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

[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]

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]

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.

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.

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. 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.

[simonbaker]

You are right in most of the conclusions, and in most details, but there are some small bits that spoil everything.
First and most important is resonant coil being a very narrowband. It isn't. It is a low pass filter, with a ripple prior to the falling slope. It is sharp and pointy at the resonance, but you can't touch it, it is off limits, too steep and unpredictable phase change. So the only mechanism attributed to the metal detecting world is impedance transformation at off resonance spot, period. All other goodies can be achieved using far more predictable RC filters, just as is done for Lobo.

There are ways of approaching the theoretical noise limit of a coil itself, and this Lobo approach is darn close.

No, I said Synchronous Detector is very narrow band -- I agree resonant coil is not narrow band -- that is my point -- even if you raise the Q of the resonant coil, you do not improve the S/N within the very narrow band of the SD. The EMI and even the coupled resistor noise are also boosted by the resonance within the SD band. Only if you consider a broader band than the coil RLC band do you achieve improved S/N metrics, I believe. I also think of resonant coil as band pass, not low pass -- why do you say low pass?

So yes, you can do the impedance transformation thing to at least optimize your resistors to the op amp you choose for noise, and perhaps choose the best coil inductance (there is a question - what do you think about RX inductance?). Now suppose we go with an "instrumentation amp" type of preamp -- does that remove the resistor optimizations because we have infinite impedance? Or is that just inherently more noisy to begin with?

As for operating at the resonant frequency of a high-Q coil tank -- I agree it is nasty, but if it had a vastly improved S/N, I would say it would be a challenge worth trying. However, I'm agreeing, based on my rough thinking, that it doesn't seem to help much because of the extreme narrow SD bandwidth, which seems to be the determining factor. But I'll continue to think about this S/N subject and am interested in the designs you are considering.

So I guess the reason you like the Lobo front end is because of the "over-damped" coil, even though it is a second order filter, (would you rather have a simple low-pass coil?). Do you like the high-impedance op amp input, or it is just a necessary evil to achieve the over-damped coil?

Regards,

-SB

[Davor]

You are right in most of the conclusions, and in most details, but there are some small bits that spoil everything.
First and most important is resonant coil being a very narrowband. It isn't. It is a low pass filter, with a ripple prior to the falling slope. It is sharp and pointy at the resonance, but you can't touch it, it is off limits, too steep and unpredictable phase change. So the only mechanism attributed to the metal detecting world is impedance transformation at off resonance spot, period. All other goodies can be achieved using far more predictable RC filters, just as is done for Lobo.

There are ways of approaching the theoretical noise limit of a coil itself, and this Lobo approach is darn close.

[simonbaker]

It is not much of a simulation, but here you go...

Thanks.

Without doing any analysis yet, I'm thinking more about S/N and usefulness of "on-resonance" design. I may well agree with your conclusions, because my preliminary intuition is that although a high-Q resonant RX should indeed increase S/N regarding EMI, resistor noise, and op amp noise when looked over a wide spectrum, the fact may be that the Synchronous Detector forms such an extremely narrow filter, that the S/N within that narrow band simply is not affected much.

In other words, a resonant RX tank increases S/N by suppressing noise outside the RLC filter band. (That includes EMI noise and any resistor noise or op amp noise that is coupled to the RX tank.) However, the SD band is tiny and well inside the RX tank band. So there is no change in S/N in the SD detection band.

In order to conclude there is no advantage to a high-Q resonant RX tank, one assumes that all resistor noise has the opportunity to resonate with the RX tank -- my question is: do some resistors, such as the op amp feedback resistor, add noise that is not "coupled/amplified" by the RX tank. If those resistors add noise that is not coupled to the RX resonance, then we might achieve some gain over that noise. Also, some op amp noise probably is not coupled to the RX tank -- likewise, we should achieve some gain over that noise.

However, because of the availability of very low noise op amps these days, the improvements due to a high-Q resonant tank could very well be too small to overcome the huge hassle of dealing with the tempermental phase caused by a high-Q RX tank. In that case, I would agree, don't waste time playing with high-Q RX tank, and probably no point in having any capacitor at all. Well, perhaps there is a small purpose of having a capacitor simply to prevent a coil self-resonance from creating a large signal at some higher frequency which might so something weird (overdrive the inputs; alias the SD, etc.).

Those are my impressions without doing serious analysis yet.

-SB

[Davor]

It is not much of a simulation, but here you go...

[simonbaker]

Just a hint, you can scale things from the known to the desired. Scaling is linear except for voltage/inductance where voltage rises with square root of inductance. Using Lobo's values you may reach to conclusion that, keeping the coil as is, the ideal op amp would have ~2.7nV/sqrt(Hz) of voltage noise. That would improve good old Lobo by ~10dB by mere op amp swap. Not bad at all.

Thanks Davor - can you attach your LTSpice simulation ".asc" files for convenience?

-SB

[Davor]

Just a hint, you can scale things from the known to the desired. Scaling is linear except for voltage/inductance where voltage rises with square root of inductance. Using Lobo's values you may reach to conclusion that, keeping the coil as is, the ideal op amp would have ~2.7nV/sqrt(Hz) of voltage noise. That would improve good old Lobo by ~10dB by mere op amp swap. Not bad at all.

[simonbaker]

Right to the point.
There are a few points to clear before continuing to the specifics.

First off, a coil as a magnetic pickup is in reality a LPF device. It produces voltage. When observed without a parallel capacitor it is a wideband device with a high cutoff that depends upon a load resistance, or as we observe it - a preamp input impedance.
You can see that cutoff is higher as the load impedance is higher, but at cost of somewhat higher noise.

While you can spoil S/N with too high load impedance, you can screw it completely by too low impedance. In microphones world the rule of the thumb suggests using loads that are 10 times the coil resistance and up, but not too much because of the noise.
These cases are shown in pictures of the non-resonant circuit. One shows AC and the other shows noise. It is important to note that real effects of the noise must be normalised for the gain/loss because they will surely reflect at the preamp output. E.g. half the noise for a circuit that gives -6dB against the other circuit results in equal final result - you must pump gain and the noise for that 6db, and puff goes your noise advantage.

In case of resonance, you obtain some virtual gains because of the impedance transformation effects, and to fully grasp the mechanism follow the red line (1k) in non-resonant and the resonant cases. Without additional noise you get to the very same normalised noise performance this way or another.

The whole difference is in preamp noise. In case your opamp has 4nV/sqrt(Hz) it means that it is well matched with a 1 kohm equivalent resistance noise. Your system noise will not go below that, and you can optimise your frontend for that.

Every preamp can be seen as an infinite impedance voltage sensor with a shunt. Every coil can be seen as an auto-transformer with 1mH primary, and a secondary/tap at desired inductance. Point to note here is that with more inductance you gain more voltage, but phase and noise get worse. Goal is to reach maximum voltage at exactly the noise equivalent to the preamp input noise.

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

Thanks for the analysis, that's what I'm looking for. I'll need to study it for a while.

-SB