lobo improved

[Davor]

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

[simonbaker]

True, because every coil has resonance somewhere unless it is damped really good. So for this Lobo thing you can say "way off-resonance"
However, what I have in mind is really an aperiodic frontend. Imagine a coil. Now imagine it picking up some magnetic flux at some arbitrary frequency, or better say any frequency. Now, extend your imagination to some Rx attached to it. It has some input impedance, and in case this impedance is lower than the critical damping - you have a real aperiodic frontend, the one that wouldn't even ring.

Now, what happens at resonance? From the coil's point of view in a resonance it sees the lowest possible impedance that is limited by coil's resistance, and that is the point where you have the best possible energy transfer - it is the voltage source after all. Because in resonance we have a sharp phase transition and we can't use it, I can attach a coil to a low impedance Rx instead and have frequency independent signal transfer, spoiled only by the coil's inductance. That kind of aperiodic.

In short - I can't use resonance, so why bother with resonance at all?

Thanks for explaining your use of term "aperiodic", I get it. I agree with Qiaozhi that the word seems not what you mean; however, I would choose "over-damped" or "low Q" or "non-resonant" or "wide-band" coil circuit. "Aperiodic" to me describes waveforms that are not periodic. Just a language thing.

I would assume the purpose of RX resonance would be to increase S/N, especially regarding EMI; the synchronous detector probably can alias wide-band signals into its detection band, though maybe not as much as I think.

Also, RX resonance can greatly increase signal gain relative to resistance noise, etc. Of course the trade-off is a loss of phase stability relative to component tolerances, as you noted. So did Tesoro find a useful compromise with the simple RLC tank?

The TGSL/IGSL operates so far off resonance, I wonder if there really is an advantage in S/N over a completely "non-resonant/wide-band" coil circuit. You seem to think not, and I can believe you may be right. We should be able to do a noise analysis fairly simply with LTSpice. And it would be useful to include some broadband EMI noise into the coil as well.

Also bear in mind that the Lobo op amp bandpass filter will also make phase shifting for off center frequencies -- why don't you object to that?

I have always wanted to try a high-Q, "on-resonance" RX circuit design to optimize S/N, even though very difficult to achieve phase stability. Put the engineering effort into stabilizing the phase and reap the benefits of the superior S/N. S/N is really the what we are trying to achieve (until we hit the "ground noise" limit).

-SB

[golfnut]

Saying resonance isnt clear.

Parallel resonance the one we use - High Z case- (Not series resonance (Low Z case))- the impedances of the L and the C are the same and are HIGH. Opposite polarity or sign.

Giving a tank Z at resonance of L/CR (R is the resistance of the L)

You can broadband the tank with extra shunt or Parallel R makes it give less phase change with ground proximity - less touchy in use.

Yuo can make a tank resonate in a parallel way - without a cap - you just need lots of winding to winding capacitance and it will work

S

[Davor]

True, because every coil has resonance somewhere unless it is damped really good. So for this Lobo thing you can say "way off-resonance"
However, what I have in mind is really an aperiodic frontend. Imagine a coil. Now imagine it picking up some magnetic flux at some arbitrary frequency, or better say any frequency. Now, extend your imagination to some Rx attached to it. It has some input impedance, and in case this impedance is lower than the critical damping - you have a real aperiodic frontend, the one that wouldn't even ring.

Now, what happens at resonance? From the coil's point of view in a resonance it sees the lowest possible impedance that is limited by coil's resistance, and that is the point where you have the best possible energy transfer - it is the voltage source after all. Because in resonance we have a sharp phase transition and we can't use it, I can attach a coil to a low impedance Rx instead and have frequency independent signal transfer, spoiled only by the coil's inductance. That kind of aperiodic.

In short - I can't use resonance, so why bother with resonance at all?

[Qiaozhi]

By aperiodic I mean lack of resonance near the frequency of interest in a coil and the immediate circuitry.

The use of the word "aperiodic" is not strictly correct in this instance. Aperiodic means a non-periodic (not occurring at regular intervals) signal. It is a more appropriate description of the receive signal on a PI detector, which is damped to prevent oscillation.

For your case, it would be better to use the term "off-resonance".

[Davor]

It is in case of aperiodic operation. Just for a moment imagine there is no capacitor to make the coil resonant.

Hi Davor:

What do you mean by aperiodic input circuitry?

Would you also compare this frontend to TGSL/IGSL (your balanced version) and explain differences and advantages?

Regards,

-SB

First off, my IGSL frontend is not much of a solution as I realised recently. Center tap coil is what makes it work fine, but otherwise the frontend's CMMR is poor. It works well without shielding, but mostly because of the center tap.

So I reverted to learning the basics of the transducers interfacing and found that the line to follow is the microphone preamp art.

By aperiodic I mean lack of resonance near the frequency of interest in a coil and the immediate circuitry. Resonance works as an impedance transformer, and most efficiently at the resonant frequency of a tank. That very situation is not used because of steep phase change. When used at frequencies lower than resonance you have a modest phase shift, which is ~the same as if you use low input impedance and no resonance at all, e.g. aperiodic. While resonant operation is superb for op amps that suck at noise, low impedance is tolerant for wild variations in Rx coils' inductance, and hence much better for an amateur coil builder.

This particular Lobo solution is asymmetric and optimised for low noise. With op amp at hand, it is overly optimised, but that's good news for the Lobo owners - they can have a better rig by simply replacing the input opamp.

Asymmetric inputs do not sort out the common mode contributions (electrostatic) from the differential ones (magnetic induction), and apart from the coil shielding there is no way of suppressing the famous "wet grass" effect. Differential inputs are much better at that, they suppress the common mode component.

With nowadays op amps boasting with low noise, it is much simpler and phase-wise predictable to forget about the resonant tank and simply go directly to a low impedance frontend. That would allow you to use coils in wild ranges of inductances, and at the same time at a wide range of frequencies. In case of a balanced input, even without the center tap you'd be able to use unshielded coils. That's my current ideal.

[simonbaker]

Different designed preamp.
Lobo is designed somewhere at end of 80-ties. I think the desiner is not Jack Gifford because the preamp circuit of Lobo differs significant from other primitive preamp circuits used by Tesoro. The differences are:
1. The preamp is formed as three stage amplifier. Other Tesoro preamps use only a single stage.
2. The resistors of first stage are designed for low noise. The resistors in other Tesoro preamp circuits have much larger resistances, ie they generate more noise.
3. RX coil is connected to form second order band pass filter. In other Tesoro circuits, RX coil forms second order low pass filter. That means they can not suppress enough the audio frequency (330 - 550Hz) and
third harmonic of mains frequency (150 or 180Hz).
However:
1. Despite its three stages, the preamp operates with low gain. I see something is written for gain in the circuit diagram attached below.
2. Resistors and RX coil have low resistances, but this is useless because opamp generates spectral noise density more than 1kohm resistor. That means the designer can decrease the weight and price of RX coil without increasing input noise..
3. The tuned circuit (band pass filter) is damped with 6kohm resistor. That means increased noise because a) the damping resistor also generates noise and b) damping forms wide band input (noises are proportional to bandwidth)
Let we make analysis, revision and redesign of this circuit.

In other Tesoro circuits, RX coil forms second order low pass filter

I don't see how TGSL RX circuit is low pass filter -- isn't it a legitimate LRC tank circuit, with center freq around 16.5 kHz or something like that? The lobo circuit here seems to show a very over-damped RX coil tank with that 6.19K resistor in there, with main bandpass due to op amps.

-SB

[simonbaker]

Try adding a common mode high voltage and of course high impedance source coupled capacitively, and you'll see what this resistor is for. From what I've seen, the whole circuit is designed for an opamp of much better voltage noise than the one that is found in a commercial Lobo. Maybe they had some turbo special variety with somewhat better guts?

Anyway, I like it. Apart from the balanced operation that it is lacking, the rest of the Lobo's solution would be my current view of an ideal frontend. It has low impedance (most probably) aperiodic input circuitry optimised for low noise. Nice.

Hi Davor:

What do you mean by aperiodic input circuitry?

Would you also compare this frontend to TGSL/IGSL (your balanced version) and explain differences and advantages?

Regards,

-SB

[Davor]

I've seen somewhere a schematic with much larger "stray" so that the coil forms a near resonance tank. That could be right.

...What will happen when we remove damping?

Try adding a common mode high voltage and of course high impedance source coupled capacitively (think of wet grass), and you'll see what this resistor is for. From what I've seen, the whole circuit is designed for an opamp of much better voltage noise than the one that is found in a commercial Lobo. Maybe they had some turbo special variety with somewhat better guts?

Anyway, I like it. Apart from the balanced operation that it is lacking, the rest of the Lobo's solution would be my current view of an ideal frontend. It has low impedance, and (most probably) aperiodic input circuitry optimised for low noise. Nice.

[Derx]

add strayC = 6n8 and replace C3 to 10p and repeat test?

[mikebg]

SPICE noise analysis shows that damping resistor R1 generates more noise than coil resistance. What will happen when we remove damping?

[Davor]

Please note that my simulation presents noise density, so feel free to multiply these results with square root of your bandwidth, and there you have it. This approach is bandwidth invariant.
Actually, the situation here can't go much better since a 1k ohm resistor produces ~4nV/sqrt(Hz) of noise, and this contraption is only marginally worse, just because that is the very voltage noise of the op amp.

The lowest you could go would be the noise produced by the coil resistance, matched by the op amp input voltage noise, and that would be something!

IMHO this frontend can become better only if configured as a differential one to fight common mode noise. Otherwise - superb!

BTW, op amps tend to introduce some THD when pressed hard on the output. In this preamp we have an op amp heavily loaded with the feedback network, while in reality the op amp input voltage noise is much larger than the equivalent feedback resistance can produce. You can observe this from 2 angles: you can put there an op amp with better noise and gain better results, or, you can increase the feedback resistances and prevent THD with strong signals ... or just leave it be.

[mikebg]

Davor, the noise situation in RX is much better. The width of noise band in your simulation is not correct estimated between 1kHz and 100kHz because
TGT signal makes modulation of carrier wave (AIR&GND signal) with maximum 16 Hz.

Conventional block diagram contains a band pass filter after synchronous demodulator. In this case, the demodulator can not see noise in a band wide more than 20Hz.

However the correct noise value is not important for design of RX coil. Important is relative noise generation by coil resistance (relative to other noise generating components - resistors and opamp). My idea is to redesign RFA to operate with increased coil resistance without significant increase of circuit noise. That means decreased weight and price of RX coil. I will simulate the idea when I have time.

[Davor]

It is simple, you just divide noise at a frequency of interest with system gain at that particular frequency. LTspice does not do that in one pass, so you'll have to do AC analysis first and see the gain at frequency of interest. To do that put "1" as AC amplitude and check gain as numeric value, not dB, fix it in "manual limits" ... click on results window, "Plot settings" -> "Manual limits" and in "Left axis" find decibels and turn them to "linear". AC amplitude 1 is OK even for devices with very small input signals, because it treats every signal as a small signal, and it does not bother checking for saturation etc.

[porkluvr]

Nice work, Davor. I went and tuned the RC values to obtain a peak more closely to 20kHz and used some additional circuitry to rotate phase back to where its needed, but I have to believe that Tesoro had reason for doing what they did. Meaning, all I did was to probably fix a non-problem, and without understanding repercussions.

Question for you: What do you have to do to normalize the noise analysis to a particular gain?