IGSL

[Davor]

I almost forgot, yesterday I bought a pair of rechargeable 9V cubes from Lidl. At 7.5 Eur for a pair - unbeatable! They are rated 200mAh and I filled them for the first time already. I hope to make the 9V mod this weekend or the next week.

[golfnut]

Talking of a better sych det to give better depth - like a modern mid price machine. Pls read attached.. S

[Davor]

There are better reasons for full wave sampling than twice the samples, like 2nd harmonic suppression, common mode suppression, and offset suppression. Please note that PWM induced fluctuations resulting from 2nd harmonic content are largely ignored in most of the designs, including my beloved IGSL.

[simonbaker]

There are better reasons for full wave sampling than twice the samples, like 2nd harmonic suppression, common mode suppression, and offset suppression. Please note that PWM induced fluctuations resulting from 2nd harmonic content are largely ignored in most of the designs, including my beloved IGSL.

If the PWM does not shift the phase of the signal, does it really matter?

Regards,

-SB

[Davor]

Oh yes, it does. There is always some residual unbalanced signal from the coil on top of which you'll find the targets' response. Any floating there will be passed forward as an amplitude change in case of half wave, but not with full wave. By floating I mean EF, 50Hz, internal voltage change ... you name it. Most of these are cancelled out by the full wave switching, and THAT is the main benefit.

[golfnut]

I guess there are the three mixer styles


in approx order of performance...



Full Quad demod - proper good machines. Tier one GMP etc £800 - £1500
Balanced type with alternate 1/2 cycle sampling - eg Cibola. £350 - £800
The single ended type like a single cmos switch used in IGSL/TGSL/IDX £200 - £350



S

[satdaveuk]

So some machines only see half the RX signal' In a ideal situation by seeing all the RX signal theres lots more to be gained but not 50% worth, nearer 10% if your lucky but by getting that your putting another 100% worth of guts into the machine together with the full quad demodulator which gives you these prices of £1500.
Its all horse for courses, lots put in little gained, but of course that extra 10% can be wether you get the hit or not so worth it to some.
Very interesting.

[simonbaker]

Oh yes, it does. There is always some residual unbalanced signal from the coil on top of which you'll find the targets' response. Any floating there will be passed forward as an amplitude change in case of half wave, but not with full wave. By floating I mean EF, 50Hz, internal voltage change ... you name it. Most of these are cancelled out by the full wave switching, and THAT is the main benefit.

I don't see how full wave detecting fixes that, because it just acts as a full-wave rectifier instead of a half-wave rectifier. Amplitude modulation of the TX signal will still come through whether it is full or half wave detected.

Even harmonics are always canceled and don't contribute to the SD output. Odd harmonics contribute to the signal, regardless of half-wave or full-wave detection.

So I don't get the idea yet, maybe a detailed example would help.

-SB

[Davor]

2nd harmonic is a source of PWM here. You'll see it in a picture below. Problem rises with offset, and there are many sources of offset, including various modulations. The way full wave switcher fixes things is by successive reversing the offset contribution, and thus suppressing it. In fact full wave switchers also suppress the carrier as well - they are balanced mixers.

All of that, of course, in case switchers are supplied with symmetrical timing. Full wave will suppress most of the artifacts even with not-so-perfect timing, but the half wave switchers will not.

Our rigs are usually made with single ended oscillators (which are fine sources of 2nd harmonic), and phase shifted AC component is compared with ground to obtain the switchers timing. Now, consider waving a coil against the rough surface and think of it as a variable loading - guess how it reflects on offset in PWM terms with changing levels of 2nd harmonic in amplitude and phase (due to the phase shifting).

We may argue that these effects are of second order, but there is a large unbalanced signal that is supposed to stop at motion compensation filter, and it's modulation will pass through. Microvolts of target signal on one side against milivolts of unbalanced Tx on other.

[simonbaker]

2nd harmonic is a source of PWM here. You'll see it in a picture below. Problem rises with offset, and there are many sources of offset, including various modulations. The way full wave switcher fixes things is by successive reversing the offset contribution, and thus suppressing it. In fact full wave switchers also suppress the carrier as well - they are balanced mixers.

All of that, of course, in case switchers are supplied with symmetrical timing. Full wave will suppress most of the artifacts even with not-so-perfect timing, but the half wave switchers will not.

Our rigs are usually made with single ended oscillators (which are fine sources of 2nd harmonic), and phase shifted AC component is compared with ground to obtain the switchers timing. Now, consider waving a coil against the rough surface and think of it as a variable loading - guess how it reflects on offset in PWM terms with changing levels of 2nd harmonic in amplitude and phase (due to the phase shifting).

We may argue that these effects are of second order, but there is a large unbalanced signal that is supposed to stop at motion compensation filter, and it's modulation will pass through. Microvolts of target signal on one side against milivolts of unbalanced Tx on other.

Thanks -- I understand it to be an "offset" problem. In other words, we want to detect amplitude changes in the RX signal, but not "offset" changes. "Half-wave" SD designs are more sensitive to "offset" modulation.

So the next question is: where do these offsets come from and how big a problem are they?

I believe "offets" arise when there is an "additive" component to the TX signal that shifts the oscillator signal up or down without changing its amplitude.

It seems we would be most worried about offsets with a frequency near the sweep frequency of our target signal (10 Hz). I would not think additive signals to the TX oscillator voltage at 10Hz would make much impact on the RX signal due to the second order RX tank (RX coil & RX capacitor) which peaks around 17 KHz and should greatly suppress components down near the 10Hz region.

So I'm still not sure it's a huge problem here theoretically, but what happens in reality is what matters. In any case, I'm all in favor of full-wave SD for designs where some extra circuit complexity is acceptable.

-SB

[Davor]

Just imagine how changing ground proximity would influence Tx coil Q and the 2nd harmonic content, and next just look at the picture above and see how 0V is not placed at a value that stands for equal off and on times.

There is a solution that uses a PLL to fix unequal timing, but I think it will require somewhat faster loop filter than it is usually set to account for the additional phase noise. I gave it a further thought and it makes sense.

Anyway, full wave is the way to go because it fixes many problems.

Imperfect timing is a different ballgame but improving it may be considered very desirable. I think I may have a trick up my sleeve, but I won't voice it until it is ripe for publishing here.

[simonbaker]

Just imagine how changing ground proximity would influence Tx coil Q and the 2nd harmonic content, and next just look at the picture above and see how 0V is not placed at a value that stands for equal off and on times.

There are two effects not to be confused. The soil can modulate the amplitude of the TX signal and it can also shift the average DC level due to the distortion of the waveform you described.

First, when the soil modulates the amplitude of the TX signal, we expect the SD to register an amplitude change, so any small correlated shift of the DC average value of the RX signal at the same time isn't big deal, it just gets combined into the overall change.

Second, let's assume the soil can cause amplitude change of the TX signal as you sweep for targets. The amplitude change can cause distortion which can alter the harmonics, which are multiples of the TX frequency, but I will assume as you say that it can also shift the DC level of the signal -- but typically at some frequency which is not a multiple of the TX frequency -- for example, the frequency at which the soil changes while sweeping for targets.

So if the soil is able to shift the DC average level by a change in the waveform shape, the equivalent additive signal is typically at a very low frequency and you should not see it in the RX signal; it would be attenuated going across the tuned transformer comprising the TX/RX coils and target.

Any second harmonic (I assume that means twice the frequency) of the TX frequency should act the same as the fundamental, only smaller, in a half-wave synchronous detector, should it not? In other words, if the TX signal is integrated to zero by the SD, then the second harmonic would integrate to zero. If the TX fundamental integrates to some voltage (due to phase), then the second harmonic would also integrate to fraction of that value -- it would behave like a small percentage addition to the fundamental.

So while I see the point of your picture, you haven't shown how that causes a problem when you follow it through from TX to target to RX to SD. I'm not saying it doesn't cause a problem, I'd just like to see more clearly how.

-SB

[Davor]

These effects are related to your sweeping speed in the same manner targets do, and while the soil response is phase shifted and reduced by properly adjusting GB, the unbalanced Tx signal modulation passes through. Point is that even due to some voltage fluctuation in the rig, this will be happily passed through to the audio, and the same thing happens with the ground proximity changing the coil Q. As a direct consequence you have varying 2nd harmonic fluctuation that directly causes offset shift PWM style, and it all passes through the motion filter as well. As a consequence you have a rig that behaves as if you can't find a proper GB anywhere. It is not severe, but I noticed it. It can be partly cured by finding the Tx oscillator bias Goldilocks point. Thanks Ivconic for a Tx trimmer - some VLFs don't have any.

In conclusion, this is not a problem for a deaf rig. Going any deeper and it catches up on you. First thing to do is going full wave, and second is running a Tx with suppressed 2nd harmonic, e.g. a symmetric one.

[simonbaker]

These effects are related to your sweeping speed in the same manner targets do, and while the soil response is phase shifted and reduced by properly adjusting GB, the unbalanced Tx signal modulation passes through. Point is that even due to some voltage fluctuation in the rig, this will be happily passed through to the audio, and the same thing happens with the ground proximity changing the coil Q. As a direct consequence you have varying 2nd harmonic fluctuation that directly causes offset shift PWM style, and it all passes through the motion filter as well. As a consequence you have a rig that behaves as if you can't find a proper GB anywhere. It is not severe, but I noticed it. It can be partly cured by finding the Tx oscillator bias Goldilocks point. Thanks Ivconic for a Tx trimmer - some VLFs don't have any.

In conclusion, this is not a problem for a deaf rig. Going any deeper and it catches up on you. First thing to do is going full wave, and second is running a Tx with suppressed 2nd harmonic, e.g. a symmetric one.

Ok, but it may not be as bad as we might think due to the other considerations I mentioned which could greatly reduce the effect of 2nd harmonic variations (distortion changes). To repeat --

1) It's not 2nd harmonics that are bad, it is modulation of the 2nd harmonic at about 10 Hz that might be a problem.

2) anything that modulates 2nd harmonics in the TX oscillator does so by modulating the amplitude of the oscillator, causing the distortion profile to change -- and anything that amplitude-modulates the oscillator signal is going to ding the audio, with or without 2nd harmonics and whether it's half-wave or not.

3) The RX circuit removes DC component from the signal and a lot of low frequencies -- so a waveform that has a large 2nd harmonic will quickly establish a new "zero" level where there is equal area on either side of the zero level -- so the SD should integrate about the same signal strength on either half-cycle (if my math is not crazy).

4) The biggest danger I see would be a low-frequency (10 Hz) "additive" differential signal coming from the RX coil. This would shift the 14.5 kHz signal up and down, and cause modulation of a half-wave SD output, and is a good case where a full-wave SD would greatly reduce the problem. Note that this would happen even without any 2nd harmonics at all.

So what might be sources of such a signal? An example might be magnetic hot-rocks, such as lodestone, which will induce a low-frequency signal in the RX coil as the coil is swept over them. I don't know what amplitudes are possible, but it would be similar to moving a weak magnet across the coil.

Another is the earth's magnetic field. As the coils are swept around or tilted, you might get different flux through the RX coil causing a low frequency signal that would add to the TX null signal, causing the half-wave SD problem Davor showed in his picture.

Again, I don't know how big those effects are, I'm guessing pretty small -- but still worth eliminating to get the quietest MD possible.

Another would be a 10Hz differential EMI signal picked up by the RX coil -- don't know how common that is.

Thoses cases would be an incentive for a full-wave SD. As for the 2nd order harmonic effect, maybe we can do a full simulation that would show how sensitive a half-wave vs full-wave SD might be to 2nd harmonic changes.

So, looking forward to some neat full-wave SD designs.

-SB

[Davor]

Yes, that mostly sums it up.
Point is that we are flabbergasted by making superb Rx-es, while Tx is the one to blame here. Quite the opposite happens with the PI lot - they happen to fondly pat Tx only.

Modulation I am speaking about happens by means of a Tx oscillator, not Rx frontend. Tx coil, being a part of an oscillator tank, is directly exposed to the ground and anything in it. Rx is directly strapped to the Tx zero crossing, which is NOT the exact half cycle as you seen it above, and it also fluctuates at, say, 10Hz-ish. These fluctuations in half wave switcher act exactly as an analogue multiplier, and that's the mechanism such modulation bypasses the Rx high pass filtering. You could build a brick wall HP filter on Rx frontend, yet this modulation would pass through.

As I can see it, having a Tx oscillator "seeing" the ground is actually a good thing, because whatever drags Tx phase around by the same mechanism drags the Rx tank in a same way so the effect mutually cancels. But that's on working frequency behaviour - the problem lurks from 2nd harmonic that becomes quite playful here. Just observe the FFT of a Tx oscillator and play with some stuff near it ... you'll see how greatly the 2nd harmonic content changes, you just can't miss it.

You may try fighting it by Tx coil being excited with an independent oscillator, such as crystal one, or a PIC, but you'll miss the mutual cancellation thing.

You may suppress 2nd harmonic by means of a symmetrical oscillator drive. If some effect is reduced enough, you'll not have much trouble from it. I'd say reconfiguring the existing oscillators into symmetrical configuration would do good. Symmetry fixes even harmonics.

There is another approach I'm about to try, and as I said it is not ripe yet. I'll not give much about it just yet - you know, the trolls...