IGSL

[axljackass]

guys whats the difference between igsl musketeer abd igsl tgsl?curios only.thanks guys

[Davor]

My musketeer IGSL oscillator is a bit different, but in a way does the same job, and I observed the FFT of these oscillations. Point is, as expected, that the purest oscillations are obtained near the border of spontaneous starting the oscillations. You actually do not need a scope to find this spot. First you establish oscillation, and then you lower the amplitude by TR3. When the oscillation stops just go back a little to the point it starts back again, and a little notch more - that's it.

I think for the next edition of IGSL some balanced oscillator will be a perfect choice. Beside the obvious advantages of suppressed 2nd harmonic (for which this tuning fuss is all about), and the lack of tuning, these can work in a very economic regime using very little current for they work.

[der_fisherman]

To set this up with a clean sine wave (the reason Ivconic put the trimmer there in the first place), you do really need a scope, but as you are getting sounds (and the correct voltages) it must be working reasonably well, so leave it alone till you have a scope available.
I don't think that using a meter on AC will help much, but you might like to set the trimmer for the highest possible voltage output for example......but don't expect much change! I have never personally tried it myself....
regards
Andy

[Davor]

Make sure your GEB is set right. Most probably it is not adjusted properly. Once you do, you may try playing with TR3 to see if it affects detection distance. Disc Sens also affects distance. A choice of opamps in gain stages as well. In case you used LM324's there, you'd lose some significant distance.

A huge advantage of IGSL over other rigs is that you may overlap indication for ferric and non ferric objects so that nowadays coins that are full of nickel sound in double tone. To make it work right, use a piece of Al foil as a target and set the Disc pots to activate both Fe and non-Fe channels. Once you succeed, mark these positions of the pots and play with the nickels ... you'll soon get the idea.

There is a mistake in IGSL that affects the adjustability of Disc sens, and unfortunately it happens on the two comparators - the pins are swapped. Your rig will work, but Fe indication will act a bit funny. When you are up to it just see my mods and do something about it.

[giuhalftrack]

No worries, inductances are close enough. Just stick with the original values.
Try finding some electrically quiet area with some half a meter distance from metals to do your initial adjustments. Make sure you tune GEB correctly, and from there you'll find the rest.

ok Davor is working... but...

I have a very poor detectiond depth (10cm - 4") on a 1 euro coin, and again strange behave.

low tone for non-ferrous sound good.

hight tone for ferrous sound "dirty" to explain this, you need to remember early age hip hop sampled drums ok?
but it goes a clear higt tone only when object is at 2 cm - 1", i suppose is something
like my silver umax: a distant target gives a "broke - crispy" sound i don't know if this sound is normal in IGSL.

I had several errors in my setup (all fix now )... ground pot for ferrous channel reversed , low power supply.


so.. my questions:

1 - is the "dirty" sound normal for distan objects ?
2 - i need to mess with TR3 trimmer ? (adjusting the "nice sinus wave", oscillator trimmer) ?

please consider that:
wires from pcb to coil connector shielded (lenght 10 cm) and some components like TR3 trimmer and tuning capacitors are on
breadboard..

[Davor]

You'll find working schematics at the beginning of the thread and not over page #20. Main difference is in adaptations for different coils, so you have Musketeer and TGSL coils. Most of the insides are identical, differences being only at the very front end.

[axljackass]

hi guys a little bit confuse here...which one is the working schematic?
a lot of rev had been did lately

thanks

[Davor]

No worries, inductances are close enough. Just stick with the original values.
Try finding some electrically quiet area with some half a meter distance from metals to do your initial adjustments. Make sure you tune GEB correctly, and from there you'll find the rest.

[giuhalftrack]

As for the trimmer, you are supposed to obtain oscillation of the Tx oscillator. Once you do, the amount of bias you are adjusting with this trimmer is supposed to be fine tuned so that the oscillation is as clean as possible, but for the time being just go with any kind of oscillation. You'll fine tune it later on.

unfortunately i'm not equipped with oscilloscope then i use a pc based software (visual analyzer) i can't see frequencies below 14 khz (due to pc sound card limitations), but i think the oscillation always take place.

I'm not familiar with your coil and to determine what capacitances are needed you'll need to know the coils' inductances. In case both inductances are as per the TGSL specification at 6 and 6.5mH just leave it be. In other instances you'll have to do some corrections. Can you determine the coils' inductances?

according to geotech1 database of coil my coil have this parameters:
Name Brown 8" ("donut" coil)
TX-L 5.59mH
TX-R (ohms) 21.2
RX-L 6.23mH
RX-R (ohms) 22.1

slightly different from igsl-tgsl coil parameter.

i can take a look inside a tesoro silver umax to read a value of tuning capacitor, (it make sense ?)

For the instability problem I'd be looking somewhere else. Most probably your lab has some noise source that your coil picks, say laptop PSU or something.
If you get tones out, you obviously have Tx oscillating. There are a few points where troubles may lurk.

First try to set R37 and R57 for less offending tone. Also when problems start try adjusting Disc sens P3 and P4 to just about stop instability. Next you'll have to adjust Ground Balance by waving some pieces of ferrite in front of your coil to find positions where waving ferrite is just cancelled. If you continue playing with Rx input capacitors you'll need to re-adjust ground balance.

Don't worry, it will work.

ok tonight i try again, now i'm going to search how to calculate capacitance needed for my coil specification (i'm out of electronics work from 20 years i don't remember much things)

thank you

[Davor]

As for the trimmer, you are supposed to obtain oscillation of the Tx oscillator. Once you do, the amount of bias you are adjusting with this trimmer is supposed to be fine tuned so that the oscillation is as clean as possible, but for the time being just go with any kind of oscillation. You'll fine tune it later on.

I'm not familiar with your coil and to determine what capacitances are needed you'll need to know the coils' inductances. In case both inductances are as per the TGSL specification at 6 and 6.5mH just leave it be. In other instances you'll have to do some corrections. Can you determine the coils' inductances?

For the instability problem I'd be looking somewhere else. Most probably your lab has some noise source that your coil picks, say laptop PSU or something.

If you get tones out, you obviously have Tx oscillating. There are a few points where troubles may lurk.

First try to set R37 and R57 for less offending tone. Also when problems start try adjusting Disc sens P3 and P4 to just about stop instability. Next you'll have to adjust Ground Balance by waving some pieces of ferrite in front of your coil to find positions where waving ferrite is just cancelled. If you continue playing with Rx input capacitors you'll need to re-adjust ground balance.

Don't worry, it will work.

[giuhalftrack]

Could you please post a part of schematic you are referring to? I have troubles finding that particular trimmer on my schemes.

schematics is the one, currently on silverdog.
stars marks the trimmer and the capacitors.

Thank you Davor

[Davor]

Could you please post a part of schematic you are referring to? I have troubles finding that particular trimmer on my schemes.

[giuhalftrack]

My Igsl tgsl behave in a strange manner...

cannot adjust trimmer TR3 (100k)

i get no sound at all or a continuous tone (mix of two tones) sometimes it works (with <10 cm
detection depth) but soon the long tone or silence return

First i try to adjust trimmer tr3 but i can't find rigth position, after i replace tr3 with multiturn
trimmer and again i can't find right position...

after i try to replace tuning capacitors (C4, C43) with 15nF (J tollerance) (like TGSL schematics) and then i return to
4n7F (K tollerance) and 10nF (K tollerance) (i try different ones).

I use the original 8" tesoro silver umax coil, (waithing to build my own coil)

anyone have similar problems ? thanks in advance !

[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...

[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