IGSL

[Davor]

Do you have a Nell coil, or you want to buy it first?

[Tompa]

I will make a igsl, now I tink about coil. Nel coil is very good bat I dont now is igsl work with nel coil and with wich model of nel. sorry about my english

[Davor]

Not quite. Both NJM2068 and NJM4580 have THD below 0.001%, but NJM4580 is not as quiet as NJM2068. Noise-wise NJM4580 is a better choice than LM833 as its 1/f corner is ~10Hz, which is better than LM833 at ~100Hz.
If it is not too expensive in your country - go for it.
Attached is a comparison of various low noise NJM opamps

[sini]

Hi,

I can not find NJM2068 in my country. I can buy only NJM4580. Is it good replacing ?

[Davor]

Of course they should be the same, it was a quick one and I missed that 220k one More so, these should be as much same as possible because their difference lowers CMMR.

Double beep is a consequence of DD coil geometry. Going from the inner overlap you have + phase, outside the overlap but inside coil perimeter you have - that you can't hear as it is in counterphase and does not pass through, and outside the coil perimeter you have + again, and this one is weaker and vanes with distance.

There is also a sharp double beep with targets that have steel core, and this one toggles between two tones on IGSL. This one is due to the target geometry, as ferrous targets respond strongest when pointing to a coil, or at ends when laying flat, while non-ferrous is strongest when laying flat. If you set your discrimination pots correctly, this kind of toggling tone can be a clear indication of junk (tin cans, beer crown caps, steel cored coins etc.)

There is yet another option for a double indication that happens on difficult grounds where GB is far off the air signal (ferrite) phase. It may not be too easy to explain, but IMHO it is the main reason why classic IB detectors fail at beaches. They are screwed either because of insufficient GB, or because of correct GB Problem is that GB is used as a phase reference for all metals, and at higher GB settings most of that reference comes at a wrong phase. With correct GB you have broken all metal reference and vice versa. Solution of this problem would be introducing an independent "all metal" channel, and a wide span GB. IGSL can do that - I already use a single GB channel, so the spare channel can become the elusive "all metal" reference.

[sini]

Hi, are you sure, R9 is 220 kohm and R6 100 kohm ? I thought, that R9 and R6 should be the same.
I have another problem. When I prospect object, which is very close, I hear double beep. What is wrong ?

[Davor]

The trip is delayed a little, so here it goes, a quick one:

[Davor]

Sure I can, only now I'm going out. However, U1, and R6 - R9 are the same on all schematics, Musketeer and TGSL. Even my mods follow original annotations.

It is preferable to use 1% or better resistors, as this influences CMMR. As I use FKK coils (naked, no shield) with centre tap, a good CMMR is a bonus, and in fact my coil does not pick any "wet grass" chatters.

[sini]

Hi Davor,

please, can you mark changes on schematic ? What schematic do you use ?

Robert

[Davor]

Just a short one, I replaced the front-end op amp to NJM2068, and replaced R7/R8 with 1k, and R6/R9 with 100k, I also placed 10pF in parallel with R9. Because of a bit more coil loading the GEB solution is at a slightly higher position of TR2, but works perfectly well. I got a little more depth, maybe about some 3 to 5 cm, but chatters are less pronounced than before. I suspect that NJM2068 is a better performer than LM833 (and also NE5532) in terms of noise and more specifically 1/f noise, and also THD. Due to a strong air signal, every offset fluctuation (1/f noise) gets modulated and later detected as chatters, so both 1/f noise and THD count in a detector performance.
I really love NJM2068

[Silver Dollar]

Look here #1404 for the complete mods vs original;

http://www.geotech1.com/forums/showthread.php?17861-IGSL/page57

[der_fisherman]

Interesting!!

Many thanks for the reply, though I would be a liar if I said I undertood it all.....don't laugh!!
Many thanks and I will look for the pin swap details here and for other bits and pieces I will wait till you have more free time.
Regards
Andy

[Davor]

Hi Andy,

My playing with IGSL and many other things is in a slow pace at this time, but I maintain my own list of things that I done, and things I wish to try.

Of all the changes I'd put "pin swap" as the most important of all. It makes the Fe channel work as it should.

I also noticed some stability issues that I'm not certain where they originate, but I have some suspicions. I guess the GB potentiometer and its neighbourhood are the culprit as the impedance there is high, and there is a minor issue of crosstalk between the comparators within the same chip. My guess is that all instability will go if, say, 10x lower impedance is introduced there.

I'm planning to turn my IGSL into a beach combing monster by (ab)using the fact that there is a channel to spare. There are 2 GB channels, and I already use only one. The other one will be used as all metal reference, and it will enable GB to adjust to any value between ferrites and wet salts. Without all metal reference this kind of detectors go bezerk on difficult grounds.

I also plan to try a simple novel Rx front end topology that would allow for operation at a lower input impedance and around resonance. Simulations say it should work fine, but real life must prove it first.

The last thing I did was AB->C class modification to Tx which results in less power consumption at Tx with a marginal improvement to signal purity.

Unfortunately I have some other more important thing to do, and my visits to my shack/lab only become less frequent.

[der_fisherman]

Hi Davor,
I've been away from the forum for sometime, so my apologies if this question has already been answered.
When I was here before, you were making good changes and additions to the circuitry, it appears from this recent post that that is still the case. So my question is simply, have you or someone else compiled a list of all the changes and placed it somewhere online? Which ones are needed? etc....
I have an IGSL that was made with the original circuit and values, that simply is so unstable that it is unuseable....I would like to try and get it working as I hope it should.....
Many thanks in advance for all of your time, trouble and expertise. I am sure I am not alone here either.
Andy

[Davor]

You can check the pics here: http://www.geotech1.com/forums/showt...824#post131824
Such coils need a proper shell, and shielding, but they all start as simple coils. When you get the desired number of turns check the inductance. It should be a little less than the goal value, as by wrapping them more tightly the inductance will rise. It is not too critical though.
Tx coil resistance is critical because of its influence on Q factor, but the Rx coil's resistance is of somewhat less importance.
Nulling the coil is a bit more interesting.