IGSL

[Davor]

I finally balanced my coil. A small step for mankind, but a milestone for my IGSL. I tuned it using a non-calibrated Visual Analiser hooked directly to a microphone input, so I have no idea how well it stands. OK, initially I placed an attenuator, but I had to hook it directly soon after. Tx coil was hooked to its oscillator at IGSL board.

At one point I got complete cancellation of Tx, but the moment I glued it - it was gone. I guess capacitances take care you have some residual signal to play with.

Both coils are unshielded, and Rx coil is bifilar wound with center tap hooked to ground. I did not notice any static related coupling, so I guess it is going to be OK this way.

Anyway, it took me several hours until I finally figured that the complete cancellation is not going to happen again, and I simply fixed things as they were after confirming a minimum with a wire curl. I was able to see amplitude changing when I waved some tools at hand, so I guess it will do. I left a small portion pliable so that I can optimise - or not - when everything else works.

I'll make another coil in a few days - just for the fun of it. If this bifilar thing works as I wish, I'll do another coil the same way. It is quite simple to form a coil this way, and gradually fix it as you go.

Things are looking up

[Davor]

This board looks good, however, I'd suggest you to hold your horses with mass production until FE channel correction is verified.

[selim]

igsl-mc maurice-pcb

[Davor]

I know, thanks. I already collected parts for Surf, but somehow I doubt it will beep this summer.

[ivconic]

OK, got it. 6 miles from one sweetspot to another.
I found a lot of medieval ceramics while diving nearby. I guess Greek beaches that you roam are not very different from our coast - what can I expect regarding ground balance etc. - do I just fix it or would placing a real potentiometer be of some use?

Placing multi turn potentiometer is better.
If you plan to dive and search than better make some small PI.
Barracuda and Surfmaster are very good for such job.
Anyone of those you choose; you will not regret.

[Davor]

OK, got it. 6 miles from one sweetspot to another.
I found a lot of medieval ceramics while diving nearby. I guess Greek beaches that you roam are not very different from our coast - what can I expect regarding ground balance etc. - do I just fix it or would placing a real potentiometer be of some use?

[ivconic]

For example; i live 12 miles away from Praesidium Pompei (Moesia Superior province)!

[ivconic]

...
Regarding the Roman sites, I heard of some random finds at places where Roman habitats used to be, but were abandoned since. Trouble is I didn't find any map indicating even approximate locations of these.
And how about the Roman roads? Are they any good to seek for?

Of course they are!
At each 6 roman miles there was "Mutatio" (station for short rest, exchange horse if need, obtain food, etc..etc..)
At each 12 roman miles there was "Praesidium" (smaller or larger settlement, or city).
Mutatio's and Praesidium's were placed usually on main Roman roads.
So there is indeed much sense to look for such remains downwards, along the roads.
Once you find one spot; it would be very easy to calculate and predict position of all others.
Of course; all this correlate only for main Roman roads.
I believe there is at least one of the main roads there in your country.
Look for connection with Sremska Mitrovica or Sisak, because those two were large cities at that period.
Good hunting!

[simonbaker]

This will require a short "ab ovo" explanation, but here you go. In these VLF Rx-es coil signal is sampled by two phase shifted LO-s and a resulting signal is amplified by respective gain block. These resulting signals are either in phase or in counterphase, and represent target response envelope. Phase as defined by LO-s will decide whether this signal is positive or negative at some moment, and that results in target discrimination. In case of IGSL and TGSL both gain blocks must supply positive signal to the comparators gates (AND function), and they turn high impedance and a GEB signal is passed to a tone producing circuitry.

So, to have a positive confirmation of a target in a specified span of angles (discrimination) both disc and GEB channels must be positive. That is the one quadrant detection: positive I and positive Q.

In all other instances there should be no firing of tone.

But... there are also noise, interferences, and phase troubles.

For noise you have a threshold control, in IGSL this is called "Disc sens". So you set it to the lowest possible setting that will not annoy you.

Interferences are a bit different because their result may rotate in phase at speed that passes through gain block filters, and their power may be quite high, e.g. two metal detectors operating nearby. I'm afraid that only a coil arrangement that cancels far field can help here for real. Or you crank up the "Disc sens".

And now - the feature content: phase troubles. Both TGSL and IGSL, and I guess many Tesoro detectors have a very specific gain block configuration with one diode in each stage. There is nothing wrong with it when you deal with low signals, but there is a phase shift with stronger signals. This shift comes from the fact that net. impedance in gain stage feedback lowers when diode conducts, and more so with stronger signals. If both disc and GEB channels are supplied with equally strong signals there would be no problem because both shifts would be ~ the same and nothing spectacular would happen. Trouble is that it is seldom the case, and being a motion compensated detector as it is, even for situations with GEB and disc being in counterphase at samplers - there are spikes at phase overlap later on.

TGSL's gain block diodes create a phase shift with rising signal, but tackle its problems by making a positive response shrink to a shorter pulse, and on top of it apply the severe "anti chatter filter" to make it even shorter. Doing so they hurt low signal detection, and also detection of signals that produce big differences in amplitude in GEB against disc. That is the main reason it got so much of my attention.

How to fix it? I think the easy way to fix the unruly gain stages phases is to remove all compression function from the first stage, and remove all LPF function from the other. In other words, a diode is removed in the first stage and placed antiparallel to the diode in second stage, and remove a capacitor in feedback of the second stage - so that you have a place for diode. This removes phase and pulse shortening problems, and some spikes appear only when first stage's output hits the rails.

I won't put the whole LTspice shebang here again because the mother of this solution is already placed in post #1289 - here is only a printscreen picture of the setup:

Ok, thanks for explanation. Yes, diodes can change pulse shapes differently between the DISC and GB channels due to different detected signal amplitudes for larger signals. That could be another thing to tweak, although strong signals are usually the least of our problems.

I sometimes wonder if a "phase linear" (pure time delay) filter/gain section over the band of interest would make the best pulse shape, but probably not practical for a number of reasons.

-SB

[Davor]

I checked your suggestions last night. Have one TGSL for such matters. Before mods it could detect coin at 32cm and after i applied mods it drops to barely 20cm.
I removed diodes from first LM358 (347 at IGSL) and removed 22nF from LM308's (also 347 at IGSL) and put another reversed diode instead there.
Audio response turned to be more sharp but detection drops significantly.
.....
Ancient Roman sites. I guess there are plenty of those, especially in North of your country.

Something else must have gone wrong. A difference you describe should be observed only for some effect that is over an order of magnitude, and these changes do not make such impact. Big difference is only for strong signals, and more so for different amplitudes in GEB versus Disc.

At small signal the only difference is one 22nF capacitor less, and a very small difference in LPF characteristic. Snappy audio indicates "Disc sens" potentiometer to be cranked up a bit, because otherwise for small signal there should be no audible difference at all. Every other difference should happen only for strong signals. In fact, removing only 22nF capacitors in the second gain stages should increase overall gain a bit, even in an original circuit, thus improving depth by little.

Regarding the Roman sites, I heard of some random finds at places where Roman habitats used to be, but were abandoned since. Trouble is I didn't find any map indicating even approximate locations of these.
And how about the Roman roads? Are they any good to seek for?

[satdaveuk]

Nice advise thanks Ivonics shall do as you advised

Regards

Dave

[ivconic]

Hi Ivonics
Can I ask your opinion between the IGSL and the TGSL.
Can you tell me the advantages between one and the other meaning is it worth the build?
As you know built two TGSL and they are a nice machine, I got the boards and most of the bits for both IGSL Minelab and Tesro TX/RX since january just dont know if its advantage to build one or the other or both or maybe another project .
As much detail as possible would be of great help.
Also did you ever complete using LCD and if so what was the results.
Lots of questions I know but would like your opinions for me to move forward in a direction.

Regards
Dave

IGSL is 2 x TGSL. Simple as that.
One "TGSL" there is in charge of processing and indicating detection of nonferrous metals and another "TGSL" there is in charge of processing and indicating detection of ferrous metals.
Both tasks can be a bit "overlapped" with adjusting Fe disc range "higher" to accept some of the "bad" alloys in lower Disc scale.
So shortly; nothing new there at IGSL, except that you have two TGSL's: one for "coloured" metals and another for "irons".
There is no "All Metal" mode as on TGSL. Only two "Disc" modes which are working with opposite logic.
Since you are having already operational TGSL with coil; good advice is you to make IGSL with which you can use same coil you are using with your TGSL.
Later if you like the "philosophy" at IGSL, you can obtain Musketeer coil and make another version too.

[satdaveuk]

Hi Ivonics
Can I ask your opinion between the IGSL and the TGSL.
Can you tell me the advantages between one and the other meaning is it worth the build?
As you know built two TGSL and they are a nice machine, I got the boards and most of the bits for both IGSL Minelab and Tesro TX/RX since january just dont know if its advantage to build one or the other or both or maybe another project .
As much detail as possible would be of great help.
Also did you ever complete using LCD and if so what was the results.
Lots of questions I know but would like your opinions for me to move forward in a direction.

Regards
Dave

[ivconic]

Yes. The ultimate test.

I concluded that matching GEB and Disc phases stand for squeezing maximum juice from the lemon, and it just can't hurt depth. Total gain is the same, if not even better this way. We'll see.

Anyway, I hear only laments about black sands, but nothing about red clays and karst, Mediteranean white (non volcanic) beaches and stuff. What can I expect? OK, not golden nuggets, but what else?

I checked your suggestions last night. Have one TGSL for such matters. Before mods it could detect coin at 32cm and after i applied mods it drops to barely 20cm.
I removed diodes from first LM358 (347 at IGSL) and removed 22nF from LM308's (also 347 at IGSL) and put another reversed diode instead there.
Audio response turned to be more sharp but detection drops significantly.
.....
Ancient Roman sites. I guess there are plenty of those, especially in North of your country.

[Davor]

Yes. The ultimate test.

I concluded that matching GEB and Disc phases stand for squeezing maximum juice from the lemon, and it just can't hurt depth. Total gain is the same, if not even better this way. We'll see.

Anyway, I hear only laments about black sands, but nothing about red clays and karst, Mediteranean white (non volcanic) beaches and stuff. What can I expect? OK, not golden nuggets, but what else?