Super TGSL With H Bridge TX Diff RX Quad Sampling

[hoadlies]

Hi Barry,

it is said that Australian gold fields were re-examined after the PI machines were introduced there. I had no chance so far to check how bad the ground there actually is, but everything indicates that if you use a VLF you'd need a machine with extended ground balancing range to find something.
This project is about building a machine with extended GB, while keeping the ferrite reference intact. This should keep iron and coloured metals apart even at very bad lands. I have no idea if that will suffice in OZ, but will surely work on the beaches.

As for centre tapped coils, there is a bit more to it than just the absent shielding. The unbalanced (and shielded) coil is internally ... unbalanced. It means that there will always be some coupling between the unbalanced Tx and unbalanced Rx, but not in a case Rx is balanced.
The other reason is only practical - it is much easier to build an unshielded coil than a shielded one, and the real good shield is a royal pain in the nether regions because it involves graphite - very messy.

HI DAVOR ,, THANKYOU for the reply & I must say your English is very very Good ,as I see you live in Croatia..?????

AS said earlia,I am 74 y/old & have just taken-up this hobby DIY-DETECTOR,, I have built many electronic projects for my other hobby Electric-Guitar ,AMPS [VALVE] Effects units,[MAINLY MULTI-TAP ECHO UNITS] solid-state but mainly from Schematics as my theory on electronics is not-really good & I do not do SMD work ..

I can obtain any componants here & also have a good collection on hand ..

I have only joined this site 8 -9 weeks ago & have learned a great deal from the Members here including ''silver-Dollar'',I successfully have built the IDX-PRO & made 3x coils for that & it works well [But not been-out on the ground detecting as yet only my Back-yard..

my computer skills are only average so cannot do spice ETC ..I have good soldering skills ,several good multimeters,an old single scope.good frequency-counter & a signal generator , ..
I have ''never'' Etched my own boards !!!But used perf-board etc etc..

OK I have not been to our gold fields to know what the mineralization is ,,But from reading articals from prospectors They mainly use The australian MINE-LAB [Made here in my State 200 ks away] but ''whites'' have made a detector VLF -called a goldmaster GMT which is reguarded highly here it has 48kz although much cheaper than M/L it is out of my budget !!!!

IT seems that somewhat higher frequency is needed for small objects as sa Gold etc..

I very much like your idea's on the c/tapped coils such as this project ,So if I was to try this circuit-out [if Silver-Dollar does not mind me copying his project???] WOULD I need to build the TGSL ,or could it be adapted to an IDX ??????..

As for Graphite shielding I finally succeded in a good mixture [after many many failures] I gring my own carbon-graphite from huge brushes used in wind turbines I get 1/2 worn ones for free ..I found that nitro-Cellulose clear furnture finnish works the best stcks to housings, & micropore-tape & is very conductive with one coat ,But as you say it is very messy to use ,but effective !!!& better than alluminium tape [less resistance] ..

GREAT PROJECT you guys are doing & I watch with great interest ,,,,..

CHEERS FROM ''Down-Under'',,,,,,,,,,,BARRY..

[Silver Dollar]

The wire I'm using comes as twisted pair so is easy to weave but I thought we didn't want the wires
together. On the TGSL thread it was pointed out that the original Tesoro units had rats nest of wires
in them. I hope it fits in a case when I'm done!

[Davor]

Amazing!
I may have a little detail that I noticed - it is always a good practice to weave the wires that go to a potentiometer. Especially if it is at a high impedance. You never know what the lose wires may pick inductively.
The next step will be to confirm if you may null the ferrite exclusion channel (all metal). Once it is done we'll see to the rest of the device, one function at a time.
The all metal channel is very important because - the other detectors do not have one.

[Silver Dollar]

Here's a picture of the project so far;



I ran out of 10K resistors or I might have had it working today. I have an order in to Mouser
but chose the inexpensive shipping so maybe next week it will be here!

[Silver Dollar]

Well I got rid of the 5pf caps for hysteresis and no more oscillation!

[Davor]

Hi Barry,

it is said that Australian gold fields were re-examined after the PI machines were introduced there. I had no chance so far to check how bad the ground there actually is, but everything indicates that if you use a VLF you'd need a machine with extended ground balancing range to find something.
This project is about building a machine with extended GB, while keeping the ferrite reference intact. This should keep iron and coloured metals apart even at very bad lands. I have no idea if that will suffice in OZ, but will surely work on the beaches.

As for centre tapped coils, there is a bit more to it than just the absent shielding. The unbalanced (and shielded) coil is internally ... unbalanced. It means that there will always be some coupling between the unbalanced Tx and unbalanced Rx, but not in a case Rx is balanced.
The other reason is only practical - it is much easier to build an unshielded coil than a shielded one, and the real good shield is a royal pain in the nether regions because it involves graphite - very messy.

[hoadlies]

Success! I took the board to work today to look at it under the scope. I found some flux
that my thorough cleaning missed but also realized one of the pots was wired wrong. I knew
on the board the center is not in the center for one side as I didn't have room to do that,
but I wired it wrong so had oscillation at both ends of the pot. Here are the pics today with
the 22N caps and pots wired correctly;

DISC Min
[ATTACH]28781[/ATTACH]
DISC Max
[ATTACH]28782[/ATTACH]
GEB Min
[ATTACH]28783[/ATTACH]
GEB Max
[ATTACH]28784[/ATTACH]

So finally we can move on and get this puppy finished!

BRAVO,,, Silver-dollar & DAVOR ,,I have been watching this project with great interest ,as now that I finished my 1st M/D DIY IDX-pro ,with I might add great help from the site members I am keen to build another ..

DAVOR,,, you often speak of a Centre-tapped coil's What advantage other than does not need shielding ????? ..
I am afraid your technical work is above my head ,[I am 74Y/old & although I can build nice circuits The theory is beyond me] ,,but I appreciate your expertese as you suggest mods to other projects on the site such as the ''S-M-W'' which seems a good unit also ..
I live in Australia & we have a few gold-fields & I cannot afford a high cost detector & would like to build a VFL [so as to discriminate mineralised ground ] to detect maybe gold ..[I guess higher frequency is needed]???????

GREAT work by the 2 of you fella's ,All the best from Australia,,,,BARRY..

[Davor]

OK, this is much better, but still there are some oscillations. This seem workable. I'll have in mind these when I design a next generation of this phase shift to:
- reduce impedance (potentiometers could be lower impedance but I did not check how "low" I can go)
- increase amplitude to the maximum that does not cause phase transfer problems.

It is a worthy goal to wipe the oscillation, because the CMOS switchers tend to inject charge at each transition, and those oscillations seem like potentially several charge injections at once. I'm even considering a true double balanced configuration to deal with charge injections the only feasible way. It would complicate the build, but may prove to be essential for going deeper. I'll see.

[Silver Dollar]

Success! I took the board to work today to look at it under the scope. I found some flux
that my thorough cleaning missed but also realized one of the pots was wired wrong. I knew
on the board the center is not in the center for one side as I didn't have room to do that,
but I wired it wrong so had oscillation at both ends of the pot. Here are the pics today with
the 22N caps and pots wired correctly;

DISC Min

DISC Max

GEB Min

GEB Max


So finally we can move on and get this puppy finished!

[Davor]

The last picture is bad. Are you sure feedback connections pins are 1-3 and 5-7?
The one before the last - did it stand still or was there some wander? I could try to push amplitude to the limit.

[Silver Dollar]

Well some success today. The bare boards I use come coated in flux to solder easily.
I tried to remove the flux with alcohol and scraping with an exacto but wasn't sure how
clean it was so I brought it to work where we have flux-off and some ultrasonic cleaners.
I cleaned the board and retested;



Maybe a little better? Then I added the hysterisis R+C;



One looks good the other is ok in the middle of the pot but
oscillates at each end. I haven't changed the 100n caps yet
I'll try that next...

[Davor]

One more thing ... the comparators inputs are seeing impedances in a range of ~20kohm and input signal is not too high. In case there is any conductivity in between the pins at all, say in vicinity of 1Mohm, and because the inverting inputs are next to the outputs, you'd have a negative feedback path and oscillations as a consequence.
Gary (Chemelec) often warns about this conduction problem, and his solution is using a water soluble flux cored solder (!) and he washes his boards in water. I tend to prefer resin flux (smells better too ) and rub the critical parts with alcohol. It is very bad to use a water soluble flux and leave it there because it is conductive.
Either way, I believe the hysteresis will fix this.

[Davor]

OK, one problem is solved then. Feedback must go between 1 and 3 and also 5 and 7. You are after a positive feedback which in this case turns into hysteresis and makes oscillations go away.
If there will be any wander (jitter) in comparators output left after this feedback thing is applied, I'll see to increase amplitude even further, but I doubt it will be necessary.

[Silver Dollar]

Well I'm using +-9V temporarily. The comparitor is wired to +9 and gnd. I did notice the inputs
to the comparitor were centered on ground (IE going above and below ground). I thought they
were at +4.5V before - I'll have to double check that.

I'm still using a LM393 and attached the 5pf and 1Meg to pins 1 and 2 and to pins 6 and 7.

I didn't change the 100n's yet, I'll try that tonight...

I didn't attempt to make the leads short but they are not super long either. Yes it's a strange
problem.

[Davor]

I see the comparator output is about 10V. What kind of stabilisation is there? The comparators are supposed to be supplied from 8V stabilised and a signal ground - not minus 5 volts. Negative supply is not required for them as switchers are 4053 that have Vss and Vee pins and support a shifted level switching. I have no idea what would happen if the comparator supplies 4053 with levels below 0. Only Vee of 4053 is supposed to go to minus 5V.

This oscillation business makes very little sense, but heck, it is going somewhere. Did you change 100n to 22n as well? Both 220ohm and 100n must be replaced to keep the same phase constellation.

A positive feedback was supposed to wipe oscillations, and not promote them, so please check if maybe the feedback was connected to the non-inverting input by mistake. Such connection would surely make it oscillate.

In case there is some external pickup of these oscillations, it is OK to place a 47pF capacitor between the inverting and a non-inverting input of a comparator. It would shift phase for additional 3° or so - no big deal.

I guess there is a problem with lead lengths in the phasing network. It is spring and it means I may reach my lab hut and try something. If you observe the top of the comparator output (logical 1), you'll notice a small "hairy" patch in a middle which is a consequence of the other channel going through transition(s) and that happens regardless of decoupling and becomes very bad if there is no decoupling. You can move the hairy patch left and right by shifting the potentiometer of the other channel. There is no way of making it disappear completely regardless of decoupling, but it is not a problem.