TGSL Experiments

[simonbaker]

I wanted to point on that subject earlier...
You noticed, in one of previous posts, an "notch" that occurs at signal when scoped. So i noticed too on few handmades. Later discovered C9 as very critical there. Must be hi quality p.p. capacitor or some very,very good ceramic. Usually i test dozen of those to find most suitable one. This turned to be very benefitial before nulling new coil on that machine (of course there is always some previously finished coil to test machine and check capacitors for C9).I do beleive that was discussed in the past. Must look at first 50-70 pages of TGS thread. Although setup at those versions is a bit different, yet i do beleive C9 is main cause of problem there.

Thanks for reminder about that C9 - now I'll pay attention to it more and check.

Regards,

-SB

[nakky]

Sorry Ivconic,forget it was your secret, loss of memory.Grt Nakky.

[ivconic]

Do I have to turn the schematic on my PCB board 180 deg,for a better discrimination ?????Grt Nakky.

Noooooo! Nakky, friend!? You just disclosed my greatest secret here!!!???

[ivconic]

I added resistor R59 (3.3k) to 180 deg disc circuit as precaution so TX coil is never connected directly to LF353 output through C9 - mainly to not drain current from TX oscillator.

May be totally unnecessary... other ways to do it also (put between LF353 output and disc pot R18?).

Note: All resistors in 180 disc circuit are subject to mods for improvement.

-SB

I wanted to point on that subject earlier...
You noticed, in one of previous posts, an "notch" that occurs at signal when scoped. So i noticed too on few handmades. Later discovered C9 as very critical there. Must be hi quality p.p. capacitor or some very,very good ceramic. Usually i test dozen of those to find most suitable one. This turned to be very benefitial before nulling new coil on that machine (of course there is always some previously finished coil to test machine and check capacitors for C9).I do beleive that was discussed in the past. Must look at first 50-70 pages of TGS thread. Although setup at those versions is a bit different, yet i do beleive C9 is main cause of problem there.

[simonbaker]

TGS 180 deg disc

I added resistor R59 (3.3k) to 180 deg disc circuit as precaution so TX coil is never connected directly to LF353 output through C9 - mainly to not drain current from TX oscillator.

May be totally unnecessary... other ways to do it also (put between LF353 output and disc pot R18?).

Note: All resistors in 180 disc circuit are subject to mods for improvement.

-SB

[simonbaker]

Do I have to turn the schematic on my PCB board 180 deg,for a better discrimination ?????Grt Nakky.

Yes, unless you are standing at the South Pole, where it doesn't matter.

[simonbaker]

Hi kt315:

2. What was your reason for changing resistors on negative supply?

I have played with resistors on negative supply also. Different ones will work, but I found you may heat transistors and waste power if not choose correctly so they will saturate very fast...
Regards,
-SB

I need to correct myself. I think the wasted power is not due to how fast the transistors switch on/off, but due to voltage drop between V+ and filter capacitor voltage. The energy has to dissipate somewhere.

But if we added a choke coil somewhere (between transistors and filter caps maybe), it should be more efficient and waste less power in the negative supply. I will look into it. Every little bit helps battery life.

-SB

[nakky]

Do I have to turn the schematic on my PCB board 180 deg,for a better discrimination ?????Grt Nakky.

[simonbaker]

"...However, I have not found easy way to mod TGSL pcb. Disc switch was a very nice jumper!..."

Am i missunderstanding this or it was your error?

It supposed to be : "... I have found easy way...". Right?

180 degrees Disc on TGSL is interesting idea. I will try that on my TGSL.

No, I had problem. It seemed easy except one thing -- I can't find easy way to run trace from pin 1 of U102a (LM393) to Gate of TR4 (J107) without major rearrangement, because switch J3 no longer there.

I can just run insulated jumper across bottom of board, but not pretty!

If you can find good layout that would be great. I am trying but getting tangled.

Regards,

-SB

[ivconic]

"...However, I have not found easy way to mod TGSL pcb. Disc switch was a very nice jumper!..."

Am i missunderstanding this or it was your error?

It supposed to be : "... I have found easy way...". Right?

180 degrees Disc on TGSL is interesting idea. I will try that on my TGSL.

[simonbaker]

180 deg Disc

Attached image is idea for 180 deg discrimination control for Tesoro Golden Sabre. I have not built yet, will be experimenting. Advantage is less parts and wires.

Also Tesoro new MDs use 180 deg disc instead of disc switch J3.

However, I have not found easy way to mod TGSL pcb. Disc switch was a very nice jumper!

-SB

Spice sim suggests the reduced gain may also solve my problem with LF353 overdriven by TX and oscillations.

[simonbaker]

simonbaker, could you to simulate up-dated one?
I hope it will good.
YOURS

Hi kt315:

It is hard to simulate digital components and signals, so I made approximations. See attached images and LTSpice circuit. I guessed that your TX frequency is approx 13125 kHz at CD4060 pin 5, square wave, +7 to -4 v p-p.

1. It looks like you are driving the TX coil with a square wave? That is actually a design I'm interested in because I think you can get higher voltage and current in TX coil if you have a very high-Q (underdamped) coil and drive with separate oscillator. However, your circuit does not seem to have a capacitor in parallel with TX coil, so not resonant at TX freq I think.

The TX voltage is still very high, big voltage spikes because of square wave, and overdrives the LF353 even more than original circuit, causing oscillations, so it does not solve the problem. My experiments indicate that it is best to put a lower voltage sine wave into C9 to get smoothest discrimination control with disc pot R18 (R5 and R6 in LTSpice circuit).

I think your circuit could be good with some modifications.

2. What was your reason for changing resistors on negative supply?

I have played with resistors on negative supply also. Different ones will work, but I found you may heat transistors and waste power if not choose correctly so they will saturate very fast. Also resistors affect the amount of voltage spikes on the the "rails" (V+, gnd, V-), although not sure it matters.

3. I am working on a 180 degree discrimination circuit that also seems capable of reducing the LF353 oscillations because it attenuates the TX signal. I will post some more about it in this thread.

Regards,
-SB

[simonbaker]

Oscillator coil current

While working on TGSL Final circuit, I wondered how the resistance of the TX coil affects the current going through it, and thus the magnetic field strength, in the oscillator circuit.

I ran some LTSpice sims trying different resistances.

Here is graph I made in Excel showing the current in the coil vs. the resistance of the coil. It may be helpful for choosing wire guage. My test coil, about 30 gauge wire, is about 30 ohms.

Interesting to me is that lower resistance does not dramatically increase current, but it does give you some boost.

-SB

[simonbaker]

LF353 disc circuit

With the TGSL Final I built I'm having some problems so I am analyzing, troubleshooting.

I get small oscillations on my U101b (LF353 discrimination output pin 1).

I made an LTSpice sim of the oscillator and Discrimination preamp and also see the oscillations.

In TGS thread, I showed simulation waveforms. http://www.geotech1.com/forums/showp...postcount=3627

(This is not meant to be a criticism of any TGS design or Ivconic's great work, just analysis for better understanding and troubleshooting - I obviously botched that, sorry.)

I see similar, but less exaggerated signals from my actual circuit. See attached photo - top trace is TX oscillator at coil, bottom trace is LF353 output pin 1.


I thought "jaggies" were due to input range of LF353 being exceeded by TX oscillator. But I experimented with LTSpice and found that the LF353 preamp circuit reacts the same to small square wave. So I suspected "ringing" was cause.

I did AC analysis of LF353 circuit and see that it has a sharp resonance at 229 kHz. That is same as frequency of small oscillation "jaggies". See attached images.

So I believe that the LF353 circuit is very sensitive to any non-linearity in the input and can ring very easily if input is not perfect sine. My TX oscillator is not perfect sine and I think that explains what I see.

I am only testing my built circuit with temporary coil, so it may be causing some of the non-linearity, although the oscillator probably saturates the transistor and some non-linearity is expected.

I am wondering if there are some other ways to make the LF353 circuit provide the same range of phase shift but have less tendency to ring. An area for experimentation.

Regards,

-SB

[simonbaker]

EZ Coil Winder

Pain in the @$$ winding coils, so needed to find an easier, faster way.

Here's a simple project that does the trick.

http://www.geotech1.com/forums/showthread.php?t=15341

-SB