Super TGSL With H Bridge TX Diff RX Quad Sampling

[Silver Dollar]

Still some trouble with oscillations. I changes the 220 ohms to 1K and took some pics;






Not too bad but a little messy. So I added the 5pf and 1M's and it got worst;




I have a .1 and .01 right on the power pins of the chip and 20uf and 1uf and .1's on the power supply.
The chip is in a socket. It's a machined pin gold plated socket but maybe it's corodded inside the holes?

[Davor]

Well, it must be well decoupled. Trouble is that comparators within a chip tend to interfere one another if each chip is not properly decoupled at shortest possible proximity. It happens.

I tweaked the values and concluded that there is a little difference phase wise, but amplitudes are some 4 times higher. The phasing network is corrected at R11 and R12, C22 and C23. C10 and C19 can be reduced, but also may remain as they are. My mistake was in a fact that the signal from the phasing network never drops below some minimum which is close to a maximum, and I simply forgot to check the minimum value. (hrkljuš)
For extra stability it is possible to add Rhyst and Chyst at 5pF and 1Mohm. I'll try these in my IGSL and see if these may improve stability. It makes sense to use hysteresis to remove uncertainty, as there is a bias at the switchers that turns every uncertainty into noise.

[Silver Dollar]

That's what I was thinking when I saw the 50mv. I just wasn't sure how to tweak it without messing up the outputs.

I had a chat with an engineer at my new job today and he suggested some filtering on the feeds to keep the comparator
happy....

[Davor]

In Bosnia there is a saying: hrkljuš (see http://www.youtube.com/watch?v=u0UqLsCCIv8 )
When deciding values I overdid it in a direction that resulted in somewhat small-ish resulting comparator input signal. I have no idea where the input branch picks that oscillation, but if given a bit more juice it will surely be less of a problem.
I'll correct the values and post the changed schematic here. There will be very few changes, don't worry about it. The linearity of a transfer function will not suffer significantly (which is the fact I had to realise sooner). The whole idea was to supply a clever arrangement of potentiometers with orthogonal signals at proper amplitude ratios, so that resulting phase transfer function goes linearly with potentiometer position. A wincy bit less orthogonal, but at a significantly larger amplitude should do the trick.

[Silver Dollar]

I'm still trying to get this going. As it stands there is only the oscillator and a LM393 in the circuit.
I have decoupled the supply's and even used batteries with no change. A funny thing, the signal
gets noisier the further along the signal path it goes and there is no noise source around. Granted
we are attenuating it a bit. I have tried using fixed resistors instead of the pots and looking at it
today (after adding a bit more decoupling) it sort of works. At one end of the pot I get a square
wave but it becomes an envelope with 1.5 MHZ oscillation it it at the other end of the pot.

I made a drawing of what signals I see. It's very small and noisy on the comparitor side of the
68K's;

[Davor]

That's correct, however, comparators in quad packs often show glitches as a consequence of mutual coupling of some kind. These are not problematic if every I/O is on low to moderate impedances.
I believe Silver Dollar decoupled these, but I inquired about what's going on at the negative rail, as that one is a bit more critical. One of these days I'll revise the circuit to include a bit of hysteresis. Most probably it will fix it for good.

[Gwil]

Oscillation is sometimes due to insufficient decoupling of the power supply rails. I used to work on aircraft systems in which every op amp package had its own 0.1uF ceramic capacitors placed as close as possible to it.

[Davor]

Can you see what is going on at the inputs of the comparator? Maybe a little hysteresis will fix it without introducing any other artefacts. I did not anticipate any instability there because of relatively large input signal and relatively low input impedance.

[Silver Dollar]

Yes the same. It seems like there is no hysteresis built in. It oscillates at 4 mhz...

[Davor]

It is odd. Is the waveform the same as before?

[Silver Dollar]

Well I wired up 4 laptop batteries for a +- 8V power supply and I still have oscillation
on the output of the comparitor. I have 2 chips and believe they are old stock genuine
parts and they both do the same thing. The only things powered on the board are the
transmitter and the lm393. I double checked the wiring, everything seems correct. I may
have to chat with the Engineers at my new job next week, see if they have some idea's...

[Oessi]

Such permanent time of filters used for example in Teknetics Mark 1

[Davor]

Sure, but it is very fast, so you have to wave with your coil very fast even when you are near the target. With high pass filter (low frequency cut-off) at lower frequency, and harder compression, you can approach a target slowly, and still have a decent indication. The only bad side is 1/f noise that becomes more pronounced this way.

[Oessi]

permanent time 16 is equal a hertz. here is better repressed signal from earth and the signal of purpose is better selected

[Davor]

This could do. I'd set time constants a bit different, I'd throw out C5, and a limiter would be much harder, but this could do.