Carl, hows the PI Circuit coming?

[Russ_NY]

Hi Carl,
Just wondering how you were doing with the PI circuit. I have most of it working, though not as well as I had expected. I am having difficulties getting the flyback to decay in a reasonable time. It seems way too long, looks fairly good going into the amp but no amount of trimming or compensation ( or lack of) seems to get the pulse to decay to zero under about 50 usec. I don't have any stock coils to try so I have wound several different ones,, tried a variety of coax. I have tried to minimize any capacitance, still long decay times. I anxiously await your latest revisions so I can compare.
Thanks and BTW the new Site Design is great.
Russ

[Carl]

Re: Carl, hows the PI Circuit coming?

I have everything working, practically ready to submit the layout to the PCB house. The only thing I wanted to do is test a couple of alternate hook-ups, and wind a custom coil. Right now I'm using a Barracuda coil.

I didn't work on the project this week because I wanted to get the new web site released (been under development since Spring). So, I will wrap it up this weekend, and post everything.

With the Barracuda coil, I get a very fast decay, maybe 10us. On the output of the 5534, I get about 50-60us for 3-tau. I believe this is about right. If you're getting a long coil decay I suppose you've overdamped, try a larger value damping resistor.

- Carl

[Charles]

Re: Carl, hows the PI Circuit coming?

Try a larger value resistor if its overdamped???
Charles

[Russ]

Re: Carl, Damping is good

Carl,
I have been very careful to adjust the damping resistor value to the point where critical damping as I understand it is achieved. ( ringing is just supressed) I also am getting about 10 to 12 usec decay times on the coil measured across the back to back 4148 diodes. it's the output of the 5534 that is 50 to 60 usec wide. You mentioned 3 tau. I assume you mean 3 time constants or 95% decay. I'm a bit confused on this point. I had assumed that the output of the 5534 would be only slightly wider than the coil decay. Am I way off base here? and if so how does this wider pulse affect the sensitivity to smaller faster decaying targets. I thought it was necessary to sample at 15 to 20 usec from the end of the TX pulse. that would have the first sample pulse occuring right in the center of the amplified flyback pulse if it is 50 to 60 usec wide. I have the distinct feeling I'm missing some key point here. Your input would be greatly appreciated.
thanks
Russ

[Carl]

Re: Carl, Damping is good

Oops, sorry, you're right, the 5534 should keep up with the coil decay. I looked at my board, and I have a 15pF cap in parallel with the 1Meg feedback R, which produces a time constant of 15us. That explains my slower decay on the opamp. If I pull the cap, the opamp pretty much matches the coil.

So I don't know why you have a slow opamp decay, unless you have parasitic problems. Are you perchance using a solderless breadboard? They have loads of capacitance.

- Carl

[Carl]

Re: Carl, hows the PI Circuit coming?

Increasing the R in parallel with the coil will move the damping from overdamped to critically damped. Further increasing the R will get you underdamped, and you will see ringing. Then you know you've gone too far.

- Carl

[Russ_NY]

Re: Carl, Damping is good

Thanks for the Response Carl
I am using an etched board. I did a board layout that gives me a bit of extra room for tweaking but it isn't all that much bigger. I will probably do a double sided board next. I also got rid of the cap across the feedback resistor as well as the compensation cap. Maybe the chip itself is bad, I will try a different one. The ones I have are from TI. Reducing the gain to about 1000 helps some, but not much. The balance works fine. I am curious as to what sort of noise level you are getting on the signal out of the amp after the flyback decay. Mine is quite jittery,, and it is directly related to the 555 TX oscillator frequency and duty cycle. You mentioned you were going to wind a custom coil. I sure would be interested in getting the details on it if it works out for you. I am also beginning to think that there may be some interference from something else such as the power substation about 300 yards from my house. Unfortunately I have been too busy to work on the circuit at all this weekend, but hope to get a couple hours in tomorrow.
Thanks Again
Russ

[Carl]

Re: Carl, Damping is good

A gain of 1000 is about all you can get on the preamp. "The balance works fine," I guess you mean the offset balance?

Because I am on my bench, with a fluorescent lamp and lotso test equipment, I see some 60Hz on the preamp output. Probably a little from the voltage inverter as well, not sure. You should not be seeing jitter from the TX oscillator as it is synchronous. The power station is more likely, try synching the o-scope to 60Hz and see if the jitter holds still.

I wound 3 coils tonight, 150mm, 200mm, and 250mm, although I didn't have the gauge wire I really wanted (24awg) and had to use 28 I think.

- Carl