Getting rid of the fuzzies

So when you're working on an amp or waveform generator or whatever, what do you do to clean up those residual cruddibits that tag along on your precious waveform? Ringing, sloppy edges, the works. What's the trick, ground plane, filtering, bypassing, pulse forming networks, ...!?

Right now I'm working on my vertical deflection amp project. I've soldered down a pretty good circuit, with bandwidth out to 10MHz or so. (I had it to 20MHz on the breadboard, where it was more stable. Yeah...) Having discovered the CRT will deflect quite nicely with just 60V supply, I'm having no problem with transistor selections.

Two problems remain. One is bandwidth: I had it out to 20MHz on the breadboard, because it was stable with some emitter compensation on the output stage (which is a cascode, 2N4401 into 2SC3597 running at 75mA). At the moment, it isn't as stable (probably the lossy breadboard stray capacitance, not to mention the ferrite beads I was using on things), so I can't compensate the output stage any more without it turning into a reasonable power oscillator at 30 or 100 or so MHz. So I need to look for a solution to that. That will probably involve shoehorning in ferrite beads.

The other concern, that I'm actually posting about: I get a straight

25-30ns rise time and, at low frequencies, perfectly flat tops. Woohoo. But after that, there are about three small things wrong: Right off the edge, there's some bouncing, to the tune of about 10% (amplitude), at 20-30MHz or so, for about a microsecond. This appears to be a complex mix of ringers. It looks weird on the (cheap?) CRT, almost to the point of looping back on itself... Over a somewhat longer scale, there's a slower bounce (about a MHz), of suprisingly high Q: when tuning a sine wave through the range, the output amplitude drops by about 10% over a narrow range of frequencies (Q ~ 10-20?). After a step, it rings down in about 10us.

The easiest (so it would seem) to tackle should be the slower ring, which seems like it would be a power supply thing. I tacked electrolytics onto the high voltage supply (+40V) and that changed things a little, cutting it in half. Now, I already have ceramic discs *everywhere* on this board, so I'm not sure just how much more I can do in terms of bypassing. But I can't think of a damn thing else that would give me this. And I've tried poking ferrite rods around the circuit and nothing seems to change the higher frequency bounce, either.

Tim

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Reply to
Tim Williams
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First you need to be sure that you're not seeing common-mode noise in the scope. Connect the return side of the scope to where you would normally make your measurement and then connect the probe tip to the return point, too. Set the triggering appropriately, and If you still see the crap then it's common-mode noise.

The most accurate way to do any measurement is with differential probes, but not everyone has them and they have limited dc offset capability (although you can externally cap couple to get rid of this limitation).

Bob

Reply to
BobW

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The new layout is faster than the breadboard and fatally delayed higher frequency components are again visible to the amp input. Fit the beads, they really do help. Failing that, add shunt C somewhere to kill the HF bandwidth. A ground plane will make the problem even worse. Weirdly shaped ringing seems to turn up with mismatched transmission line. Local LC ringing looks pretty. Tx line stuff looks to be composed of parabolic curve segments.

Reply to
john jardine

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