T
Tim Williams
- Jan 1, 1970
- 0
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
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