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| Idea for reasmpler to view high frequency waveforms on a soundcard. |
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| David Hess:
--- Quote from: magic on July 08, 2019, 07:28:33 am ---Precisely, it's opened when the trigger is armed and closed when the circuit has been triggered and the sample delay timer expired. Then it stays closed for half µs to transfer the sample to another capacitor and it opens again. Each sample is therefore taken completely individually and anew. To get good results, the state of the capacitor right before the gate is closed must reflect what the waveform is now, not a nanosecond ago. So a really low time constant is needed, much shorter than effective sample time. It's a tradeoff. The sampling head must track much faster than Tek's, but its drive circuitry can be slow and easy to build. --- End quote --- Oh, well, that is a problem. Still, the author got pretty good results considering that limitation. It would pay to implement a better sampler allowing S-1 or S-2 (or S-3) like operation but you are on your own. Do not ask me to do the work. A traveling wave gate might be easier. --- Quote from: David Hess on July 08, 2019, 04:01:48 am ---What you say about advancing a sequential sweep slowly enough to render τ irrelevant is another option, but one which requires very short drive pulses to isolate the sample from preceding waveform. --- End quote --- Tektronix did this to implement "smoothing" mode in their sampling sweeps which reduced noise but it naturally only worked with sequential sampling. |
| magic:
--- Quote from: David Hess on July 10, 2019, 04:20:53 am ---Oh, well, that is a problem. Still, the author got pretty good results considering that limitation. --- End quote --- Well, he got some results, after compensation, testing with an unknown and uncalibrated DIY edge generator, and there is still some overshoot and irregular ringing and we have no idea how much of it originates from the generator or the sampler. And he didn't show what happens when another step follows 1ns later, but I think I know. --- Quote from: David Hess on July 10, 2019, 04:20:53 am ---It would pay to implement a better sampler allowing S-1 or S-2 (or S-3) like operation but you are on your own. Do not ask me to do the work. --- End quote --- No problem, I'm not really building anything, just offering a bit of free criticism on the Internet :) The time constant issue and sending RF through TL072 die and decoupling caps are probably responsible for at least some of that junk which follows his rising edge and partly responsible for the undershoot in uncompensated version. In fact, he didn't even say anything about decoupling and layout around that opamp, which is probably quite important given that it constitutes the sampling capacitor's ground return. |
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