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Re: Russian Tunnel Diodes

Chuck Harris
 

I don't want my comments to be a source of disparagement of Leo's
work. I haven't spent a great deal of time studying what he has
done, but in all of the scope pictures he has presented, there is
very significant leading edge overshoot and ripple. To my weathered
eyes the peaking and ripple appears to be 20-30% of the pulse
amplitude.

In calibrating high performance scopes, you need a fast risetime,
flat-topped pulse. You take that pulse, and adjust the scope under
test's pulse forming networks for a sharp leading edge with no
overshoot, and a flat top. Usually you can't achieve perfection,
but I always have been able to do much better than what I see in
the scope pictures, using the Tektronix tunnel diode pulser, or
the Tektronix CG5011 calibrator.

Generally what one sees when a pulser, with a nearly ideal shape
(so says a sampling scope with 10x the bandwidth of say a 7104...),
is used for calibration, is a risetime that appears slower than
the sampling scope showed, and a curved leading edge. Not a
30% peak, unless the peak was already there.

In other words, the peaks and ripples get filtered out by the
slower scope's networks, not accentuated. That is the whole point
of the transient calibration.

If Leo's pulser is intended for measurement of the risetime of
wide and varied scope models, as a method of displaying and calculating
the theoretical bandwidth, then it is perfect.

If it is intended as a calibration device, I have worries that it
won't do the job as well as the Tektronix TD pulser, or the CG5011
calibration generator. Probably all it needs is a little better
matching network between the comparator switch and the DUT.

-Chuck Harris


Reginald Beardsley via Groups.Io wrote:

Consider the time scale on Leo's plots and the response of the scope he's using which is what you are seeing in the calibration plot. I can't find my plot right now as my bench is undergoing at upgrade and everything is a huge mess. But IIRC the entire length of the step response Leo provides is less than the sample rate of anything most of us are likely to own.

I have put the BNC version on a new MSOX3104T. 436 pS rise time and 7% overshoot. I discussed with Keysight support which confirmed what I was seeing. A 750 MHz low pass filter reduced the overshoot to well less than 1%. But I could not inline the filter.

I then had an RTM3104 on demo. It arrived with 350 pS rise time and 3% overshoot. If I applied a 1 GHz LPF I had no visible overshoot. I was ecstatic. But for some legal reason the K18 option is not available in North America and the available FFT was completely useless. It was suggested that I install the 1.300 FW update. After that it had 10% overshoot. Restoring 1.100 did not restore the original response.

I've looked at 4 other scopes with Leo's pulser. I have no doubt that the waveform I saw is the true step response of the instrument. Now if you have one of the new Keysight 256 GSa/S 111 GHz DSOs, you probably need a better signal source. I suspect it would require one to give an accurate picture of the step response of Leo's pulser.

There are a large number of plots of the pulser output made on a wide variety of scopes in this thread.



My big problem at the moment is he's developed a version that produces 100pS wide pulses. I'm trying to come up with a justification for buying one.

I was amused that there were two posts in succession about Leo's pulser.
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