Re: 11801 question

Albert Otten

Hi Reg,

I did similar with a CSA803A. This scope has the slower Calibrator, about 120 ps rise time.
First I tested an SD-22 with the cal signal. 20 ps/div and 10 mV/div, which gives a reasonable steep mid part of the trace. About the 50% point centered at the grids center. During more than half an hour from startup I could not detect any shift in the trace.
Next I followed your splitter setup, with an SD-20 and an SD-26. In Enhanced Accuracy I set the SD-20 as delay reference and had the SD-26 delay calibrated to this. Displayed 20 ps/div and 5 mV/div. Made the mid region of the traces coincide by a small fine adjustment of Vertical Position of one of the traces. Again I saw no systematic shift of one trace w.r.t. the other. However there 2 occasions (which I noticed) where there was a short duration shift (more or less jumping) of one of the traces over 2-3 ps. That could jst as well be a shift vertically of course. I attributed this to a not firm enough cable connection somewhere since the trace returned nicely to the proper position. Not sure though.
Altogether I can't share your "pessimism" about the performance.


On Thu, Mar 28, 2019 at 12:52 AM, Reginald Beardsley wrote:

I've got the unit running with a pair of SD-22s fed by a 2 port splitter from
the calibrator into ports 5 & 7. I was finally able to figure out how to
manually adjust the skew so the two traces overlaid. The cables are about 1/2
mm different lengths

I'm seeing the displacement between the two traces vary from 0 to about 10 ps
over the course of a few minutes. Is this normal or is there a fault? I've
read the user manual once, but it's not great. Certainly no match for a 465
or 485 manual.

Does anyone know why this is happening? I've got the internal clock output
connected to an HP 8356A and I can see the period varying. I don't have the
8356A connected to a GPSDO at the moment, but it has the OXCO option and is
very stable.

Is there any reason to think that replacing the 200 MHz master oscillator with
one of Leo Bodnar's GPSDOs would eliminate the interchannel drift?

I discovered this shortly after making an offer for four SD-26 heads. My
intended use is to measure actual FPGA word skew of a DSP stream, so this
would be a serious problem.


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