Re: Tektronix 7854 Digitization Issue

Albert Otten

On Tue, Sep 17, 2019 at 03:03 AM, Raymond Domp Frank wrote:

It acquires waveforms, but it's as though it's acquiring two different
waves, one at the correct frequency, and one approximately 1/5 the period.
I posted a video to YouTube showing an acquisition, you can clearly see
another superimposed portion of the wave across the entire sweep.
The "low frequency" that you see is actually the retrace (return of beam to
left of screen). Have a look where it "ends" at the right side of the screen:
Always where the real trace stops, so it's actually the starting point for the
retrace. It should normally be invisible but blanking isn't set correctly for
the digitisation.
It's likely that adjusting the "grid bias/blanking" for the digitisation part
will solve your problem so check and if necessary adjust Z-axis levels. The
Z-axis circuits are on the board mounted at the back, where the GPIB and
battery backup connections are. It's not a dangerous adjustment in the sense
that you may/will lose calibration.
Hi Raymond,

I agree with your interpretation, retrace samples appear in the acquired waveform. But I think your suggested remedy is not working. The Z-axis blanking is itself a result of internal signal. The only logical output of the Z-axis circuit is the GSF signal which inhibits digitizing/storage of extreme Y values. During retrace there must be some "inhibit" signal active which prevents digitizing/storage during retrace.
It's won't be easy I think to pinpoint the cause. I spent a few minutes reading the Theory of Operation and looking at some diagrams, but oh boy!


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