Re: Another scope 7854

Joe Laffey

On May 6, 2017, at 4:19 PM, David @DWH [TekScopes] <TekScopes@...> wrote:

On Sat, 6 May 2017 14:45:56 -0500, you wrote:

At the timebase plugging, measuring the rails on the trigger board I see

-15V rail. 0.32miliVAC RMS
+15V rail 0.73miliVAC RMS
+5V rail 2.07milVAC RMS

When the plugin in NOT triggering the ripple on the +5V goes to 0V

I will see about removing the power supply for the scope and testing it. This whole thing may go on hold for a week due to work.
The peak to peak high frequency noise matter more than the low
frequency RMS noise in this case.

Ah... well I did put a 1X probe on the 5V rail and Peak detect mode on the 2232 I saw edge peaks of around 75-90mV P-P. For grins and experimentation/learning tried clipping in various sized capacitors on the 5V rail and it did not alleviate the issue.

I do not think the common mode rejection of the trigger circuit path
in the 7B53A is all that good so it is possible that your 7A18 is
producing a lot of common mode noise which would not be immediately
apparent. Comparing the trigger signal waveforms at the output of the
7A18 to the 7A26 might reveal something.

I may compare these just to see what I see.
It may not be worth spending too much time on this. The 7B53A is not
a very good timebase to use with the 7854; the timebase itself
triggers too slowly to show the leading edge with the 400MHz 7854's
delay line which is only 65 nanoseconds versus 120 nanoseconds of the
100MHz 76xx mainframes. And like I said earlier, all of my 7B53As so
this to one extent or another; adjusting the trigger level and
coupling is usually enough to solve it.

Of course some of us have a lot of fun tracking down obscure problems
like this. :)
I am learning stuff in the process-- so that is good. In the real world I would just enable HF reject in the trigger filter and it is stable.

I will seek out a better timebase.

Thanks for all your help. I really appreciate it.

Joe Laffey

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