Re: 7904A readout bounce


bobh@joba.com
 

David,

I haven't tried to look at this effect (moving the beam position) and
its relation to the readouts.� Does it depend on which scope you are
using and the range selected.� Seems like higher ranges (if available)
could have more affect on the operating point.� I would have thought the
diff amps would be tolerant of this kind of effect.

Your analyses are usually a couple grade levels above me but I would
like to understand in order to apply to troubleshooting. And, it seemed
like potential for a nice on-topic discussion.

Bob.


On 12/14/2017 6:18 PM, David @DWH [TekScopes] wrote:

Thermal offsets are literally that, offset voltage drift in the
differential pairs through the horizontal and vertical signal paths.
They are caused by the changes in operating points with signal level
which result in a change in power dissipation and temperature in the
individual transistors.

At higher frequencies, the changes in power dissipation average out
but alternate mode occurs at a frequency determined by the sweep
lengths so it is slower than any individual sweep and slower yet than
the vertical signal.

If you want to see the effect in a controlled way, watch how the
readout shifts when X-Y mode is used and the beam is moved to
different points on the display. This is (usually) not the result of
electrical leakage between the beam position and readout; it is caused
by changing the operating point of the transistors ultimately changing
their temperature which changes their current gain and base-emitter
voltage.

On Thu, 14 Dec 2017 21:56:26 +0000, you wrote:

On further investigation this is vertical bounce due to ALT mode
being invoked either on the plug-in or the mainframe when lowish scan
rates are used. I didn't see anything about this matter in the manual;
I wonder if there is a supply decoupling problem somewhere?

Regards

Nigel


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