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From: "Harvey White" <madyn@...>
To: "tekscopes" <[email protected]>
Sent: Thursday, December 30, 2021 9:42:50 AM
Subject: Re: [TekScopes] 7704A - Noise on sine-wave traces

Do not do anything.

There is one beam.? It has to be "borrowed" to write the readouts.

There are two ways:

1) Borrow the beam when it's needed.? This chops tiny holes in the
trace.? it's going to be obvious at some sweep speeds, but will work on
slow sweeps.

2) refresh the readouts on the sweep retrace.? This eliminates the
"noise" but for slow sweep speeds, you will get a flickering readout.

There's generally a switch on the readout board to control this behavior.

Harvey


On 12/30/2021 10:14 AM, n4buq wrote:

When displaying a sine-wave where several cycles/division are present, when the
READOUT is switched on, there is noticeable "noise" (for lack of a better
definition) in the trace. It appears as tiny blank spots in the trace that
move rapidly along the trace. The noise is not in sync with the triggering and
appears at many points along the trace. Switching the readout off stops the
effect.

The power supply has fresh filter caps as well as the horizontal board and the
HV board so I don't think this is PS ripple and more likely digital noise from
the READOUT circuitry but that's just a guess.

I've looked for places in the READOUT board and/or interface board where there
might be bypass filtering components that might have been designed to eliminate
this but I don't see anything. I do note that switching on the READOUT
connects it to the +15V rail and possibly that's the source of the noise.

I've mostly observed this with sine-waves but it might be present with other
waveforms. As a side note, with mostly square-waves, with the GRATICULE
ILLUMINATION turned up, I can see a much smaller presentation of similar noise
in the trace. This is much less noticeable but could be related.

Any ideas what I can look for as a cause for this?

Thanks,
Barry - N4BUQ