As far as I can determine this appears to be caused by a big spike as IC403
(74LS02 on the earthy processor board) switches. You can see the spike at TP 3
on the earthy processor board. This spike goes below ground by about 1.4V
every 0.66uS.
This is interesting. I don't see this at all. I just probed the tp and
it's a very clean TTL signal, no undershoot at all. I'm using a 300 Mhz
scope (Tek 2465A). Not only is the tp clean, the +5 is also clean. I'm
using the ground point very near the tp.
Silly question - have you checked your probe compensation?
There is a very small rf burst every 0.66 uS I can see on the floating
board, but it's not on the digital power line, I checked that on one of
the ICs on the floating board. I'm seeing about 20 mv bursts at the AC
buffer test point you mentioned before, the burst is quite short, and
the burst frequency is much higher than any of the clocks in the meter,
so something is ringing, but it's not getting coupled out very far. I'll
probe around some more.
--
Bill Ezell
----
They said 'Windows or better'
so I used Linux.
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