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Re: ScopingSSTC



Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

At 03:04 PM 12/15/2005, you wrote:

Don't today's scopes have a differential mode select? I haven't used a scope in a long time - but when I needed to scope signals that had a 60 hz reference I just used a probe from each channel and set it to differential mode. Of course you need a two channel scope to do this.
The problem with that is that the two channels don't always match 
perfectly. You need a very good match indeed to view a small wanted 
signal riding on top of a big common mode signal. The matching 
usually gets worse at high frequencies, so if the common mode signal 
is HF, like the output of an inverter, it hardly works at all. 
Adjusting the compensation trimmer on one probe (while both probe 
tips are clipped to the same signal) can help minimise the 
breakthrough but it never seems to get rid of it completely.
I have an old battery-powered analog scope which comes in very handy 
indeed for floating measurements. It was given to me free... because 
it was broken :-< but I managed to fix it up. It runs for a couple 
of hours off internal NiCd batteries. When floating it, I like to 
sit it on top of a large 33kV line post insulator to give a little 
visual reminder that it may be live :-O
Steve Conner
http://www.scopeboy.com/
Steve pretty much sums it up.  One often needs to go to 100:1 probes 
and you are looking for say a 25V signal on a 600V signal.  The 
probes, attenuation, leads, noise, phase, parasitics, 
digitizing...  The poor little gate signal just gets lost in the hash...
The trick is to hook both probes to the same point and fiddle with 
things until the signal looks like 0V or at least you get a good idea 
of what the noise looks like.  Then you sneak one lead over to the 
test point and hopeful you can get an idea of the gate signal.  But 
on DRSSTCs, the switching noise is really bad and you often want to 
use the other channel anyway.  Even "real" differential probes need 
some care so as to minimize the noise pick up.  Of course, we are 
looking at fairly high frequency signals too which just makes it all 
that much worse.
I must admit that the differential mode on scopes has never been 
useful at all unless the signals are slow, low voltage, and 
clean.  Other than that, the mode just "looks good" in the sales 
brochure...   Unfortunately, good HV differential probes are very 
expensive!!  When we tried to make our "cheap version", we found out why =:O
Cheers,

Terry