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 :-OSteve 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