Re: ScopingSSTC

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>


> 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/