CRO trace drift.

I have noticed this phenominon with al the analog CROs that I have used.

When I start the CRO, I ground my channel and Y-adjust the trace to the centeral graticule (X asis).

After a few minutes of operation, when I ground my channel again, I observe that the trace has moved up slightly and I have to readjust the zero.

Is this common with all CROs... Has anyone else observed this?

Why does this happen?

It is just the equipment coming up to operating temperature. As the components heat up to operating temperature their values are likely to drift slightly. Any time you need to make critical measurements you should let the test gear "stabilize" by allowing it to get to operating temperature.

Crystal oscilators are a good example... you can get ones that have their own little "ovens" to hold the crystal at a known temperature to keep them stable and accurate.

Yes, I've witnessed this in all three types of analog scope that I use including a Leader, a Tek 465 and an HP17xx series. As mentioned above, its all about analog circuits warming up. There is some DC bias drift, some gain drift, some DC offset drift and some balance drift in the internal circuits and its pretty normal. However, you would expect that better designed scopes have less of this and I find that to be true.