Answer 8—Sounds like an interesting project! However, using a sound card to do the D/A and A/D is probably not going to work out well, mainly because of the anti-aliasing/anti-imaging filters found in the audio converters and the lack of DC response. For an SEM, you want to be able to set the beam position quickly, precisely, and repeatedly to a particular spot and then read the target current and move on. The huge delays of the low-pass filters (several tens of sample periods, typically) are going to make it difficult to correlate a beam position with a particular video pixel value. If you slow down enough to make that work, then the lack of DC response is going to kill your repeatability.

As an experiment, just try hooking your sound card outputs up to a scope configured for X-Y display and see if you can generate the kinds of rasters you want. You’re probably not going to be happy with what you see. You’ll want to build or buy yourself a good laboratory type D/A-A/D board that has high absolute precision and very little if any filtering.

Contributor: David Tweed

Published September 2003