Applying Multi-Stage, Multi-Rate Digital Filtering

Figure 1: The input signal shows both the desired 63-kHz signal
along with a 60-Hz component. Zoom trace Z1 shows the
60-Hz component in detail.

A while back, we posted some basics on how to apply digital filters to sort out signals with undesirable elements riding on top of them, i.e. a square wave that's being corrupted by a sinusoidal signal creeping in from somewhere in your system design. Now, let's look at how to extend the range of cutoff frequencies for digital filters, allowing them to be used even more effectively.

Using Digital Filters

Figure 1: Using a band stop filter to remove a 5-MHz sine
wave from a 4-MHz square wave

In a perfect world, all of the signals we wish to view on our oscilloscopes would be as pure as the driven snow. However, in most scenarios, reality intervenes. As we know, everything in a system is an antenna, some being more efficient than others in terms of radiation and/or reception. Hey, crosstalk happens, and if you've got a sine wave and a square wave co-existing in the same general vicinity, it could make a true evaluation of the square wave difficult to do. But how do we get that nasty sine wave coming from one circuit to stop jumping on the  back of the square wave in another?