Although mixer theory and design are beyond the scope of this book, Figure 4.4 shows a simple mixer that uses two diodes and could be used for two logic- level signals. Nearly any nonlinear device will work as a mixer to one degree or another. Off-the-shelf mixers are available, such as the Philips NE612. This example used a two-stage L/C low-pass filter. In some applications, you might want to use a more sophisticated filter or a bandpass filter. You could even use a DSP to perform the filtering in software, although that is a significant increase in overall complexity. The mixer approach looks like a simple way to multiply the frequency shift, but it does have some drawbacks: • The mixer approach multiplies the frequency shift you want to measure, but also any other frequency shift. This includes drift caused by component heating, noise, etc.
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