Book Analog Interfacing to Embedded Microprocessors
would make the low-pass filter simpler. The farther the unwanted mixer fre-
quencies are from the desired frequency, the easier they are to filter out.
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
annaabi.ee 
