3. Lab 3 - Mixers

posted Feb 17, 2010, 5:58 AM by Bill Bai
(Fundamentals, harmonics, and cross products of 100.5MHz and 110.7MHz in a non-linear mixer)
So lab 3 involved mixers.  There are two kinds of mixers: linear and non-linear.  You put two or more signals into a linear mixer and you get an output that consists of the two fundamental frequencies of the inputs (i.e. an op-amp summer).  A non-linear mixer does some interesting things.  Rather than just adding up two signals, a non-linear mixer produces all the harmonics of each input signal.  Furthermore, the cross products of the input signals are also generated (spectrum analyzer screen capture above shows fundamentals, harmonics, and cross products of 100.5MHz and 110.7MHz).  It is a bit complicated to explain, but the important thing about non-linear mixers is that the cross products of the signals include the original information embedded in the carrier frequency.

Here's the rundown: humans can hear 20Hz to 20kHz, i.e. music.  This 20Hz to 20kHz signal is impressed onto an FM radio frequency, 88MHz to 108MHz; this process is called modulation.  When the signal is picked up by our FM radio receiver and gets to the mixer, our local oscillator is set to a frequency 10.7MHz higher than the frequency we want to tune to, this is called high side injection.  The intermediate frequency (IF) of FM radio is 10.7MHz by definition.  Because we set out local oscillator (LO) frequency 10.7MHz above our desired radio frequency (RF),  there will always be a cross product signal present at 10.7MHz on the mixer's output.  

As I said before, every cross product output from the mixer contains all of the information originally encoded on the RF carrier signal.  This 10.7MHz frequency down-conversion allows us to extract this information from the carrier signal.  Remember, the final goal is to extract the 20Hz to 20kHz music/sound information, the mixer gets us halfway from the 88MHz-108MHz FM band down to 10.7MHz, and the demodulator (later on, see diagram below) will extract the 20Hz-20kHz music/sound information.

(From Wikipedia)

Anywho, anyone who actually checks this website will probably go TL;DR on this post, but mixers are just a bit too complex and technical to explain easily.  My lab report attached to this post.  100% because I'm just full of win. =D
Bill Bai,
Feb 17, 2010, 6:35 AM