Bill Bowden said:
I've never really understood how a mixer circuit in a typical radio
receiver produces the sum and difference frequencies between
the incoming RF signal and the local oscillator. I understand
it requires a non-linear circuit, but I can't quite see
how the signals subtract to produce the IF frequency.
For example, if the RF input is 1 mHz and the IF is 455 kHz, the local
oscillator should be running at 1.455 mHz. How do we combine
1 mHz and 1.455 mHz to get 455 kHz?
-Bill
'Mixers' are just phase sensitive rectifiers.
If you want physical insight as to how that difference frequency somehow
turns up, It can be handy to just to draw out 10 cycles of a 50:50 square
wave a piece of squared paper (2 square high, 2 squares low).
Pretend this is the local oscillator running at say 1MHz. This now switches
the rectifier device that will be rectifying the incoming RF input signal
and somehow generating those sum and difference frequencies (and many
others).
The rectifier can be something as simple as a CD4066 ON/OFF switch. The
rectifier does not *have* to be a diode or some strange device with a non
linear bend in it. In fact, the ON/OFF type switch is the best of them all.
For the RF input signal, draw a line of (say) 7 square waves underneath the
local oscillor signal, 3 square high, 3 squares low, =666kHz. Start them at
the same point but make sure they are correctly sized (phased) relative to
the top line.
Now for the rectification/mixing/multiplying/modulating action ...
Draw a third "0V" output line under the previous two.
Every time the local oscillator is high then the RF signal passes through
to the output (is rectified), so just copy to the output line, the segment
of the 'RF' square wave that sits under the local oscillator during it's
high periods.
After doing all 10 then look at the resulting mishmash of blocks and half
blocks and visually average them (a human RC low pass filter).
Notice there is a low frequency undulating component present (about 3 cycles
over the run ='340kHz')). This is the 'oddball' I.F frequency.
regards
john