Re Question For Craig Markwardt

"ralph sansbury" writes:
I would accept your explanation if you could explain why the
law
of superposition does not apply here and how other filters
at the output can produce each of the two input gate frequencies
and
how electric fields multiply.

The principle of linear superposition requires a linear medium.

However, the mixer is a nonlinear electronic device, and therefore
superposition does *not* apply.

I confess ignorance of what precise components are inside the mixing
circuit. The key point is that the mixer is *designed* to multiply
the incoming signal by a reference signal. Since the reference signal
is oscillatory, the result is the beat, or frequency difference, which
is desireable because such an intermediate frequency is more
straightforward to digitize and analyze.

An ideal mixer simply shifts the frequency; it does not filter the
signal. However, for the purposes of digitization, one must
bandpass-filter the IF signal.

The 1 MHz bandwidth of the filter is very large. It is larger than
any expected signal Doppler shift in the solar system, which is of
order +/- 400 kHz [based on earth motion of ~30 km/s]. Thus, there is
nothing in the data acquisition system which assume a particular
expected sky frequency.

CM