The Radiotelescope Mixer
The job of the mixer is to lower the frequency of the signal from the preamplifier.
We do this for a couple of reasons. First, it is hard to build good amplifiers,
filters, and other other components for higher frequencies, (though this
is getting easier with new technologies). Secondly, if we do all of our
amplifying at the frequency which we are receiving, there is a good chance
that some of the amplified signal will escape back into our antenna and
produce feedback. Its the same problem you have when you get the microphone
too near a public address speaker. The mixer combines the signal
from the local oscillator with the signal from the preamplifier (input
signal) and produces two additional outputs, one at the input signal frequency
minus the local oscillator frequency, and one at the sum of these frequencies.
We pick the lower of the two outputs by passing the mixer output through
a filter in the IF amplifier.
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