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In the present context, heterodyne refers to receivers where the
frequency of the input signal is shifted to lower frequencies. This
is done by adding to the (small) input signal a (relatively) strong
monochromatic signal, called the local oscillator and passing the sum
through a non-linear device, whose output contains (among other) the
difference frequency. Although a non-linear device is involved, the
transformation from input to output is linear for the small signal
(see S.Guilloteau lecture, Eq. 2.8).
This
process is called mixing or downconversion. The output signal is called
the intermediate frequency. Actually the complete signal processing
at a radiotelescope can involve up to four heterodyne conversions.
The first reason why heterodyne downconversion is needed is that only
few signal processing devices exist at millimeter frequencies, and
definitely not the fully parallel spectrometers (as opposed to multiplex
devices such as FTS) that are routinely used for spectroscopic
observations.
Then arises the question of where in the signal processing chain to
operate the down conversion. Basically we have no choice, because hardly
any amplifiers are available in the millimeter range, except in the 3mm
band, where they do not match the low noise properties of SIS mixers (to
be discussed below). So we must perform a downconversion before we
can amplify the signal.
Next: 3.4 Local oscillator system
Up: 3. Receivers : an
Previous: 3.2 Coupling optics
S.Guilloteau
2000-01-19