Why we need heterodyne receivers

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. 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.