Practical radio astronomy receivers incorporate frequency conversion because it is technically more convenient to amplify, filter, delay and cross-correlate signals at a fixed intermediate frequency (i.f.) than at their original, changeable (and often higher) radio frequencies. The frequency conversion is done in one or more mixers, in which a non-linear element (usually a diode) forms signals at the sum and difference frequencies between the astronomical signal and a tunable local oscillator (l.o.) signal.
microwave signals are shifted down in frequency by combining them with a locally generated signal in such a way that the difference in the frequencies is generated. The local signal is generated by the local oscillator. In project Argus type systems, the frequency is shifted from around 1420,000,000 Hz, to somewhere around 1,800 to 20,000 Hz (centre). There are many reasons for doing this, but if it weren't one would still need an accurate local references of the same precision, which needs to be significantly better than the bin width in 1420,000,000 Hz.