The instrument uses a local oscillator to produce its reference signal. This is mixed with the signal from the source object, and the result is a signal frequency much lower than the original and much easier to process with electronic circuits.

Musicians will recognize the heterodyne principle, since it's what they use when they tune their instruments, as they listen for the beat frequency. For HIFI, the reference signal must be extremely stable and pure. The advanced local oscillator source unit being contributed by Canada has unprecedented precision and stability.

Canadian scientists developed the “tuner” (above) for Herschel’s HIFI instrument, which will explore the chemistry of our galaxy.
(Source: Michel Fich)