This is an image of the Heterodyne Instrument for the Far Infrared's (HIFI) focal plane unit. Clearly visible are 7 of the 14 mixer sub-assemblies, in which the signal received by the telescope is mixed with the signal generated by the local oscillator.
HIFI is a very high-resolution heterodyne spectrometer. The heterodyne detection principle involves translating the frequency range of the astronomical signal being observed to a lower frequency where it is easier to perform the required measurements.
This is done by mixing the incoming signal with a very stable monochromatic signal, generated by a local oscillator, and extracting the difference frequency for further processing.
HIFI observes in seven bands covering 480 to 1910 gigaHertz.