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The term heterodyne spectrometer is used for a particular type of spectrometer suited to the submillimeter wavelength range (in the general range of a terahertz frequency), which uses EMR interference to produce an EMR signal at a lower frequency that is practical to analyze using electronics, e.g., below 300 GHz. The EMR is combined with that of a local EMR producer (e.g., a synchrotron maser) that serves as a local oscillator (LO). The combined result includes lower frequencies (beat frequencies) that have an amplitude proportional to the received signal, i.e., it retains characteristics of the received signal but at the lower frequency. A sensor (typically a photodiode) capable of reacting at this lower frequency produces an electric signal which can be analyzed using microwave-frequency electronics. This differs from typical radio electronics in that the beat frequencies are produced while the signal is still EMR rather than after it is converted to an electronic signal by an antenna.
The term heterodyne spectrometer is also used for lower-frequency (radio or microwave) spectrometers doing the latter.
The term spatial heterodyne spectrometer refers to a different type, which includes a modified Michelson interferometer, with reflectors that are gratings. It splits the EMR signal, disperses it two different ways, then combines the result.