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Heterodyne detection is a method of detecting frequency-modulated radiation by non-linear mixing with radiation of a reference frequency.

The radiation in question is most commonly either radio waves (see superheterodyne receiver) or light (see interferometry). The reference radiation is known as the local oscillator. The signal and the local oscillator are superimposed at a mixer. The mixer, which is commonly a (photo-)diode, has a non-linear response to the amplitude, that is, at least part of the output is proportional to the square of the input.

Let the electric field of the received signal be

${\displaystyle E_{\mathrm {sig} }\cos(\omega _{\mathrm {\mathrm {sig} } }+\phi )\,}$

and that of the local oscillator be

${\displaystyle E_{\mathrm {LO} }\cos(\omega _{\mathrm {LO} }).\,}$

For simplicity, assume that the output of the detector I is proportional to the square of the amplitude:

${\displaystyle I}$${\displaystyle \propto }$ ${\displaystyle \left(E_{\mathrm {sig} }\cos(\omega _{\mathrm {sig} }t+\phi )+E_{\mathrm {LO} }\cos(\omega _{\mathrm {LO} }t)\right)^{2}}$

${\displaystyle =}$

${\displaystyle E_{\mathrm {sig} }^{2}(1/2)\left(1+\cos(2\omega _{\mathrm {sig} }t+2\phi )\right)}$

${\displaystyle +E_{\mathrm {LO} }^{2}(1/2)(1+\cos(2\omega _{\mathrm {LO} }t)}$

${\displaystyle +E_{\mathrm {sig} }E_{\mathrm {LO} }\left}$

The output has high frequency (${\displaystyle 2\omega _{\mathrm {sig} }}$ and ${\displaystyle 2\omega _{\mathrm {LO} }}$) and constant components. In heterodyne detection, the high frequency components and usually the constant components are filtered out, leaving the two intermediate (beat) frequencies at ${\displaystyle \omega _{\mathrm {sig} }+\omega _{\mathrm {LO} }}$ and ${\displaystyle \omega _{\mathrm {sig} }-\omega _{\mathrm {LO} }}$. The amplitude of these last components is proportional to the amplitude of the signal radiation. With appropriate signal analysis the phase of the signal can be recovered as well.

See also

• Homodyne detection

• Waves