Heterodyne detection
Encyclopedia
Heterodyne detection is a method of detecting radiation by non-linear mixing with radiation of a reference frequency. It is commonly used in telecommunications and astronomy
for detecting and analysing signals.
The radiation in question is most commonly either radio waves (see superheterodyne receiver
) or light (see Optical heterodyne detection
or 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.
The the received signal can be represented as
and that of the local oscillator can be represented as
For simplicity, assume that the output I of the detector is proportional to the square of the amplitude:
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
for detecting and analysing signals.
The radiation in question is most commonly either radio waves (see superheterodyne receiver
Superheterodyne receiver
In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
) or light (see Optical heterodyne detection
Optical heterodyne detection
Optical heterodyne detection is an important special case of heterodyne detection. In heterodyne detection, a signal of interest at some frequency is non-linearly mixed with a reference "local oscillator" that is set at a close-by frequency...
or interferometry
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
). The reference radiation is known as the local oscillator
Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
. The signal and the local oscillator are superimposed at a mixer
Frequency mixer
In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...
. The mixer, which is commonly a (photo-)diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
, has a non-linear response to the amplitude, that is, at least part of the output is proportional to the square of the input.
The the received signal can be represented as
and that of the local oscillator can be represented as
For simplicity, assume that the output I of the detector is proportional to the square of the amplitude:
-
-
The output has high frequency (
and 
) and constant components. In heterodyne detection, the high frequency components and usually the constant components are filtered out, leaving the intermediate (beat) frequency at 
. The amplitude of this last component 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
- HeterodyneHeterodyneHeterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...
- Homodyne detectionHomodyne detectionHomodyne detection is a method of detecting frequency-modulated radiation by non-linear mixing with radiation of a reference frequency, the same principle as for heterodyne detection....
- Optical heterodyne detectionOptical heterodyne detectionOptical heterodyne detection is an important special case of heterodyne detection. In heterodyne detection, a signal of interest at some frequency is non-linearly mixed with a reference "local oscillator" that is set at a close-by frequency...
- Heterodyne
-