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The harmonic mixer and subharmonic mixer are a type of frequency mixer, which is a circuit that changes one signal frequency to another. The ordinary mixer has two input signals and one output signal. If the two input signals are sinewaves at frequencies f1 and f2, then the output signal consists of frequency components at the sum f1+f2 and difference f1−f2 frequencies. In contrast, the harmonic and subharmonic mixers form sum and difference frequencies at a harmonic multiple of one of the inputs. The output signal then contains frequencies such as f1+kf2 and f1−kf2 where k is an integer.
The classic frequency mixer is a multiplier. Multiplying two sinewaves produces just the sum and difference frequencies; the input frequencies are suppressed, and, in theory, there are no other heterodyne products. In practice, the multiplier is not perfect, and the input frequencies and other heterodyne products will be present.
An actual multiplier is not needed. The significant requirement is a nonlinearity, and at microwave frequencies it is easier to use a nonlinearity rather than an ideal multiplier. A Taylor series expansion of a nonlinearity will show multiplications that give rise to the desired higher order products.
Design goals for mixers seek to select the desired heterodyne products and suppress the undesired ones.
Overdriven diode bridge mixers. Drive signal looks like odd harmonic waveform (essentially a square wave).
A classic design for a harmonic mixer uses a step recovery diode. The mixer's subharmonic input is first amplified to a power level that might be around 1 watt. That signal then drives a step recovery diode impulse generator circuit that turns the sine wave into something approximating an impulse train. The resulting impulse train has the harmonics of the input sine wave present to a high frequency (such as 18 GHz). The impulse train can then be used with a diode mixer (also called a sampler).
Subharmonic mixers are often used in direct-digital, or zero IF, communications system in order to eliminate the unwanted effects of LO self-mixing which occurs in many fundamental frequency mixers.
A subharmonic mixer that has two switching stages is used to improved mixer gain in a direct downconversion receiver. The first switching stage mixes a received RF signal to an intermediate frequency that is one-half the received RF signal frequency. The second switching stage mixes the intermediate frequency to baseband. By connecting the two switching stages in series, current is reused and harmonic content from the first stage is fed into the second stage thereby improving the mixer gain.
- 8410B Network Analyzer; 8411A Harmonic Frequency Converter, Hewlett-Packard, 1968, 08410-90521. RF input is 0.11 to 18.0 GHz; LO 62 to 154 MHz; output 20.278 MHz. See page 8-39. For step recovery circuit and power spectra, see page 8-35.
- Grove, Wayne M. (October 1966), "A DC TO 12.4 GHz Feedthrough Sampler for Oscilloscopes and Other RF Systems", Hewlett-Packard Journal 18 (2)
- http://www.microwaves101.com/encyclopedia/mixerssubharmonic.cfm Just hints at elements of SHM design
- Donsi, Tony (2002), Subharmonic Mixer Design with Ansoft Designer, Ansoft Roadshow, Ansoft, Presentation 9. Describes some theory, use of antiparallel mixer diodes, odd harmonic selection