Harmonic mixer

Lore & Background

The harmonic mixer emerged as a practical alternative to the classic multiplier-based mixer, which ideally produces only sum and difference frequencies but in practice suffers from imperfect multiplication and unwanted heterodyne products. At microwave frequencies, a nonlinearity is easier to implement than an ideal multiplier, and a Taylor series expansion of a nonlinearity reveals the multiplications that generate higher-order products. Design goals for harmonic mixers focus on selecting desired heterodyne products while suppressing undesired ones.

One classic design uses a step recovery diode (SRD) driven by a subharmonic input amplified to about 1 watt. The SRD generates an impulse train approximating a series of narrow pulses, which contains harmonics of the input sine wave up to frequencies such as 18 GHz. This impulse train is then fed into a diode mixer, also called a sampler. The SRD can achieve a very high frequency multiplication ratio and forms the basis of a comb receiver, which monitors several harmonically related frequencies simultaneously—a principle used in simple bug detectors for detecting transmissions on unknown frequencies.

For lower frequency multiples such as doubling, tripling, or quadrupling, Schottky diode circuits are more common. The conduction angle, adjustable by changing drive level or temperature, determines which part of the I/V curve is used and thus the relative strengths of harmonically related outputs. An anti-parallel pair of diodes can suppress odd local oscillator contributions when an even multiple is desired, provided the diodes are identical and experience the same source impedance. Unlike a normal mixer, there is a fairly clear optimum drive level; above it, conversion loss increases.

Reader's Guide

The harmonic mixer's significance lies in its ability to avoid the complexity of generating a microwave local oscillator, making it a common and reliable frequency extender for low-frequency designs. In direct-digital or zero-IF communications systems, subharmonic mixers—a form of harmonic mixer where the LO is provided at a submultiple of the frequency to be mixed—are used to eliminate unwanted effects of LO self-mixing that occur in many fundamental frequency mixers. The design is also employed in frequency synthesizers and network analyzers.

A variation with two switching stages improves mixer gain in a direct downconversion receiver. The first stage mixes the received RF signal to an intermediate frequency at half the RF frequency; the second stage mixes that intermediate frequency to baseband. By connecting the stages in series, current is reused and harmonic content from the first stage feeds into the second, improving gain. The legacy of the harmonic mixer includes its role in comb receivers and bug detectors, where it enables monitoring of multiple harmonically related frequencies without prior knowledge of the transmission frequency.

Did You Know?

• One classic harmonic mixer design uses a step recovery diode to generate an impulse train with harmonics up to 18 GHz.
• A subharmonic mixer variant with two switching stages reuses current and improves gain in direct downconversion receivers.