What is a Harmonic Mixer?

A harmonic mixer is a type of mixer that uses a harmonic number of the applied local oscillator (LO) to mix with a radio frequency (RF) signal to generate an intermediate frequency (IF). The most commonly used harmonic mixers operate at an even harmonic of local oscillator. The behavior of a harmonic mixer shown in Figure 1 operating at even harmonics of LO is described by Equation 1.

IF=RF ± m x LO                   (m=2, 4, 6...)        Eq.1

Figure 1 Harmonic Mixer

The advantage of using a harmonic of the LO is that it allows the user to utilize a lower LO frequency than those required to operate a fundamental or sub-harmonic mixer. The topology, therefore, offers a simple and cost-effective solution for extending the frequency range of spectrum or signal analyzers where the LO frequency range is limited by the equipment. On the other hand, the compromise when using the harmonic mixer over a fundamental or sub-harmonic mixer is that the conversion loss suffers as the LO harmonic number increases. Degradation in conversion loss will result in a reduction in the sensitivity of a spectrum or signal analyzer.

Figure 2 shows a diagram of a harmonic mixer being used to extend the frequency range of a spectrum analyzer. The LO supplied by the spectrum analyzer is used to pump the anti-parallel pair of diodes. This well-known diode configuration is utilized to lower conversion loss and oscillator sidebands as well as provide protection against peak inverse voltages [1]. 

The LO and RF mix, according to Equation 1, and generate the IF which is fed directly into the IF input port of the analyzer. This bypasses the internal mixer of the spectrum analyzer but utilizes the remaining receiver chain to process the IF.

Figure 2. Harmonic Mixer Diagram

Unlike in sub-harmonic and fundamental mixers, the Harmonic mixer LO is low enough in frequency to be supported by a coaxial connection and share an impedance matching and low-pass filter (LPF) circuit with the IF. Using a coaxial connector and cable to supply the LO rather than a waveguide allows the user more spatial freedom on the work bench for testing their device. A diplexer is however required to separate out the LO and IF signals, this can be integrated into to the mixer shown in Figure 3a or require one externally as shown in Figure 3b.

 Figure 3. Harmonic Mixers 3a) Internal diplexer 3b) External diplexer required

When using a harmonic mixer with a spectrum analyzer it is important to enter the conversion loss values which will be supplied with a mixer and any multiplication factor being introduced by the mixer to ensure accurate measurements.

Summary:

Harmonic mixers are simple and cost-effective devices that have their strengths in extending the frequency range of test equipment. They are capable of working with lower LO frequencies than those required for fundamental and sub-harmonic mixer but that comes at the cost of increased conversion loss.

References:

[1] M. Cohn, J. E. Degenford, B. A. Newman, ”Harmonic Mixing with an Antiparallel Diode Pair”, IEEE MTT-S Vol. 23, No. 8, pp. 667-673, August 1975.