Diode Mixer vs. Harmonic Mixer: Key Differences Explained

This article highlights the differences between diode mixers and harmonic mixers, two important components in RF and microwave systems. We’ll explore their characteristics, applications, and how they operate.

Diode Mixer

A diode mixer utilizes a silicon crystal diode to produce a low-frequency Intermediate Frequency (IF) signal. This type of mixer is commonly used at frequencies around 1 GHz. Diode mixers are known for their low conversion loss and low noise performance.

Figure 1: A coaxial line single diode mixer can be used up to frequencies of 3 GHz. This circuit is suitable for applications where signal levels are significant compared to noise power.

For scenarios where signal levels are not always large, balanced mixer configurations using 2 or 4 diodes in hybrid or magic-T configurations are employed to improve performance.

READ MORE about Balanced mixer configurations

Harmonic Mixer

Harmonic mixers find their niche in frequency calibration setups. These mixers use a nonlinear device, such as a crystal diode, to generate harmonics of a standard low-frequency signal. These harmonics are then heterodyne with a microwave test signal to produce an IF signal.

Figure 2: A harmonic mixer constructed using a crystal diode and other components.

It’s important to note that the harmonic amplitude or power is inversely proportional to the harmonic order.

Harmonic Mixer Circuit Operation

Let’s break down how a harmonic mixer circuit functions:

• The Local Oscillator (LO) signal, represented as Vpcos(Wpt), generates currents within the crystal. These currents contain the fundamental frequency and its harmonics (n*Wp, where n = 1, 2, 3,…).
• The incoming Radio Frequency (RF) signal, Vscos(Wst), interacts with these frequencies, resulting in sum and difference frequency components (n*Wp +/- Ws).
• The IF amplifier is tuned to extract the difference frequency component.
• In essence, a harmonic mixer utilizes a harmonic of the LO signal along with the low-frequency LO to generate the IF frequency.

Similar to a standard RF mixer, the conversion loss of a harmonic mixer is defined as:

Conversion Loss (dB) = 10 * Log10 (IF output power / RF input power)

Key Differences Summarized