Diode Mixers

Introduction: Receiver and transmitter systems require the use of frequency mixers to up/down convert signals. The mixer is able to convert a desired signal from one frequency to another while maintaining the original signal’s modulation information.

Diodes are often used in microwave and millimeter-wave mixers because of their low cost and simplicity. The configurations are nearly endless. However, in general they use at least one balun and some type of filtering. This mixer uses a very simple transmission line quasi-balun circuit and a diplexer.

Requirements: The primary requirements were low cost, small size and low conversion loss. Table 1 outlines more of the detailed requirements.

Mixer Design: The mixer chosen for this application was a singly balanced diode mixer. This mixer uses two matched diodes in a single, low cost plastic package. The LO/RF isolation was achieved by using a balun on the LO drive input. Actually, the balun for this
application is more accurately described as a quasi-balun since it does not achieve very high isolation between the balanced outputs and ground. However, for this application, that characteristic of ideal baluns was not required [1].

Figure 1. Mixer circuit layout.

The RF and IF are separated using a diplexer. The diplexer was realized using lumped element inductors and capacitors. Figure 1 shows the layout of the mixer.
Performance: Figure 2 shows the simulated conversion loss of the mixer.

Figure 2. Mixer simulated performance.

Note that the simulated conversion loss is well within the requirement of 10dB. I use Microwave Office (MWO) from Applied Wave Research for all microwave and RF circuit design [2]. The simulations used MWO and diode models from the manufacturer. Figure 3 shows the simulation schematic for the balun. Note that the schematic includes the transmission lines, bends, line width steps and tee junctions. The balun was also simulated using electromagnetic analysis. The simulation results showed good agreement with the circuit simulations.

Figure 3. Balun schematic in MWO.

The mixer was fabricated on RO4003 board 0.008” thick. It was then epoxy attached to an aluminum test fixture. SMA connectors were attached to the aluminum block with 2-56 screws. The center pin of the mixer was solder attached to the substrate. Figure 4 shows the test fixture.

Figure 4. Fabricated mixer in test fixture.

Measurements were performed on the mixer. Figure 5 shows the measured conversion loss for the mixer. Note that the conversion loss is well within the requirement.

Figure 5. Mixer measured conversion loss.

Conclusions: The purpose of this white paper is to illustrate a circuit developed by Rick Sturdivant for a Ku-Band system. It provides an overview of a mixer design and the measured conversion loss.

[1] R. Sturdivant, “Balun Designs for Wireless…Mixers, Amplifiers and Antennas”, Applied Microwave and Wireless, Summer 1993.
[2] Applied Wave Research.

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