High-Performance X-Band Quadrature Mixer |
Overview
Single-sideband converters, Doppler radar, and interferometric microwave audio surveillance devices often require the use of a RF mixer that has its Local Oscillator (LO) feed in "quadrature." What this means is that the incoming local oscillator RF signal is split in two, with one of the LO signals undergoing a 90 degree phase-shift. The incoming RF port signal is also split in two, but these two signals remain in phase. The new RF and LO signals are then sent to two separate double-balanced mixers. The IF output signal from the phase-shifted LO mixer is referred to as the quadrature (Q) output. The IF output from the other mixer is referred to as the in-phase output (I).
In Single-Sideband (SSB) up or down converters, quadrature mixers are used to reject the image product of the mixer to allow the use of just the upper- or lower-sideband. In Doppler radar applications, quadrature mixers are used to determinte both the target's speed and direction (moving toward or away from the radiating antenna), as compared to a regular police radar which can only determinte speed. In interferometric microwave audio surveillance devices, using a quadrature mixer allows the elimination of "range deadspots" in your target beamwidth. If the quadrature IF output has a poor signal, you can try switching to the in-phase output and vice-versa. Another remote surveillance application of quadrature mixers is the ability to perform direct phase demodulation by comparing the slight phase differences between the two I & Q IF signals. For example, this demodulation method should allow you to detect a sub-millimeter vibration in a remote target by comparing the phase differences between the transmitted and received RF carrier. The phase of the demodulated signal(s) varies with the motion of the target. The I & Q outputs therefor occupy a frequency band related to that of the motion being detected.
Normally, to construct a quadrature mixer at microwave frequencies, you would require the use of some external RF splitters, a 3 dB hybrid to provide the 90° phase shift, and two mixers. Individually, these parts are difficult to find and can be quite expensive. Thankfully, Hittite Microwave Corp. has the HMC527 I/Q mixer which covers 8 to 14 GHz with an IF output that works from DC to 2 GHz. The HMC527 is completely passive, requiring no external voltage, but it does require a fairly high LO power between +17 to +21 dBm (50-125 mW). RF power into the RF and IF ports should not exceed +20 dBm (100 mW). Another thing to watch out for is that the HMC527's IF outputs can only source/sink around 3 mA of current. You'll need to take this into consideration when using the IF outputs in DC or low-frequency applications.
HMC527 Application Circuit

Pictures & Construction Notes

Mixer parts overview.
The HMC527 evaluation board is off to the left.
SMA connectors and cables were salvaged from various hamfests. I only had one bulkhead-mount SMA jack so I had to improvise using two SMA female-to-female adapters which, believe it or not, are available at Radio Shack.
The RF input to the HMC527 will be via the SMA bulkhead and connects to the evaluation board board using a SMA male-to-male adapter.
The use of the HMC527 evaluation board is highly recommended for this project. Proper microwave construction techniques need to be used with the HMC527 in order to help maintain the high RF/LO isolation of the mixer.

Adding a standoff the HMC527 evaluation board.
A small hole was drilled in the evaluation board for a #4 screw to attach the board to a small threaded standoff.

Mounting the HMC527 evaluation board in the project case.
Align the board properly so you can route the LO connection and two IF outputs.
The HMC527's RF input is via the SMA bulkhead.

Finished LO and IF connections.
The holes for the SMA adapters on the IF outputs should be counter-sunk slightly to allow the male SMA connector to be tightened properly.

Final case overview.
The HMC527's RF input is on the left, LO input is on the right.
The two IF outputs are on the bottom, with the "Q" output on the left and the "I" output on the right.