Originally posted Jul 1, 2014
Where should your first mixer be when you’re making high-frequency measurements?
In Torque for Microwave & Millimeter Connections, I complained that engineering was inherently more challenging at microwave and millimeter frequencies. One reason: many factors that can be ignored at lower frequencies really begin to matter. Therefore, it’s important to consider all the tools and approaches that can help you optimize measurements at these frequencies, and this includes external mixing.
In my years of working at lower frequencies I knew about external mixing, but I always thought of it as a rather exotic and probably difficult technique. In reality, it’s a straightforward approach that has significant benefits, and modern hardware is making it both better and easier.
I also realized that I had been using external mixing for years, but at home: the low noise block (LNB) downconverter in my satellite dish. Satellite receivers use external mixing for many of the same reasons engineers do.
For satellite receivers and signal analyzers it’s a matter of where you place the first mixer. In analyzing microwave and millimeter signals, the first signal-processing element—other than a preamplifier or attenuator—is generally a mixer that downconverts the signal to a much lower frequency.
There’s no requirement that this mixer be inside the analyzer itself. In some cases there are benefits to moving the mixer outside the analyzer and closer to the signal under test, as shown below.
In external mixing, the analyzer supplies an LO signal output and its harmonics are used by the mixer to downconvert high frequencies from a waveguide input. The result is sent to the analyzer as an IF signal that’s processed by the analyzer’s normal IF section.
External mixing has a number of benefits:
- Flexible, low-loss connection between signal and analyzer. The vital first downconverting element can be placed at the closest and best location to analyze the signal, typically with a waveguide connection. The analyzer can be located for convenience without a loss penalty from sending high frequencies over a distance.
- Frequency coverage. External mixers are available for frequencies from 10 GHz to the terahertz range, in passive and active configurations.
- Cost. Signal analysis may be needed over only a limited set of frequencies in the microwave or millimeter range, and a banded external mixer can extend the coverage of an RF signal analyzer to these frequencies.
- Performance. Measurement sensitivity and phase noise performance can be excellent due to reduced connection loss and the use of high-frequency and high-stability LO outputs from the signal analyzer.
Some recent innovations have made external mixers easier to use and provide improved performance. These “smart” mixers add a USB connection to the signal analyzer to enable automatic configuration and power calibration. The only other connection needed is a combined LO output/IF input connection, as shown below.
Agilent’s M1970 waveguide harmonic mixers are self-configuring and calibrating, requiring only USB and SMA connections to PXA and MXA signal analyzers.
The new mixers enhance ease of use, including automatic download of conversion loss for amplitude correction. Nonetheless, they can’t match the convenience and wide frequency coverage of a one-box internal solution that has direct microwave and millimeter coverage. And because external mixing doesn’t include a preselector filter, some sort of signal-identification function will be necessary to highlight and remove signals generated by a mode—LO harmonic or mixing—other than the one for which the display is calibrated (more on this in a future post).
External mixing is now a supported option in Agilent’s PXA and MXA signal analyzers. This is described in the new version of Application Note 150 Spectrum Analysis Basics and in the application note Microwave and Millimeter Signal Measurements: Tools and Best Practices.