In passive diode mixers, all three ports are interchangeable, with a caveat: performance will change significantly. While any port can be used as the LO port, and any other port can be used as the signal port, the performance of the mixer is most likely much worse than it would be if it was used in the designed manner. For example, all mixers are optimized for L-R isolation. If the IF port is used as the LO port, then significant harmonic content will appear at the RF/LO ports. Similarly the mixer is usually specified to operate in the lowest conversion loss over the broadest bandwidth in it’s standard configuration. In the opposite configuration (swapping the LO and RF ports), you can use less LO power, but with more conversion loss. When you are battling spurs, however, it almost always makes sense to flip the mixer around and take a measurement to see if the performance is better in reverse. On our modern MMIC mixers we call this ‘Configuration B’ vs. ‘Configuration A’, and either can be used for optimum performance in your system.
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