I picked up a used e4406a off of ebay. Seemed to be working fairly well. Downloaded the KE5FX phase noise utility and it works pretty well! A nice cheap phase noise measurement system.
But, when testing it out I noticed some odd spurious outputs on the e4406A display. At 640 MHz in, there was a spurious sideband displayed about 2.8 MHz higher than the carrier. As I tuned the input frequency, the spur crossed over the carrier, so it acted like a N*LO -M*RF spur. It was around 40 dBc down.
Oddly, it did not show up in the KE5FX acquisition phase noise plot!
Everything else in the VSA looks good. Accurate power readings, etc. I tried various sources, and checked on other spectrum analyzers, and the source does not have the spur.
So what inside of the E4406A is causing a spurious product like that? Any suggestions, any way to cal something inside to remove it? Or do I have a semi-blown front end?
The E4406A is a transmitter tester that is fundamentally mixed to 321.4 MHz. It doesn’t have a super heterodyne design that mixes to a higher frequency and then down to the IF. So, the E4406A’s design doesn’t work well for frequencies around 321.4 MHz, and there are other harmonics that don’t work well. The E4406A was designed to be a transmitter tester that works well for in-band measurements in the 800 MHz, 900 MHz, 1800, 1900 MHz, etc. cellular bands. The KE5FX phase noise utility probably doesn’t see the spur issue you see because it probably uses a narrower bandwidth and steps the center frequency. In general, the E4406A is very stable and is extremely accurate measuring power such that you can just about use it as a power meter.