Hi Tom, to reply to your questions: (1) Yes, I was also surprized at how good the 5065a is. It's a stock unit that has apparently never been repaired or modified. I'm guessing it was turned off for a long time (the seller said it was stored in an unheated container in Alaska). Leo's lab is very quiet, it's in a subbasement of a new building with good temperature control. The 5065a was installed there in November and had run a month before the tests. Before that it was on for > 6 months in my shop. (2) The simultaneous test would be much better, but we were worried that digital noise from the fitter would leak back through the power splitter into the 53230a and provide noise. The ADC chip in the fitter is connected directly to the input line using a wideband Minicircuits transformer, and might be noisy. An isolation amplifier would fix this. Still this is a good suggestion and we should try this out. The fitter is running continuously and we have about a dozen 200K second runs stored. I have one of Corby's barometric chips and a data logger, so we trying to correlate the temperature and pressure with the results. Ralph