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VMC measurements of a two stage downconverter in fixed IF mode

Question asked by LOlson on Jul 7, 2012
Latest reply on Jul 7, 2012 by Dr_joel
I have been running through the setups for measuring a two mixer downconverter. I have managed to do the setup and calibration of both fixed and swept SMC measurements but the stickler is trying to do a VMC measurement with a Meta LO that allows the reference and cal mixers to be a single stage. I came in to the development after the basic setups were agreed to during conversations with a couple of Agilent applications engineers. Their suggestion was to simplify the setup by using a meta LO to convert the Source output to the 2nd IF. So after receiving the commands needed to setup a PSG generator to sweep over a frequency range to get the same 2nd IF as a two mixer DUT, I tried doing a calibration. It took a while to get through it all but at first I thought I was had it working. I ran the mixer characterization and then the VMC calibration and then looked at the response of the cal mixer using the Meta LO as the LO source for it. I had a fairly stable phase response and a reasonable conversion loss. But when I hooked up a simulated DUT with two mixers, using the SRC 2 Out 1 port for the swept 1st LO and a 83712B generator for the fixed 2nd LO, I saw a phase response that just looked like noise and was constantly varying. I then used a spectrum analyzer to verify that the each signal had the right frequency range or fixed frequency. The magnitude response did look OK. All the instruments are using the PNA 10 MHz output as a reference clock. I found that both the reference mixer and cal mixer IF outputs were varying about 1 MHz (between the desired frequency and 1 MHz below it) while the IF output of the simulated DUT had a constant frequency output. I tried another PSG generator as the meta LO and saw the same issue. I'm using a two port PNA-X N5242A with a second internal source.
Obviously this is a high priority as I'm writing this on a weekend. Should this setup work or do I need to use two mixer stage reference and cal mixers as shown in the help files?  

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