Thread: VMC measurements of a two stage downconverter in fixed IF mode

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?

Dr. Joel,
The DUT has 2 LO external inputs. For verifying I could make a valid cal and measurement, I used a simulated DUT with two mixers and low pass filters. There are a lot of different cases that I have needed to verify but one that is causing a problem is the first VMC fixed IF case I tried.

I am using single stage reference and cal mixers using a PSG to generate the single LO with a splitter to provide signals to both mixers.

An example setup for this case is a down converter with a 7700 to 8100 MHz RF input, 10800 to 11200 MHz LO1, 3100 MHz IF1, 3800 MHz LO2, 700 MHz output, the Meta LO would be swept from 8400 to 8800 MHz to provide the same 700 MHz IF out of the reference and cal mixers.

There are a total of 4 sources used. The PNA-X source 1 for the RF Stimulus and whatever else it is used for, SRC 2 OUT1 for the LO1 signal, a uncontrolled 83712B for the LO2 signal, and a PSG 8257D for the Meta LO.

The way I get it to change frequencies is to use commands I received from an applications engineer to control it; if the generator has been named E8257D_LO_Gen and is the fifth source the commands are as follows:
SENS:FOM:RANG5:COUP 0
SENS:FOM:RANG5:FREQ:STAR 8400 MHZ
SENS:FOM:RANG5:FREQ:STOP 8800 MHZ
SOUR:POW:LEV:IMM:AMPL 10,'E8257D_LO_Gen'
SOUR:POW:MODE ON,'E8257D_LO_Gen'

I send those commands after setting up the mixer parameters and before attempting a mixer characterization. I don't change the mixer setup from two stages to a single stage.

The eventual use of this design is classified but I'll give the designers a break and assume they have a good reason for this measurement but I wouldn't do if I could avoid it.

I sent the s2p and s2px files to the applications engineer. Although they aren't classified I rather not post them on-line. I think I still have your email so I should be able to send you a copy.