Phase Locking the Up and Down Converter Modules

Question

I am interested in phase locking the 5660 and 5670 modules, but from what I understand...this might not exactly be possible.
Using the 10MHz references...it seems that I can frequency lock the two together (i.e. - I can eliminate any phase error due to the two LO's drifting away from each other).  However...am I correct in assuming that there is no way to actually access the individual oscillators such that I can remove the phase difference between them?
For example...if I want to measure the phase added by a DUT....I would have to measure the phase without the DUT...then meausre the phase with the DUT...then look at the relative difference in the two phase measurements (as calculated from the IQ samples).  However, if I were to repeat this measurement by first measuring the phase without the DUT....then turn the generator off...then back on again....then measure the phase with the DUT...I'd expect to get a different answer, since by turning the device on and off, I have no idea or control over the phase relationship between the two devices.
Is this correct?
Finally...does simply connecting say, the 10MHz Out of the upconverter, to the Freq Ref IN of the downconverter automatically frequency lock the two together...or do I have to go into LabView and programatically tell the things which reference to use?
Thanks!
Brandon

Answer 1

Hi Brandon,
You are correct in what you are saying. The PXI-5600 and PXI-5610 Down/Upconverters each have their own synthesizers internally generating the LO signals for the mixers inside, and these LO signals cannot be exported from the modules. For these modules to be phase locked, they would need to share the same LO signals.
By sharing the 10 MHz reference you can achieve frequency lock where the phase difference between the two modules will stabilize, but the phase difference will not be zero - it will be some unpredictable phase offset which will not be increasing or decreasing (i.e. frequency lock). Whenever the 5600 or 5610 is retuned, the phase difference will also be different.
With regards to the 10 MHz signal, both use the same component for the reference so it doesn't matter which module you lock to the other as they are equivalent in quality. You do need to make a call in software to lock the destination module to the source module. If you are driving the PXI backplane with the source module, you'll also have to make a SW call to do this. This is very easy though, just look for VIs called Configure Reference Clock or something similar.
Regards,
Andy Hinde
National Instruments