PXI
Do you need to calibrate the internal reference clocks?
Solved!
I am working with a PXIe-1088 chassis plus 2x PXIe-6355 (multifunction IO) modules and a PXIe-6739 (AO) module. Both of these have their own internal clock sources. Right now I'm looking at my synchronization options.
I believe I can share my clocks from one device to another through the backplane so they all share one timing source. But my PXIe-1088 chassis already has a built-in 100 MHz reference clock, so it seems like it would be more straightforward to use that one.
However, I can't find any information on whether or not this internal reference clock can be traceably calibrated. If I'm using it as my sample clock source, shouldn't it need to be calibrated to have NIST-traceable timing? The modules themselves CAN be calibrated, and I assume their clocks are calibrated as part of that process.
Perhaps I'm misunderstanding how the internal clock sharing is used? My end goal is to have calibrated timing on my measurements synced between all of the modules, and I'd prefer to use the chassis clock rather than have one module be the "master".
Whatever clock you are using as the master reference clock should be calibrated, regardless of where it is coming from. In our systems, we tend to use Rubidium clock references that can be used by all of the equipment, including the PXI chassis. That reference clock is calibrated.
@crossrulz wrote:
Whatever clock you are using as the master reference clock should be calibrated, regardless of where it is coming from.
That's what I figured, and is why I'm confused about the lack of ability to calibrate the internal reference clock. I'm wondering if the intention is to always use an external clock and do some sort of PLL lock of the internal reference... it seems odd to me that they'd make a big deal about the PXIe chassis providing a stable, universal clock, but provide no means to calibrate it.
@crossrulz wrote:
In our systems, we tend to use Rubidium clock references that can be used by all of the equipment, including the PXI chassis. That reference clock is calibrated.
Can you use that external clock as your internal reference clock source? Or do you just import the clock via a PFI port and share it across the backplane, and ignore the built-in reference clock?
Edit: Just read through the calibration procedure for the X series cards (the 63xx ones) and it confirmed that the internal clock can be verified but not calibrated. So, perhaps I'm using an incorrect term here- I assume you can verify the internal clock of the PXIe chassis, but likely cannot calibrate it.
@BertMcMahan wrote:Edit: Just read through the calibration procedure for the X series cards (the 63xx ones) and it confirmed that the internal clock can be verified but not calibrated. So, perhaps I'm using an incorrect term here- I assume you can verify the internal clock of the PXIe chassis, but likely cannot calibrate it.
That is a distinction I have recently become all too familiar with lately. It just can down that our RF attenuators have to go through calibration. I questioned it stating there's nothing they can do to calibrate them. The response was that they have to verify them. My response was then that we have to "calibrate" our RF setups anyways (either the NetAn port calibration or measure line loss with RF Generator and Spectrum Analyzer). "It doesn't matter" was the idiotic response I was given.
With that said, I am not sure if the reference clocks are actually calibrated or verified. As far as importing the reference clock, there is often a BNC or some other connector on the back of the PXI chassis to import (and often export) a reference clock. It may be a special option. If not, use a timing card that can bring in an external reference clock or act as one itself. If you inject one, the chassis autodetects and tunes to it. This will give you the accuracy of the external clock on the chassis clock. Each card then needs to be explicitly told to use the reference clock in the chassis instead of relying on its own reference clock.
Thanks- that's extremely helpful. It looks like it's a special option, but I think I can export one of the internal verified clocks from one of my modules to use as a "calibrated ;)" reference source.
I also wonder if I can simply pipe an external source through one of my cards' PFI inputs to the backplane. I don't NEED to do this but I'd like to know for future reference. Maybe I'll get a little time to play around with clocks before I have to do "real work" with it 🙂
@BertMcMahan wrote:
I also wonder if I can simply pipe an external source through one of my cards' PFI inputs to the backplane.
Only if your card is in the special Timing Slot.
Some links you may find interesting:
PXI Timing and Synchronization course. I don't know what would make you eligible to take the course, but it might be something to look into.
PXI Synchronization Modules. These are what I would recommend for importing and/or being your reference clock. You do need to put them into your PXI Timing slot to be able to set the PXI reference clock.
Awesome, looks like that class is bang on for what I'd like to do, and not super long. Much appreciated!!
