How to output sample clock with PCI 6220

[pgraebel]

I am trying to output the AI sample clock task via a channel on my RT Desktop system equipped with PCI 6220 and PCI 6723. I found a post discussing this (Forum Link) and using the "DAQmx export signal" VI:

My 1 GHz LeCroy oscilloscope does not see any sample clock signal on PFI7, but if I change from "sample clock" to "10 MHz timebase", I do get a sine wave signal. What is the problem here? Is it depending on my specific PCI card?

[SFK]

Hi pgraebel,

a 1GHz scope should be fast enough to trigger on and visualize the 50ns sample clock pulses. Your card should also be fine if you can export the reference clock on the PFI pin and read samples with your VI. To reproduce your setup, I have used NIs 2GS/s PXI-5152 scope to produce this image of the sample clock signal using a simple 1:1/1:10 probe (1:1 setting) on a simple CB-68LP connector block connected to a PXI-6224 through a SHC68-68-EPM cable:

In parallel to the probe, I have connected the sample clock output on PFI7 to PFI8, which is the default ctr0.scr input. To verify the ouput, I have let the counter count the sample clock pulses, which proved the routing to function correctly.

I would recommend you to do the same: connect the PFI7 and PFI8 on your breakout box, and run the ctr0 test panel as shown above. Can you see the edge count increase when running your VI?

Regarding your observation that the 10MHz Reference Clock looks like a sine wave: this is almost expected behavior (and the same reason as for the "worn-out" signal form of the sample clock): the normal SHC cables and connector blocks are not designed for high frequency digital signals, which causes the observed signal quality degradation.

If your LeCroy scope features switchable input impedances and is set to 50Ohms, please try to set it to a higher impedance, like 10MOhm or something like that. You can also check if DC coupling is enabled...

The professional solution for outputting these signals for synchronization purposes would be the use of a PXIe chassis: the sample clock or reference clock can there be routed through the RTSI bus to the PFI outputs of a PXIe-6672 timing module, which features 50Ohm SMB connectors suitable for high frequency outputs.

Best regards,

Sebastian

[pgraebel]

Thanks for the hint. I did connect PFI7, PFI8 as you said and used the MAX test panel to read ctr0. The counter did increase according to the "number of samples" value specified in the VI. I got a signal like this:

Unfortunately the scope does not offer switchable input impedances.

Now that I am successfully exporting the AI sample clock, I tried to create a clock based on the AI sample clock to run a timed loop. I think I cannot export the RT Desktop Onboard clock which is usually running timed loops. So I would like to use the AI sample clock as a common basis. But this experiment failed, as you can see below (error -200088):

What's the problem here? Is it possible to do at all?