Appear to be missing samples.

[Rimfire]

I am running a pair of PXI DAQ cards (6070 and 6259) and sampling 16 AI channels at about 80kS/s. The cards can handle the rate and I get no errors while running. I am using an external AI sample clock that is generated by an encoder on the shaft of an engine and sampling a number of signals on the engine (pressure, spark, etc.) When I look at any signal relative to the sample clock (with a scope) signals are staying aligned as is expected.

I sample 720 degrees of data which works out to 5760 samples with my encoder (2880 pulses per 360 degrees). As I read the samples back with the MX read block and plot them to a graph, the signals all drift to the right.  One of the guys here made the suggestion that I was "dropping" samples.  I experimented a bit with the number of samples I read back, I can get the signals to stabilize momentarily, but they will start to drift again and I have to adjust. All 15 sampled channels exhibit the same problem.

Does anyone have any suggestions?

Thanks,

Dave

[Laura F.]

Hello Dave,

It sounds like you could be experiencing some aliasing in your data.  Aliasing occurs when you do not sample your inputs fast enough compared to the frequency of the signal.  Please check out this tutorial for more information about this concept.  You did not mention the frequency of the engine signals, only that you are sampling using the engine encoder at 80 KHz but I think that you may be experiencing this problem.  Try increasing the sample rate.  I am not sure how this will affect the other signals, since your sample rate is dictated by the engine itself. 

Thanks,

Laura