Can I configure niScope Fetch Binary 16.vi to grab I,Q data from PXI-5620 ?

[Andy_Hinde]

Hi Chris,
The NI-Scope sample rate set in niScope Configure Horizontal needs to be set to 64 MS/sec regardless. I believe this may be the issue. I'm attaching a simple example that acquires binary IQ data using the PXI-5620 (DDC enabled) and that makes use of the subVI I attached earlier in this thread. It also shows how to convert this binary data to scaled voltage data for plotting on an I vs. Q graph. Hopefully this answers your questions!

Regards,
Andy Hinde
National Instruments

[cdschmit]

Andy,

Yes, that was exactly it.  I modified your code (as best I could) to perform continuous acquisition.  I can't figure out how to get it to fetch whatever is available...I thought setting the timeout to 0 and setting the minimum sample requirement to 1 would work, but it seems to fetch exactly 1 data point and allows the Backlog to grow.

Chris

[Andy_Hinde]

Hi Chris,
If you set the number of samples to fetch equal to -1, NI-Scope will return whatever is in its backlog at that point in time. Hope this helps!

Regards,
Andy Hinde
National Instruments

[cdschmit]

Andy,

Actually, I did that too.  It grabs exactly one sample (the minimum sample length).  Code attached.

Chris

[Andy_Hinde]

Hi Chris,
You're right - I see the same behavior although I fetch 2000 points every time when I set this field to -1. I think this is because we're in continuous acq mode and the default acquisition size is 1000 samples (2000 samples treating I and Q as separate 16 bit samples).
In any case I think this is probably a moot point since you really do not want to use a small fetch size in continuous acquisition mode. The smaller the fetch size the smaller the ratio of samples fetched in proportion to the fixed costs of the driver calls and everything that must happen regardless of how much data is returned. Using small fetch sizes will prevent you from being able to keep the Fetch Backlog from growing and eventually overflowing your onboard memory.
In my experience a fetch size between 100,000 and 300,000 samples allows for highest throughput. I modified the example I posted before to incororate continuous acquisition (and took out all SW processing and graphing from the loop) and am able to use the max DDC BW / Sample Rate and keep the Fetch Backlog down when using a fetch size of 100,000 samples. If I use 10,000 or below I start to overflow.

Let me know if you need more help on this!

Regards,
Andy Hinde
National Instruments

Message Edited by Andy Hinde on 03-15-2007 06:17 PM