Dual Channel Waveform Measurements

[Clinton Thomas]

Hey,

I'm relatively new to LabView, so bear with me if this question is trivial.

I am trying to capture two channels with a PXI-5922. Channel 0 is the I-Data, Channel 1 is the Q-Data. I want to perform spectral measurements on both channels together (since they are just two parts of the same signal) so I am using the Dual Channel Spectral Measurements VI. Originally, I tried to use the multi-waveform fetch, sort the array coming out into two individual arrays, and pass each of those to the DCSM vi. When doing this, I'm getting an "Error -20305 (waveform index 0 of 1)" and the possible reasons are "Two time signal waveforms contain different number of data points", but I do not understand why this would be the case if both channels were configured identically.

I also tried seperating the Fetch after initializing the digitizer, Fetch Waveform for Channel 0 and Fetch Waveform for Channel 1. Again I receive the same error.

Can this VI be used with the NI-SCOPE digitizer functions? Perhaps I am missing something obvious here, but after investigating this I have been unable to arrive at a solution. Any help is greatly appreciated. Thanks


CT

[Clinton Thomas]

Here is my current code.

[S_Hong]

Hi Clinton,

It seems that the problem may be with the data being passed to your Dual Channel Spectral Measurements VI. The error you are getting is referring to the fact that the two waveforms passed to your Express VI may be of different size. Currently, your system will be fetching all available samples in your buffer as fast as it can since you have not set any wait functions in your loop. And since you have your fetch commands occurring one after another, it may be possible that on one iteration, your first fetch returns some data and your second fetch returns nothing due to the fact that the data for your second channel may have already been emptied at the previous iteration.

To fix this problem, you should control how much data is being fetch at every iteration by wiring the number of samples to acquire in your niScope fetch. Also, you may want to increase your fetch timeouts to ensure that there is enough data to be fetched on both channels at every iteration. Another recommendation is to place a wait timer in your loop in order to alleviate your processor for other tasks.

I have made these changes to the attached code. You will notice that I have also changed your trigger from immediate reference to software since on immediate, you are set to acquire only a 1000 samples in your record by default and stop. Therefore, you will have your spectral measurements plot update at the first iteration with your fetched data and every subsequent iteration will show a blank plot due to the lack of new data.

I hope this helps,

[Yardov]

Hi CT,

Notice as well that you can aquire two channels at once, and more important, that you want to calculate a spectrum of complex data (I and Q). Therefore, you need to combine your signal into a complex number. Here is an example:

Hope this helps,


Message Edited by Yardov on 08-06-2008 07:42 AM

[Clinton Thomas]

Gerardo,

Thanks for the suggestion. I have been trying to come up to speed on RF Signal Analysis and Aquisition since starting this project, but my progress has been slow. One question though, when I combine the two wfm signals into a single complex signal, I would like to re-bundle into a cluster so it can be imported into the SMT Zoom Power Spectrum function. Since both channels of the 5922 are being capture synchronously, can I assume that the timing data (xIncrement,relativeInitialX) are the same for both channels? Attached is an example of what I am talking about. Basically, I want to see the power spectrum FFT of both channels combined.


CT

Message Edited by Clinton Thomas on 08-06-2008 11:50 AM

[Yardov]

Hi CT,

Did not know you had SMT, this is great. Your assumption is totally correct, the two channels were acquired at the same time and therefore you can use any of the "dt" as the "dt" of your cluster.
You are not as slow as you think