my analog in freq is 10X my out

[mrochon]

I am using cDaq with 9263A0 and 9215AI modules.  Simple test setup at the moment, AO0 directly into AI0.  The problem is that my aquired signal's frequency is an order of magnitude larger than the one I am generating.

HELP

[Raajit_Lall]

Hi,

This could possible be occuring because of aliasing.  What is the actual frequency that you are generating from the 9263 and what are .  Also, what is the sampling rate that you are using on both devices?  Are both of these cards on the same cDAQ chassis or different ones and how have you connected grounds on the two cards.

Also, are you using LabVIEW to make these measurements or are you using some other environment.  Are you always seeing 10 times the frequency on the input side?  I would need a few more details to be able to help you out.

Thank you,

[mrochon]

I think I figured it out but some affirmation would be great.

I was generating the waveform at any given frequency less than 2X the sample rate, at 1kHz with 100sample buffer size.  So 10 buffers of 100 samples each every second, right?  Then was doing the D/A with the 9263 at 10kHz with a 1000sample buffer size.  So 10 buffers of data of 1000 samples every second?

So what I figure is that to the 9263, every buffer has 1000 samples, which are made up of 10 buffers from the signal generator of 100 samples, thus the 10X frequency difference.

I guess my next question is that does the LabView signal generator have to be at the same sample rate and buffer size as the analog output module?

[Raajit_Lall]

Hi,

You are right.  As soon as you are generating/acquiring at lesser than the Nyquist frequency you will be seeing aliased signals.   What is really happening with the rate and the number of samples is a little different though.  The two parameters that you specify are rate and number of samples per second.  If you specify the rate as 100KS/s, that just means that you are acquiring 100,000 Samples every second on the hardware side of things.  Let's say that we put in 1000 as the number of samples, that means that your programming environment, LabVIEW for example, is going and fetching 1000 samples, whenever it can, from the buffer (which is getting filled at 100,000 samples per second).  This will just mean that if you have a while loop around your DAQmx Read, that while loop is going to run at 1000 times every second.

As far as matching the output modules sampling rate with the input modules sampling rate, that is not absolutely necessary, as long as you are not violating the Nyquist theorem anywhere.  So if you are generating a sine wave of a frequency of 100Khz (keep in mind that this is different from the output sampling rate), your input sampling rate should be atleast 10 times that for a good accurate signal, i.e. 1000Khz.  So as long as you are meeting those requirements you should be fine.

I probably repeated a lot of things that you said but I hope this helps.  Please post back with any further queries.

Regards,

Message Edited by Raajit L on 10-24-2007 04:25 PM