Ni-5406 & Ni-5112 frequency sweep

[ratus]

Hello,

I have a Frequency generator (5406) that I use to generate a frequency Sweep (1 ms per Frequency).

When I read the signal with a Digitizer  (5112) it looks like the duration at each frequency is wrong.

I have attached a picture that (I hope) help to identify the problem.

Here, the sinus starts at a frequency of 20 kHz at 1710 (coordinates, left panel).

Because I sample at 3 MHz and because each Frequency step is 1 ms, I expect to see the next step at 4710.

Which is not the case (right panel) 

Any help appreciated

Also attached to this message is a copy of part of the block diagram.

Thanks 

[Jennifer O · Accepted solution]

HI there,

The explanation for what you are seeing is that although you ar setting your NI-5112 to sample at 3MS/s, you are actually sampling at 3.03MS/s. 

If you refer to the device specifications  you will note that this board has a maximum sample rate of 100MS/s and can sample at a decimated rate of 100MS/s /n where n is an integer between 1 and 100e6.  If the sample rate you request is not achievable by the digitizer, the driver will coerce the value to the next achievable sample rate which is greater than the one you requested.  In this case n = 33 and your actual sample rate is 3.03MS/s.

So by your calculation for 1ms, you would acquire 3000 samples at sample rate or 3MS/s.  This would increase to 3030 samples at a rate of 3.03MS/s.  I believe this matches what you see in your image.  To be certain of the actual rate your board is sampling at, try using the VI "niScope Sample Rate" to query the actual sample rate of your digitizer.

- Jennifer O.

Message Edited by Jennifer O on 01-08-2009 07:47 AM

[ratus]

THANKS

It looks like this is the correct answer.