How can I output a frequency modulated signal following a function on an NI 5421?

[labJunky]

I would like to output a sin wave with a frequency that follows the below function, and would like to know if the NI 5421 can do this.

frequency(t) = Sqrt( b(t)^2 + [ C Sin(wt) ]^2 )

where b = 40 (MHz) - 39.5 (MHz/min) t 
           C = 2 MHz

and the t (time) goes upto 1 minute playtime.

If the NI 5421 can't do this, is there a different product that can? Also what are the memory requirements on the card for this.

Thanks,
Paul C

[Otis]

Hi Paul,

The NI 5421 is an ARB and as such can generate any arbitrary data that you give it.  Therefore, to generate something that appears to be a sweeping frequency then you will need to generate some code that will essentially create all of the data points that based off your formula and then load all of those data points onto the ARB.  The ARB will then output the entire signal that you gave it.

If you have a 5411 or 5431 at your disposal then you could take advantage of DDS and simply load up one sine wave and then clock it faster and faster essentially making a swept frequency signal. 

For the time being though you will need to generate the data points you want and then output that arbitrary waveform.

Regards,

[labJunky]

I require about 100 points per cycle which would need a huge amount of memory because I need the nonlinear frequency sweep to last 1 minute. So I think generating all the points and then sending them to the card to play is out of the question, no? Maybe I could perform some double buffering of the memory or playing the waveform from the computer memory?

Your second suggestion is a good idea. With these cards and the NI 5406 or NI 5404 can I change the frequency and amplitude, phase-coherently, on the fly? Or for example, can I create a list of frequencies and durations, instead of the whole set of data points, and have the card play a sin wave changing the frequency in a phase-coherent way? I may also need to change the amplitude of the sin wave. For example, ramping the amplitude up at the beginning and then down near the end, is this also possible?

Thank you for your help, its very useful!!
Paul C

[Otis]

Hi Paul,

Yes, with the 5404 or 5406 you can change the Frequency and Amplitude, phase-coherently, on the fly.

I hope this solution works better for you.

Regards,

[kirsch]

The NI 5406 supports Frequency List Mode, so you can download a list of Frequency-Duration steps. NI 5404 does not support this Output Mode (and is only available in PXI, in case you wanted PCI).

Keep in mind that the amplitude changes would need to be software controlled (as opposed to HW-timed like the Frequency List).

Marcos