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Cant generate frequency lower than 5 HZ on PCI6723

I need to generate sine wave down to 0.05Hz.

However the frequency seem be distorted when the frequency lower then 5Hz.

my seting on freequency generator as follow:

-Samples per second(Hz) = 50000

-Number of samples = 20000

- Run as fas as possible

How can I do this using LabView and the NI 672c card?

canany one help?

Many Thanks!

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Hi,

  you have to remember that the 6723 is an 800kS/s 13 bit board.
That means on full range (+-10V) the voltage steps are 2.4mV apart.
If you want to run at 0.05Hz, and your time setup is 50kHz, then your time resolution is 1/50kHz = 20uS.
Translating that back to impose on your 0.05Hz signal, it's going to be very blocky, which means that each element you output is going to be a group of signals stepped together. If you do a power spectrum, then you would see the steps as high frequency content.

You'd need to place a low pass filter on the output in hardware to get a less distorted signal out I'm affraid.

Have a look at the attached code which shows effectively what's going on by simulating a signal and then breaking it back down into the representation for a I16.
You can see the stepping around the minimum and maximum values quite clearly. If you stop the code and then zoom in on the zero crossing section, you should see the continuous flat segments. (About 80 elements long = 80x20uS = actual time resolution at the fastest of 1.6mS and the slowest changes (at the peaks and troughs of about 350 x 20uS = 7ms)

Hope that helps

Sacha Emery
National Instruments (UK)

 

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