Output frequency is different then the set value

[MAnees]

I am generating sine wave using "simulate signal" block and sending it to DAQ Assistant to excite a shaker. The settings of Simulate signal and DAQ assistant are shown and the VI is also attached. The frequency being sent to DAQ is same. BUt when I am measuring the output frequency of shakr it is always coming higher. I am using BNC NI 9260 card and the shaker is a pssive device.

Can someone please guide where the problem is?

Thanks and regards

[MAnees]

I am generating sine wave using "simulate signal" block and sending it to DAQ Assistant to excite a shaker. The settings of Simulate signal and DAQ assistant are shown and the VI is also attached. The frequency being sent to DAQ is same. BUt when I am measuring the output frequency of shakr it is always coming higher. I am using BNC NI 9260 card and the shaker is a pssive device.

Can someone please guide where the problem is?

Thanks and regards

[RavensFan]

How do you know that the shaker is at a higher frequency?

[MAnees]

I am measuring it using a laser displacemnet sensor and getting voltage through Labview in another computer

[Ben]

The frequency (samples/second) of the output signal have to match the speed of the hardware device output update rate. Check to see if you can verify the two match-up.

Ben

[LocalDSP]

How much off is your measured frequency (in %, ppm or Hz) and is the error within the accuracy of your interferometer and/or DAQ board?

... and why is your DAQ Assistant configured to generate 1001 sample blocks?

[Henrik_Volkers · Accepted solution]

Looking at the datasheet you will find:

So, if you create samples for a different samplerate, not supported by the device, the resulting frequency will differ 🙂

Your example uses 1k SPS , and I don't know how the driver will coerce this.

Look for the examples, I bet there is a vi 😉

Finally, you will always find some differences between the frequencies, because the two units you use (one for generating, another to measure) don't share the same clock.

[MAnees]

My frequency is approx 1.61 times higher and the error is almost negligible. i just kept one sample extra to write to start the new wave with 1st point of new cycle however i m not sure about this. 

[LocalDSP · Accepted solution]

As Henrik Volkers pointed out earlier your module can only sample at specific rates and as stated on Henrik's datasheet screenshot, the lowest rate is 51200 / 31 ~= 1.652 kS/s. That is why it is running ~1.65 times too fast.

To generate a theoretically perfect 1 Hz you either need to generate (and repeat) 31 periods of signal using 51200 samples or probably easier, 1 period of 51200 samples but change your rate (on both your Create Signal and your DAQ Assistent) to 51.2 kS/s

[Henrik_Volkers · Accepted solution]

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@MAnees wrote:

My frequency is approx 1.61 times higher and the error is almost negligible. i just kept one sample extra to write to start the new wave with 1st point of new cycle however i m not sure about this. 

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According to the spec the lowers possible samplefrequency is 1.652 kHz , your output is a distorted sinewave with 1001 points...(assumed, I can't open your vi ) ...  so I would expect 1.6503 kHz ... almost fits 😄  

How do you measure the frequency?

Try it with a samplerate of 1652 Hz and generate 1652 sample 😉 

(for 1Hz, if you need to generate 0.8Hz you need to recalc the number of samples 😉 )

EDIT: LocalDSP... beat me with 5min 😉     

MAness: You should really learn some basics on digital signal processing. The hint with a higher samplerate is a good one!