wave frequency

[fadetoblack]

Hello

I am using a NI USB 6211 DAQ and Lbview to acquire AC voltage. Before I test in the equipment that I want to measure, I'm testing it at an AC Voltage supplier (230 V, 50Hz)

My question is about how the frequency of sampling rate can afect the result of the frequency of the AC wave. I've seen that with sampling rate of 10kHz my values of wave frequency are far away from the 50Hz.

Using 1 or 2 kHz gives me the desired 50Hz... I'm using Tone to capture Amplitude, Frequency and Phase...

Also, I'm not sure about the value of phase...

Thanks for the help

[MattBradley]

Hmmmm..... doesn't seem to me you should be seeing anything different based on the info you've posted.  As long as you are sampling above Nyquist freq (which in your case would be 100 Hz), the frequency should be fine and as long as you are sampling above 2.56*50 or roughly 130 Hz, you should be able to reconstruct amplitude as well.  Since you are well over 10x the frequency of interest, it should work fine.

So, a couple of questions:

1) Did you look at the spectrum and can you verify that the frequency is at 50 Hz or not?

2) Do you have any hardware filtering?

As for phase, if you have a 50 Hz signal you would need to get the phase (from the phase component of the spectrum) at 50 Hz.  The phase at frequencies where there is no signal is virtually meaningless.

Any chance you can post your code as well?

[fadetoblack]

Hello

First I would like to thank for your reply

I am sampling far beyond Nyquist frequency so, as you say, there shouldn't be problems...

Answering your questions:

1) As I mentioned, I'm doing tests, so I'm using a domestical electrical plug - 220V and 50Hz -, but I'm not looking to the spectrum. Another question: What is your suggestion to analyze the spectrum? What command should I use?

2) No filters. Just a simple voltage divider with 1/100 relation to connect it to the DAQ

Assuming that domestic plugs really have 50Hz, I verififed the influence of sampling freq, but, for phase I don't have a clue if it is correct or not.  

I attached the code as bitmap. I connected the DAQ Assistant dialog box to Tone Measurement box, where I take max Amplitude, Frequency and Phase.

Thanks for your help