Correct sampling rate and number of samples

[Pukhtun_Yum]

Hello everyone,

I have an incoming signal at different frequencies which i am acquiring through NI-USB 6441( minimum 1000 Hz sampling rate ).

The signal frequency starts from 4 Hz and goes until 40 Hz in 1 Hz increment. What sampling rate I should choose so that the number of samples remain same every time.

[Bob_Schor]

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@Pukhtun_Yum wrote:

I have an incoming signal at different frequencies which i am acquiring through NI-USB 6441( minimum 1000 Hz sampling rate ).

The signal frequency starts from 4 Hz and goes until 40 Hz in 1 Hz increment. What sampling rate I should choose so that the number of samples remain same every time.

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It seems to me that you do not understand digital sampling of a (continuous) analog signal.  Your signal "exists at every instant of time", but you measure ("sample") it at discrete "instants" of time.  For example, if your sampling rate is 1 kHz, it means that 1000 times/second you "sample" the signal, which means you get its value at 0.001", 0.002", 0.003", ...  You don't have data of what is happening between your samples, but assume (and "hope") you are sampling faster than anything of interest.

There are three "variables" to consider in sampling a signal -- sampling frequency (f), number of samples (N), and sampling time (T).  They are related, because sampling frequency, f, = samples (N) per (or divided by) sampling time (T), f = N / T.  

What most of us do is to decide on a fixed sampling frequency and number of samples for a given task, hence this means we also fix the sampling time (see the above formula).  In your example, you could sample at 1 kHz and collect 1000 samples at a time, which would mean you'd get a set of samples every second:  if f = N / T, then T = N / f = 1000 samples / (1000 samples/sec) = 1 sec.

Bob Schor