Question on sampling frequency/low pass filter

[hob]

Hi all,

I have a question relating to sample rates and data filtering.

I have a (relatively) high sampling rate of 100Hz, there is some high frequency "noise" present that acts over the range of 1-15Hz.

My question is, given that I am interested in much lower frequencies (slow movement around 250 seconds /4x10^-3 Hz) is there anything to be obtained from higher frequency sampling?

I.e provided my sample rate is sufficiently high (say 0.5Hz) and I apply a low pass filter (at 0.25Hz) to prevent aliasing, is there any more to be gained compared with higher freq logging with a low pass filter at 1Hz?

 

[AlephZero]

The answer depends on the power spectrum of the measured data, and the effects of aliasing when you sample the data.

As an extreme example, suppose the signal you are interested in at 0.004 Hz has an amplitude of about 1, but there is a noise signal at about 10 Hz with amplitude 10.

if you sample at about 0.5Hz, the sampled data will be mainly the noise, and the 10 Hz will be aliased to some frequency between 0 and 0.25Hz (the Nyquist frequency). That may cause problems when you filter the data.

On the other hand if you sampled at 100 Hz, if would be trivial to filter out the noise at 10 Hz.

Actually, in real life the best technique is often to use an analog filter before sampling, to make sure there that aliasing won't happen. If you did that, a sample rate of 0.5 Hz (or even lower) would probably be OK.