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I have a quick question on sensor data decimation, which I'm sure is pretty easy but thought I'd check. I have a sensor that is sampling at 25Hz and the data is being sent across a serial RS232 connection to an external data logger, which is logging the data at 10Hz.

I think if I want to recover a true 10Hz signal, I should I pass the original 25 Hz signal through a decimation process (i.e. filtering followed by down sampling). Is this correct?

If it is correct, I was thinking that I should decimate the original 25Hz signal by passing it through a low pass filter with a cutoff frequency of ~10 Hz, to remove the higher frequency components. The filtered signal would then be down sampled to produce a final signal. This down sampling would be achieved by extracting a value every 2.5 samples from the filtered signal.

So in other words, the 1st value of the final signal would be the first sample of the filtered signal. The second value of the final signal would be the average of samples 2 & 3 from the filtered signal. Then the third value of the final signal would be sample 5 from the filtered signal, and the fourth sample would be an average between samples 7 & 8 etc.....

Hopefully that makes sense. I think that would provide me with a clean 10Hz signal.

Many thanks for your time and efforts on this, they are very much appreciated

Cheers

George
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The type of filtering you should use will depend in part on what you are using this signal for and how noisy the captured data is.

In general, you should not change the sampling frequency of the filter constantly, this could introduce artificial periodic noise in the captured data. My guess is that for this process you are sampling something that doesn't change rapidly. You might want to just take a rolling average of the last 3 samples, even though some of the data averaged into each logged sample will overlap.

Paul S
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