Hello everyone out there ,

Is it my turn now? I have been on this list for a few months; find it very interesting, and have learnt a lot. I also hope I have added a few cents worth...

Competence is very high amongst contribituors, and I guess the following issue can be solved - or already have been solved by a few of you: A suggestion for approoach would be appreciated:

I would like to measure the frequency of signals in the 1 -- 20 Hz range, not using a DSP or anything other that the already engaged PIC 16C711. It uses a timer interrupt routine, that is it idles most of the time, but every millisecond it jumps out of the loop and polls the AD RES. The A/D is of course picking up the low freq. "AC" (actually 0 -- 5V) to be measured.

My idea was to see on each sample if the previous sample was smaller. If so, the voltage is ascending. When descending (implementing 3 bits of noice reducing "backlash") I have found a point in time when the peak was found. Correspondingly, I find the "valley". The idŽa is based on a counter that is stepped up every ms, and is used to measure the time between peaks and valleys. In other words, if there are 100 counts between a valley and a peak, we have ¸ wavelength being 100 ms long. 5Hz - right ?

I have 20 registers. If the frequency is 1.5 -- 2.5 Hz, I add one to the "2 Hz register". If the frequency is 18.5 -- 19.5 Hz, I call it 19Hz. After a few seconds of measurement, I should have a handfull of values in the registers, to correspond to the frequency components of the signal.

This should work for pretty and clear sinewaves. But these signals are actually noisy with - for instance - long "flats" to consider, when finding a correct point in time. Actually the signal can be considered a random pink noice.

The signal is in other words a bit complex for finding frequency components in a FFT-type fashion. If it is 5 Hz with a dent in it, it is maybe 80 % 5 Hz, 15 % 15 Hz . . so to speak. This is not imortant in detail. I would like to know that the base is 5 Hz.

Now to the problem. It doesn't  __  work. Am I too optimistic ? Should I realize that a PC or a DSP of some quick sort is needed for this? Am I better off "zero cross-detecting" at 2.5 Volts? 

So - the question is -  has anyone been walking this path before ? Am I on the right track ?

Sven