HELP:Low-pass filter on frequency counter

Jan 21, 2011 1 Replies
C

Hi all, I have to made some measurements with a frequency counter (model VC3165) on low frequencies (up to 60 Hz) .I know that in case of low frequency measurements a low-pass filter is needed in order to obtain stable and precise readings. My frequency counter has only an AC/DC button (to be used with a low-pass filter as said in the service manual) but it doesn't have an integrated low-pass filter (many counters have it).



I built some simple RC filter ( using a resisitor and a capacitor) with various cut-off frequncy ( I tried 15 Khz and 50 Khz) but I alway obtain floating and not precise readings on low frequencies. How can I solve this problem?Maybe using a low-pass filter like these(very expensive IMHO)?:



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or building a proper one (but as said before simple RC filters didn't work)?


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R

You may want to look at the wave shape. If it's changing slowly and if there is high frequency noise riding on the input (there's always some) then you may be getting multiple trigger events as the signal crosses the trigger level.

If this is the case, you might improve things by inserting a comparator with some hysterisis between the signal and the counter. Using the comparator may give the counter a sharper edge to work with and the hysterisis will improve your noise immunity.

Rich Webb Norfolk, VA
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