HELP:Low-pass filter on frequency counter

[caius]

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)?:

http://cgi.ebay.com/AGILENT-HP-10856A-LOW-PASS-FILTER-KIT-5KHz-50KHz-500KHz-/120668081898?pt=LH_DefaultDomain_0&hash=item1c1860ceea

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

 

[Rich Webb]

On Fri, 21 Jan 2011 19:07:56 +0100, "caius" <briccus@yahoo.it> wrote:

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)?:

http://cgi.ebay.com/AGILENT-HP-10856A-LOW-PASS-FILTER-KIT-5KHz-50KHz-500KHz-/120668081898?pt=LH_DefaultDomain_0&hash=item1c1860ceea

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

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