Why does my frequency measurement double

[Adrian_S.]

I have a Low-Frequency 1 Counter measurement in a VI that I wrote. There are two of them in the same program, and one of them reads accurately at ~118 Hz. The other one occasionally doubles the frequency. The true signal is ~30Hz, but the display reads 60Hz half the time. What am I doing wrong and how can I fix it?

[nitad54448]

you may need to look for "aliasing"

[Adrian_S.]

To my knowledge "aliasing" occurs to do undersampling a signal, and results in an apparently lower frequency. If the program has no problem distinguishing 118 Hz it is not undersampling 30 Hz. The frequencies stated have been measured by oscilloscope.

[Ben]

Try tracking the delta time between your freq checks. I fmemory serves me that VI accepts a delta time input that if not provided by the caller will be calcualated by the driver based on when it THINKS it was last checked.

Trying to help,

Ben

[Ben]

adding to the above...

a low pass filter (if availabel on borad for your widget) may help if the is a mismatch between the output/input impeadance resulting in reflection or "rining" which could give you multiple counts per unit time.

Ben

[altenbach]

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Ben wrote:

a low pass filter (if availabel on borad for your widget) may help if the is a mismatch between the output/input impeadance resulting in reflection or "rining" which could give you multiple counts per unit time.

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That's exactly the issue I had about 15 years ago reading pulses from an instrument. A simple termination resistor at the input eliminated the problem.