Frequency Measurement

[rgarcia]

I´m using a Laptop with a Daq Card AI-16XE-50 and LabVIEW 4.1 and
NI-DAQ 6.5.1.
I have a lot of SCXI modules (1122, 1160, 1162, 1180) and their
terminal blocks.
I still want to measure the frequency of the pulses I have from an
indutive sensor of proximity. I tried using the counter (measure
frequency). But I can´t put it to work and if I could, I couldn´t
count more than 2 counters. I need more than 6. I tried using the
digital port (the sensor outputs a digital sign) and I constructed a
VI, using a loop and sequence, that´s working. But when I use others
VIs using analogic channels at the same time, I realized that the
digital VI have some delay.Now I want to know, again, how should I do
to measure my frequency. Should I transform the digital VI that´s
working to analogic? Should I use some VI to measure the frequency
assuming that my digital sign is analogic? (I tried the threshold peak
detector before).

[A Rafiq]

hi rgarcia,

Measuring frequency using the digital lines of the board will not be
the accurate way of measureing frequency. But those digital lines read
the pulses based on software timing. Counters are best for measureing
frequency but since you have only two counters, you can measure
frequency of two signals.

Now the best option would be to use the SCXI-1126 frequency module to
measure 8 frequency signals (from 0 to 128KHz range). You will get
accurate frequency information based on hardware timing.

One more workaround would be to acquire the time domain signal using
analog channls off of DAQCard then using the software routines, you
can extarct frequency spectrum information from it. The example of
such technique is "Benchtop Spectrum Analyzer.vi" under signal
processing in labview.

A Rafiq
National Instruments
http://www.ni.com/ask

[Stu]

If you end up acquiring the digital signal with analog channels,
insure that the time resolution of the sampling rate is adequate for
your requirements. Counter Timer resolutions are much better than
analog sampling. I would not suggest the benchtop spectrum analyzer
to measure the frequency content of a digital (square wave) signal.
You are really looking for leading edge to leading edge period and
this is a threshold delta time type calculation.
Stu

[markwysong]

If you can use the digital port, you can probably use the counter. If
you have access to the PFI lines of your DAQ board, you can multiplex
these PFI lines to the gate of 1 counter, and though the readings
would not be simultaneous, you could read the frequency of up to 10
digital signals with one counter.

Check out the item on the NI website:

http://zone.ni.com/devzone/devzoneweb.nsf/opendoc?openagent&4229043E5852EC998625683A000A5410&cat=33E541E2AEBC0B5986256786000FC83D
This should show you the VI to do this.

Mark