Measure frequency of 50 Hz with at least 3 digits

[dsasorin]

Hi.

I have Labview 8.2 , and I want to measure the network frequency (50Hz) with at least 3 digits and good accuracy.

Is there any solutions to do that in Labview , easy? I also have a DAQ 6210.

Thanks.

[johnsold]

Look at the examples which come with LV for measuring frequency or period.

The accuracy will be dependent of the accuracy of the timebase in your DAQ device. The resolution will depend on how you set up the measurement. To measure 50.0 Hz by direct frequency counting you would need to count at least 500 cycles of the signal, which will take 10 seconds. If you measure the period (~20 ms) you will get an answer much sooner. The timebase frequency would need to be high enough to produce the resolution you need. The period at 48.0 Hz is 20.83 ms. At 48.1 Hz it is 20.79 ms. To measure the difference of 0.04 ms would require a clock frequency of at least 25 kHz. You should do the math for the range of frequencies and resolutions you need.

Be sure to couple the signal safely to you DAQ device. Connecting the DAQ device directly to the line will destroy it!

Lynn

[hrh1818]

On May 12, 4:40 am, dsasorin <x...@no.email> wrote:
> Hi.
> I have Labview 8.2 , and I want to measure the network frequency&nbsp;(50Hz)&nbsp;with at least 3 digits and good accuracy.
> Is there any solutions to do that in Labview , easy? I also have a DAQ 6210.
> Thanks.

The DAQ 6210 is not appropriate for measuring the frequency of a
signal. A counter timer such as the NI6601 is the preferred choice
when using Labview to measure the frequency of a signal. If you are
trying to measure the frequency of a 50 Hz power line you will need to
attenuate the signal before it is applied to the input of a NI 6601
counter timer. The maximum input rating of the NI 6601 is 5 volts. To
meet your resolution and accuracy requirements measure the frequency
of your signal for a gate time of 20 seconds or longer.

Howard

[dsasorin]

Thanks, I will look for it.

[Nathan_R]

Hi dsasorin,

You can use your 6210 to measure the frequency of your signal. The 6210 has 2 - 32 bit counters on-board. It has built-in timebases of 80, 20 and 0.1 MHz available for use with the counters. You will still need to attenuate the signal. The maximum input rating for the 6210’s counters in 5 volts. They are TTL digital inputs.  Also the counter inputs are not designed for negative voltages. So you may need to rectify your signal as well as attenuating it. Examples of frequency measurements using the DAQmx drivers can be found in the NI Example Finder. In Labview go to Help >> Find Examples… >> Hardware Input and Output >> DAQmx >> Counter Measurements >> Digital Frequency.  Please let me know if you have any questions and take care.

Thanks,

[BruceAmmons]

If converting your signal to a digital signal is too difficult, just get it down to +/- 10 V and use a regular analog input daq channel.  Acquire it at the maximum rate possible, then measure the time between zero crossings.  If you measure the time between two rising zero crossings, you will have the period of the signal.  You can even interpolate if you want more accuracy.  It takes a little bit of work, but it is extremely accurate.  For even more accuracy, measure the time between 10 rising zero crossings and divide by 10.  On the hardware end, it only takes a couple of resistors to make a voltage divider.

Bruce

[mishklyar]

May be just to save your time...

We did something similar,and saw that network frequency has a little time of coherence,

in other words,we could measure accurately only a mean  frequency,but never could thay "Today the freq. of a network

is 50.007 Hz".

Best regards,

  Michael