Minimum Amplitude Needed to Measure a Signal's Frequency Using NI-DMM

Updated Apr 28, 2020

Reported In

Hardware

  • PXIe-4081

Driver

  • NI-DMM

Issue Details

I'm trying to measure a signal but it is giving me inconsistent results.
  • I am able to measure both the frequency and the amplitude of a 1 kHz signal when the amplitude is 150 mV.
  • Once I lower the amplitude to 50 mV I'm unable to measure the frequency of the signal.
  • I correctly set the digitizer range when changing the amplitude.
This has generated some questions:
  • Is there any relation between the frequency that I can measure and the amplitude of the signal?
  • Is there a minimum amplitude value that will allow me to measure a signal's frequency?

Solution

The device's frequency range is 15 Hz to 500 kHz

This error can appear if you don't set the correct value in "niDMM_ConfigureFrequencyVoltageRange". If you don't set up this value in the program it will default to 5 V. To correctly measure the frequency in this scenario you should lower this value to 150 mV and 50 mV.

Amplitude and frequency measurements are performed independently in the card.
  • Amplitude measurements are determined via digital signal processing.
  • Frequency measurements are determined using a zero-crossing detector and a counter.
You will also need to take into account the minimum amplitude when adjusting the digitizer range.