FCnt is used to accurately measure the frequency of a tone, for example, the tone generated by a local oscillator. The rate of change in the instantaneous phase of the time domain signal determines the signal frequency. The signal is acquired for a duration specified by the measurement interval and is digitally filtered by an RBW filter.

The following list describes the basic properties that you can configure for an FCnt measurement:

  • RBW: Configure RBW to specify the 3 dB bandwidth over which time domain signal needs to be analyzed. Ensure that RBW does not exceed the instantaneous bandwidth of the signal analyzer. NI recommends using no filter or a flat RBW filter. Refer to the Zero Span section of the Spectrum topic for information on the sample rate required by the measurement.
  • Measurement Interval: Configure the measurement interval to specify the duration of acquisition.

Allan Deviation

Frequency stability of oscillators is affected by different types of noises such as flicker noise and random walk noise apart from stationary white noise. Allan deviation is a measure of the frequency jitter arising from these types of noises. If are instantaneous frequencies measured at regular time intervals, Allan deviation is computed as two-sample standard deviation of this data as shown in the following equation.