The Spectrum measurement allows you to view signal in the frequency domain within the frequency span that you have configured.

The following list contains the basic properties to configure in a Spectrum measurement:

  • RF Center Frequency and Span—Configure RF center frequency and span to observe the frequencies of interest around the RF center frequency. Alternatively, you can specify the start and stop RF frequencies.
  • Resolution Bandwidth (RBW)—Configure RBW based on the required resolution of frequencies and DANL.
  • RBW Filter—Configure RBW filter to Gaussian or Flat in order to digitally emulate the RBW filters used in traditional swept analyzers. NI recommends using FFT based RBW filtering to get the best speed performance.
  • FFT Window—Configure the FFT window type when you are using FFT based RBW filtering based on the tolerance in spectral leakage requirements of the application as mentioned in the Spectral Measurements topic. When using other RBW filters, NI recommends using the Flat Top window. 

The amplitude of the frequencies is reported in dBm, a logarithm scale referenced to 1 mW. Other units supported are dBm/Hz, dBW, dBV, dBmV, dBuV, W, V, and V2.

The following image shows the RF center frequency and span for a Spectrum measurement.

Zero Span

Zero span is a special type of spectrum measurement in which you set the span to zero. The spectrum measurement returns the time-domain power trace as seen through an RBW filter. Sweep time specifies the duration of the acquisition.

The RBW specifies the acquisition bandwidth and cannot exceed the signal analyzer’s maximum device instantaneous bandwidth.

Zero span acquisition bandwidth is calculated using the following formula:

Zero Span Acquisition Bandwidth = RBW + Excess Span required by RBW Filter

Zero span acquisition sample rate is calculated using the following formula:

Zero Span Acquisition Sample Rate = 1.25 * Zero Span Acquisition Bandwidth

The following figure shows a zero span acquisition.

NI recommends the following settings to configure a zero span measurement:

  • Span: Set span to 0 Hz.
  • RBW: Configure the RBW to specify the 3 dB bandwidth over which the time domain signal needs to be analyzed. Ensure that the RBW does not exceed the instantaneous bandwidth of the signal analyzer.
  • RBW Filter: NI recommends using the flat or FFT-based RBW filter when using a zero span measurement. The FFT-based RBW filter implies an all pass filter.
  • Sweep Time: Configure the sweep time to specify the duration of acquisition.