Phase Noise Spectrum

Menu Path: Trace > Data > Phase

The Phase Noise Spectrum is the spectrum of the Phase Error Time trace, computed using the frequency/phase analysis parts of each selected pulse. It is computed similarly to how the FM Frequency Modulation Meas Spectrum is computed from FM Meas Time:

• Each selected pulse is analyzed separately.
• Each selected pulse may contain multiple frequency/phase analysis parts, depending on the modulation detected on the pulse. For CW and Linear FM, there is only one frequency/phase analysis part. For Triangular FM there are two. For Barker Phase modulation there is one frequency/phase analysis part for each BPSK Binary phase shift keying - A type of phase modulation using 2 distinct carrier phases to signal ones and zeros. bit detected.
• Each frequency/phase analysis part of each selected pulse is analyzed separately, and the results are RMS averaged together to produce the Phase Noise Spectrum trace.
• The FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. length is the maximum of either:
  • the Max Spectrum FFT Length
  • the largest power of two smaller than the number of points in the entire Acquisition Time trace
• The data window is specified by the Spectrum Window.
• If the frequency/phase analysis part is shorter than the FFT length, the data is windowed and then zero-padded to the FFT length.
• If the frequency/phase analysis part is longer than the FFT length, multiple FFTs are done to cover the entire frequency/phase analysis part, with the window applied to the data going into each FFT. No zero padding is done. The results are RMS averaged together.
• The spectrum data is scaled so that it has units of dBc/Hz amplitude in a 1 Hz bandwidth relative to the carrier, and by default it is displayed using a log X axis. (These are different than the FM spectrum traces which are not scaled this way and which default to linear X axis.)

If measurement averaging is enabled, then the Phase Noise Spectrum trace is averaged from one measurement to the next, while the Inst Phase Noise Spectrum is the spectrum of just the most recent measurement. If no measurement averaging is enabled, the Phase Noise Spectrum and Inst Phase Noise Spectrum are the same.