Model LO Phase Noise

A mixer transfers local oscillator (LO) phase noise directly to its output.

The preceding figure shows the transfer of phase noise from fLO1 to fIF1.

Create a Model with Phase Noise

The model ex_simrf_phase_noise introduces phase noise into the model from the section Create a Model with RF Interference. The first mixing stage downconverts the RF and image to fIF

To open this model, at MATLAB® command line, enter:

addpath(fullfile(docroot,'toolbox','simrf','examples'))
ex_simrf_phase_noise

View Simulation Output

The model uses subsystems with a MATLAB Coder™ implementation of a fast Fourier transform (FFT) to generate four plots.

  • The IF1 Display plot shows a power spectrum centered at the first intermediate frequency, measured between the first and second stages.

    The figure shows that the LO phase noise has been transferred to the image. The RF signal on the carrier fIF1 is not visible in the figure because its power level is below the phase noise power of the downconverted image signal. The two visible peaks are at the same power as the IF shown in the previous section, Create a Model with RF Interference.

  • The Output Display plot shows the downconverted RF with the images removed.

    The LO phase noise has been transferred to the receiver output. The peak signal power is the same as in the previous section, Create a Model with RF Interference.

If you have DSP System Toolbox™ software installed, you can replace the MATLAB Coder subsystems with Vector Scope or Spectrum Analyzer blocks.

Shaping the LO Noise Skirt

To simulate phase noise, the model phase modulates pink noise generated in the LO with Phase Noise subsystem.

The subsystem contains the following blocks:

  • A Random Number block outputs a Gaussian random number at discrete time steps to generate white noise.

  • A Gain block scales the signal by a factor of

    where fLO1 is the LO frequency and Prel is the relative noise power density in dBc/Hz.

  • A Discrete Filter block filters the uniform white noise to generate 1/f noise.

  • A Magnitude-Angle to Complex block phase modulates an input signal ϕ. The output is of the form exp().

  • An Inport block models a controlled voltage source in the SimRF™ environment, modulating the carrier fLO with the input signal exp(). The circuit envelope equivalent signal is exp[j(2πfLO1t + ϕ(t))].

If you have Communications System Toolbox™ software installed, use the Phase Noise block to add phase noise to a given input signal.

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