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How to Measure Sine Wave Phase Noise

When amplifying or modulating a sine wave with electronic equipment, the output signal will unfortunately always have noise components introduced. Through the use of filtering components on the output of a circuit, engineers can produce a signal that represents the input as accurately as possible by blocking unwanted noise and static. These filters can be designed using the phase noise values found when comparing the output signal against the input.

Things You'll Need

  • Oscilloscopes
  • MATLAB
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Instructions

    • 1

      Connect two oscilloscopes to an open terminal on the circuit, with one measuring the voltage on the input and the other on the output. As the sine wave readings are an AC signal, the oscilloscope test probe must be connected with the tip touching the live terminal and the black alligator lead attached to a grounded voltage point. Although the oscilloscopes will be measuring separate voltages, both probes should be connected to a common ground point.

    • 2

      Switch on the circuit and take readings from the input and output using the results displayed on the oscilloscopes. Before copying results from the readings, make sure that the axes on the displays of each of the oscilloscopes are set to the same scale using the adjustment knobs found on the face of the equipment. If your oscilloscope has a USB connection and can send data plots to a PC, it is preferable to take advantage of this method as the results will have a greater degree of accuracy than those taken by hand. You should aim to take 50 measurements at the input and output, so doing this by hand may take at least an hour if you are not familiar with the process.

    • 3

      Import your data sets into MATLAB, creating four separate arrays to hold the x- and y-axis data for both the input and output measurements. Use the FFT() fast Fourier transform function along with the plot() graph function to convert the data to the frequency domain and display the results on a graph. For example, plot(([1:X-1]/X), FFT(Y,X)) would produce a frequency domain plot where X and Y represent the arrays holding data for the x- and y-axes.

    • 4

      Create plots for both the input and output and analyze the results side by side. The input plot should show a single impulse and represents the frequency of the sine wave entering the circuit. The output plot will show the same single impulse, but will also display a number of smaller impulses. Each of the impulses represents the frequency of noise components, and their phase noise values in radians can be calculated by using the formula [((Fi ̵1; Fn)/Fi) * 2pi] where Fi is the impulse frequency of the input signal and Fn is the impulse frequency of the noise component.


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