Signal Analyzer

The frequency of a signal source always has some random fluctuations in its phase of frequency. This can be examined and measured with the Signal Source or Phase Noise Analyzer. It measures the power present at a certain frequency distance away from the carrier frequency, relative to the total carrier power. A typical result looks like this:

 

Measurement principle

The simplest way to measure the phase noise is to compare the Device-Under-Test (DUT) to the source of a spectrum analyzer. With the SA set at the same frequency as the OUT, you see the sum of the SA's and DUT's sideband-power spectrum on the SA display. It is a simple and straight forward method well suited for free running VCO's where the SA is easily an order better.
You can improve on this method by establishing a Phase Lock (PLL) between the DUT and the Local Oscillator and create a zero-IF or base-band spectrum analyzer system.

A better method is to mix to DC with the DUT's signal (1) and the same frequency, but delayed (2) in such a way that the phase noise becomes de-coupled or uncorrelated between the 2 signals.

But the best method is using a cross-correlation principle. Here you measure the same signal twice while it is being mixed down to DC via 2 branches with similar, but distinct, reference sources. Then the cross-correlator analyzes the 2 DC+noise spectra and filters out the difference components (from the 2 different sources!), leaving over the common noise contributions in both branches, so the phase noise of the DUT.

With this method you can easily measure DUT phase noise levels which may be approx. 20dB LOWER than the reference sources!