Any transmitted signal must have "clean" waves, with no jagged edges or voltage spikes. The transmitter itself may put forth a clean signal, but somewhere along the line from the transmitter to the receiver, the signal can get frayed. Something as simple as a wind-bent antenna introduces phase noise into the transmitted signal.
Highly sensitive test equipment is designed to look for phase noise. First, a reference standard produces a clean wave. The clean wave is introduced into the testing equipment alongside the suspected wave. The testing equipment compares the two signals, and produces a display of the suspected noise. Testing equipment used includes an oscilloscope and a frequency analyzer.
The test equipment can only tell the technician the noise is there. It's up to him to find the root cause, and eliminate it. He usually runs what's called a circuit trace. Starting at the transmitter, he uses other testing equipment to trace the signal as it travels through the system. When he finds the source of the noise, he can replace defective parts.
Sometimes, the testing procedure itself introduces noise, or eliminates it. If a technician is not careful, the testing equipment will give a false reading. The Telestrian Limited company points out the reference source itself may be flawed. Myriad reasons exist for flawed testing. Warren F. Walls of Femtosecond Systems Incorporated states that finding phase noise is not easy. He recommends averaging the findings to increase the confidence level.