Harmonics induced uncertainty in phase noise measurements

RF signals in VCOs, PLLs and synthesizers are characterized by signal power, harmonic content and phase noise. These parameters have to be accurately measured in order to guarantee the system performance. This paper focuses on the relationship between the signal´s harmonic content and the precision of the phase noise measurement in test equipment. The phase noise measurement´s accuracy is degraded when the measured signal has high harmonics. We demonstrate a case in which a 3^rd harmonic level of −7dBc results in a 12dB variation in the phase noise measurement´s accuracy. Based on our studies, the phase noise test setup should include filters between the source and the test equipment to bring down the harmonic level to −20 dBc max.