An approach for phase noise measurement of microwave signal sources based on a microwave photonic frequency down-converter is proposed. Using the same optical carrier, the microwave signal under test is applied to generate two +1st-order optical sidebands by two stages of electro-optical modulations. A time delay is introduced between the two sidebands through a span of fiber. By beating the two +1st-order sidebands at a photodetector, frequency down-conversion is implemented, and phase noise of the signal under test can be calculated thereafter. The system has a very large operation bandwidth thanks to the frequency conversion in the optical domain, and good phase noise measurement sensitivity can be achieved since the signal degradation caused by electrical amplifiers is avoided. An experiment is carried out. The phase noise measured by the proposed system agrees well with that measured by a commercial spectrum analyzer or provided by the datasheet. A large operation bandwidth of 5-40 GHz is demonstrated using the proposed system. Moreover, good phase noise floor is achieved (-123 dBc/Hz at 1 kHz and -137 dBc/Hz at 10 kHz at 10 GHz), which is nearly constant over the full measurement range.
A photonic-assisted scheme for the phase noise measurement of microwave signal sources is proposed based on the optical delay-line method. In the proposed scheme, all the microwave signal processing is implemented in the optical domain, and the electrical devices that would limit the operation bandwidth and measurement sensitivity are avoided, leading to a large operation bandwidth and a high sensitivity. The feasibility of the proved phase noise measurement system is experimentally verified. A large operation bandwidth of 5-40 GHz is achieved, and a phase noise floor as low as −140 dBc/Hz at 10 kHz offset is obtained.
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