Abstract
A wideband microwave phase noise measurement system is proposed based on quadrature phase demodulation of the mixing components of a signal under test (SUT) and its delayed replica. The time delay is introduced by a low-loss optical fiber, which can be sufficiently large to achieve a high phase noise measurement sensitivity, and the quadrature phase demodulation is achieved by photonic-assisted in-phase and quadrate (I/Q) mixing together with digital signal processing. Thanks to the optoelectronic hybrid quadrature phase demodulation, the use of feedback loops, which are usually required in conventional photonic-delay-line-based phase noise measurement systems, is avoided, and the measurable frequency range is expanded. An experiment is implemented. Accurate phase noise measurement of SUTs in a frequency range of 5-35 GHz is demonstrated. With a 2-km single-mode fiber serving as the photonic delay line, the phase noise floor is as low as -131 dBc/Hz at the offset frequency of 10 kHz. The proposed scheme can be applied for evaluating the performance of microwave systems using low-phase-noise and wideband tunable microwave sources.
Highlights
Phase noise is an important parameter for evaluating the short-term frequency stability of a microwave signal source in communication, radar and other systems [1]
A feedback loop is required in these systems to ensure that the signal under test (SUT) and its delayed copy are quadrature to each other before sent to a frequency mixer, which is usually implemented by dynamically controlling an adjustable electrical or microwave photonic phase shifter incorporated in the feedback loop
One promising method to eliminate the use of the feedback loop and the tunable phase shifter is to apply quadrature phase demodulation, in which the phase information is acquired from the in-phase and quadrature (I/Q) components of the signal [12,13]
Summary
Phase noise is an important parameter for evaluating the short-term frequency stability of a microwave signal source in communication, radar and other systems [1]. To measure the phase noise of signals in a large frequency range, the bandwidth limitation of electrical phase shifters and frequency mixers should be overcome, which can be solved by implementing microwave phase shifting and frequency mixing in the optical domain [8,9,10,11]. Thanks to the use of low-loss optical fiber as the photonic delay line, a large amount of time delay can be introduced, leading to a high phase noise measurement sensitivity. The 90° hybrid is replaced by a 180° hybrid to let the two driving signals applied to the DP-MZM be out of phase In this case, the output voltages from PD1 and PD2, denoted by v7(t) and v8(t) respectively, are calculated to be v7 (t ) = VDC + Q(t). Optical fiber delay line is introduced to the phase noise measurement system based on quadrature phase demodulation, which can provide a large amount of delay and helps to improve the measurement sensitivity
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