Instantaneous frequency measurement scheme employing dual-parallel phase modulation

A scheme for instantaneous frequency measurement based on two phase modulators (PMs) in parallel with coupled dual-output is designed to instantaneously obtain frequency information of the unknown signal over a wide frequency range. In the proposed scheme, two parallel PMs are used to modulate the unknown signal with a tunable electrical time delay; the frequency information of the unknown signal is converted to phase shift because of the introduction of the electrical time delay and then detected by two photodetectors after a 2×2 optical coupler; an amplitude comparison function is established by shaping the corresponding relation between the output power ratio of the two photodetectors and the microwave frequency; and so the frequency information is measured. The simulation results illustrate that a frequency measurement range of 0.25-21 GHz with an error of less than 60 MHz is obtained. The frequency measurement range can be shifted by adjusting the electrical time delay, and so the frequency measurement with high performance can be obtained by utilizing a two-step measurement method that is the joint operation of a large range rough measurement and a small range precise measurement.