Microwave frequency measurement technology plays an important role in various defense and civil applications, such as electronic warfare, radar, and wireless communications. Microwave frequency measurements can be achieved by conventional electrical methods. Although high-resolution multi-frequency signal measurement is achieved, the frequency measurement range is limited by electronic bottlenecks, and the measurement system is susceptible to electromagnetic interference, while photonic-assisted frequency measurement based on photonics can not only overcome such problems but also has the advantages of high flexibility and high speed. In general, photonic-assisted instantaneous frequency measurement systems work in three ways, which are frequency power mapping (FTPM), frequency time mapping (FTTM), and frequency space mapping (FTSM). FTPM-based schemes have the advantages of good real-time performance and high accuracy, but most of them have great difficulties in measuring multi-frequency microwave signals. FTTM-based schemes are easy to implement the measurement of multi-frequency signals, but they are not applicable in scenarios with real-time requirements. FTSM-based schemes are suitable for handling multi-frequency signals while obtaining accurate frequency information in real time, but they often require filter arrays or wavelength division multiplexers (WDM), which can increase the system complexity and reduce the flexibility of the system. To address the problems of the FTSM-based scheme, we propose a multi-frequency signal transient detection scheme without optical filtering.

The experimental system is built by using optical simulation software. The system uses a non-flat optical frequency comb modulated by a sawtooth wave to determine the frequency range of the signal by using the beat-to-beam power ratio of the signal to be measured and the optical frequency comb as a reference and then calculates the exact frequency of the signal to be measured from the demodulated frequency information. The system uses the multi-frequency signal to be measured and the sawtooth wave to be modulated together, and the generation of non-flat optical frequency combs is realized while loading the electrical signal. The frequency information can be processed by a computer such as fast Fourier transform, and the multi-frequency measurement system can be realized in this way. In this paper, an electrical spectrum analyzer (ESA) is used to obtain the beat frequency results.

Firstly, the system is verified with single-frequency signal and multi-frequency signal, and the results are shown in Fig. 4 and Fig. 5, respectively, which are in accordance with the theoretical results. Then the frequency measurement is carried out within the measurement range of the system in steps of 300 MHz, and the measurement results are shown in Fig. 6, with the absolute error within 40 MHz, but two of the sampling points could not be measured because the signal frequency to be measured is in the middle and border position of the channel, resulting in the inability to get paired electrical signals after tapping the frequency. For such problems, another branch with different channel width is added to the original system. In addition, as the system is susceptible to noise, it is easy to cause the power ratio of weak signals to be unstable, so the influence of the power of the signal to be measured on the measurement results is analyzed, as shown in Fig. 9. The power ratio of the signal to be measured starts to stabilize from -10 dBm. Due to the internal waveguide structure and material of Mach-Zehnder modulator (MZM), the static operating point of its direct current (DC) bias voltage will shift with the change of external factors such as temperature, which will greatly affect its operating characteristics and system performance, so the bias voltage drift of MZM is discussed. It can still be measured accurately within the floating range of 1% up and down, and the results are shown in Fig. 12.

In summary, a channelized multi-frequency measurement system based on a non-flat optical frequency comb is proposed and analyzed. The sawtooth wave spectral power decreases step by step, and it is modulated into the optical domain by suppressing the carrier bilateral band modulation, which can form the non-flat optical frequency comb required by the system. When the signal is measured, the signal to be measured will be co-modulated with the sawtooth wave. This way can avoid the unbalanced change of multi-branch detection while reducing the complexity of the system and improving the stability of the system. By changing the sawtooth wave frequency, the measurement range of the system can be adjusted, which has a certain degree of flexibility. The simulation achieves multi-frequency measurements in the range of 0.3-40 GHz with an error of less than 40 MHz and a relative error of better than 4%. In addition, the power variation of the radio frequency signal to be measured and the effect of MZM bias voltage drift are analyzed, and the results show that the power ratio of the signal to be measured starts to stabilize from –10 dBm, and the frequency judgment is accurate. The system has a certain tolerance to the MZM bias voltage drift, and it can still measure accurately within ±1% variation.