Ultra-broadband multi-tone frequency measurement based on the recirculating frequency shift of a frequency modulated continuous wave.

We proposed an ultra-broadband multi-tone frequency measurement (FM) approach based on frequency modulated continuous wave (FMCW). This work aims to achieve wide-range multi-tone FM without image interference, using electrical components with narrow bandwidth and low sampling rate, while maintaining high FM accuracy. The FM range is largely increased by extending the bandwidth of the optical FMCW through a recirculating frequency shift (RFS) loop, from 0.001 GHz-16 GHz to 0.001 GHz-437.5 GHz. The bandwidth-extended optical FMCW coherently beats with a continuous wave (CW) light modulated by the signal under test (SUT) at the balanced photodetector (BPD). The following low-pass filter (LPF) outputs pulses at the time when the frequencies of FMCW and SUT are equal, constructing frequency-to-time mapping (FTTM). Owing to the zero-intermediate-frequency (zero-IF) architecture, image interference is avoided. In addition, the up- and down-chirps of FMCW are used to achieve self-reference, avoiding the utilizing of reference signals, which realizes high FM accuracy. In the experiment, a FM within 0.1 GHz-43.5 GHz is demonstrated using an available microwave generator (MG) with a maximum output frequency of 43.5 GHz. The FM errors are kept within ±10 MHz for all frequencies with a mean and standard deviation of -0.3 MHz and 3.17 MHz, respectively. The multi-tone resolution is about 60 MHz at the FMCW chirp rate of 3.1998 G H z/µ s, which is consistent with the theoretical result. According to the theoretical derivation, the multi-tone resolution can be improved to 1 MHz by lowering the FMCW chirp rate.

Photonic-assisted fast broadband microwave vector network analyzer based on FMCW.

In the paper, we propose a photonic-assisted fast broadband microwave vector network analyzer (FB-VNA) based on frequency modulated continuous wave (FMCW). A photonic recirculating frequency shift (RFS) loop is used to extend the bandwidth of optical FMCW. The bandwidth-extended optical FMCW beats with the continuous wave (CW) light to generate the broadband electrical FMCW, which serves as the incident signal of the device under test (DUT). The response signals of the DUT are modulated on the bandwidth-extended optical FMCW to perform de-chirping. After coherently beating the de-chirped light with the CW light, the broadband response signals of DUT are down-converted to a single-tone intermediate frequency (IF) signal carrying the frequency response of DUT, and the scattering parameters of DUT can be obtained. The single-tone IF signal relaxes the demand on the bandwidth and sampling rate of the electrical backend. Thanks to the RFS loop and the short period of FMCW, the measurement frequency range is highly extended and measurement speed is greatly accelerated at the same time, which can be applied in monitoring sudden changes of DUT features. A bandwidth multiplication of the FMCW from 6-18 GHz to 6-498 GHz is experimentally implemented. With available photodetectors (PDs) and Mach-Zehnder modulators (MZMs), a 6-54 GHz FB-VNA is demonstrated, and the S parameters of a 25-GHz low-pass filter (LPF) is measured within 6 µs. The sudden changes of S21 parameter of DUT simulated by fast adjusting the bias voltage of the MZM used for de-chirping are also characterized by the proposed FB-VNA. The sudden changes as short as 0.01 µs can be captured.