An Innovative Real-Time Nonlinear Frequency Modulation Signal Generator for Airborne SAR

Synthetic aperture radar (SAR) typically transmits a linear frequency modulation (LFM) waveform. It utilizes weighted windowing to depress the peak-to-sidelobe ratio (PSLR), which results in a decrease in signal-to-noise ratio (SNR). In contrast, a nonlinear frequency modulation (NLFM) signal can reconstruct the power spectral density (PSD) to achieve lower sidelobes without compromising the SNR. Considering the substantial challenges posed by hardware constraints in the real-time generation of complex NLFM waveforms, this letter proposes a piecewise linear (PWL) algorithm to optimize resource utilization while realizing parallel processing via pipeline execution and the Horner method. Meanwhile, an improved predistortion approach integrated with the PWL algorithm is utilized to compensate for the phase errors in the SAR system. The high-level synthesis (HLS) technique is applied in the waveform generator based on the radio frequency system-on-chip (RFSoC). The performance of the proposed method is validated through simulations, and practical experiments are conducted on an X-band airborne SAR system.