Abstract
Abstract A high-quality broadband radio frequency linear frequency modulation (RF-LFM) signal generation method based on the optical frequency quadrupling and the direct digital waveform synthesize (DDWS) with time-domain pre-distortion compensation is theoretical analyzed and experimental demonstrated. In this method, the optical frequency quadrupling is realized by beating the higher-order sidebands using a dual-parallel Mach–Zehnder modulator (DPMZM) so as to achieve the RF-LFM signal output. Since this optical frequency quadrupling link will induce the amplitude and phase distortions for the LFM signal, a time-domain pre-distortion compensation technique is used based on polynomial fitting. The experimental result shows that a LFM signal with the bandwidth 12 GHz and pulse width 10 μ s at the central frequency 22 GHz can be obtained. With the pre-distortion compensation the output LFM spectrum flatness is improved from 6dB to 2dB, the phase error is reduced from 30°to 5°, and the peak side lobe ratio (PSLR) is improved from -10.12dB to -13.01dB, which is close to the theoretical value -13.26dB.
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