A C-Band FMCW SAR Transmitter With 2-GHz Bandwidth Using Injection-Locking and Synthetic Bandwidth Techniques

A synthetic bandwidth architecture utilizing an injection-locked oscillator to enhance the bandwidth of a linear frequency-modulated continuous wave transmitter is presented. A 500-MHz narrow bandwidth chirp from a direct digital synthesizer is upconverted to four adjacent carrier frequencies with 500-MHz spacing to obtain an effective synthetic bandwidth of 2 GHz at the C-band (43%). The four adjacent frequencies are obtained through the injection-locking technique to achieve fast frequency switching. This effectively results in an improved range resolution of 7.5 cm. Single-sideband mixing is employed to reduce in-band spurs and to avoid the use of switched filters. The architecture demonstrates ranging capability with a standard deviation of about 0.8 mm (1.36σ_CRLB, 26-dB SNR). Synthetic aperture radar images obtained from the proposed transmitter show sidelobe levels of less than -27 and -36 dBc in range and azimuth, respectively.