A coherent range and range-rate detection Ladar with large ranging dynamic range and high-repetition-rate (II) : Signal Modulation

For the "dual-band dual-modulation dual LO (DFDMDL)" range and range-rate detection ladar with large ranging dynamic range and high-repetition-rate, we analyzed the LFM modulator linearity of the signal, the "amplitude-frequency " characteristic of the local and emitted light signal, the period stability of the symmetrical triangular linear frequency modulation (STLFM) signal, which are needed to be considered in the ladar design. The signal modulation scheme for the system was proposed by comparison, simulation and measurement of different parameters. In the desgin, we used the acousto-optic frequency shifter (AOFS) to provide external modulation for high linearity STLFM signal, and the direct digital frequency synthesis device (DDS) to drive AOFS. We also proposed and used the DAC to adjust the RF drive source power to compensate the non-flat "amplitude-frequency" characteristics of the AOFS, the High Precison FPGA based TDC measures the STLFM period and the time between multiple STLFM signals. The result demonstrated that the DFDMDL system has a high accuracy of ranging and has the potential ability of pulse integration by using this scheme.