  We designed and implemented a signal generator that can simulate the output
of the NaI (Tl)CsI (Na) detectors pre amplifier onboard the Hard X ray
Modulation Telescope (HXMT). Using the development of FPGA (Field Programmable
Gate Array) with VHDL language and adding random constituent, we have finally
produced the double exponential random pulse signal generator. The statistical
distribution of signal amplitude is programmable. The occurrence time intervals
of adjacent signals content negative exponential distribution statistically.
