All digital FPGA based PWM modulator for radio frequency transmitters

A complete all digital pulse width modulation (PWM) modulator for AM or DRM (using Kahn envelope elimination and restoration technique) radio transmitter has been developed, implemented in FPGA, and measured. The modulating process strictly uses digital signal processing and only a switched MOSFET power stage followed by a low-pass LC filter, which compose a source of an “envelope” supply voltage for a switching power stage of radio transmitter. The advantages of fully digital signal processing bring the possibility to simply implement the dynamic configuration and signal correction. However, the digital naturally sampled PWM implementation puts not only high demands on the discrete time resolution, but, consequently, it requires exceptional accuracy of interpolating algorithm not to cause unacceptable signal distortion within the modulator. Effect of the both phenomena, the time resolution and the interpolation accuracy, has been studied. The results are compared with the measurements of the PWM modulator experimental implementation, which demonstrates the system is applicable in AM or DRM radio transmitter.