High Efficiency VHF Transmitters Based on Polar Modulator and Enhanced PWM Power Converter

This paper focuses on the design solutions needed to improve the global power efficiency of a VHF transmitter. The proposed approach is based on a polar modulation architecture and uses an enhanced PWM power converter (patent pending, PCT/IB2006/051827) to realize the envelope modulation. A custom algorithm is adopted to dynamically control the output envelope of the RF signal. In order to operate in real-time, the algorithm has been implemented in a FPGA. The AM to PM distortion of the polar transmitter has been carefully investigated and corrected exploiting either a feed-forward or a feed-back technique. As a result, a transmitter efficiency (final stage and amplitude modulator) of 62% was measured at 119 MHz for a 16 W average output power. In this experiment the VDL2 modulation standard was used (D8PSK modulation with lO.Sksps) obtaining an EVM of 0.30% and an ACPR less than -63 dBc. A final discussion on the possibility to export the proposed solutions to other RF communication systems and platforms is given.