Linear Wideband VHF/UHF Quad LINC Transmitter System

This paper presents the design, build and measurements of a wideband quadrature out-phased transmitter system based on Linear Amplification with Non-linear Components (LINC) that covers the 30 to 450 MHz VHF/UHF band with 185 Watts output power. The key in this transmitter design is the decomposition of the baseband signal into in-phase (I) and quadrature (Q) components that get digitally processed and up-converted, resulting in two constant-envelope signals each that can be amplified efficiently with non-linear class-D amplifiers. After amplification, the output signals get combined to reconstruct the amplified, original up-converted, non-constant envelope signal. The result is improved linearity of the transmitter system and reduced sensitivity to components mismatch as opposed to conventional LINC technique. This transmitter system has potential to enable Joint Tactical Radio System (JTRS) to use highest throughput "non-constant envelope" waveforms such as Orthogonal Frequency Division Multiplexing (OFDM) and Quadrature Amplitude Modulation (QAM) to provide the warfighters access to higher data rates and greater communication range. The superior linearity and efficiency of the transmitter system is demonstrated in this paper for various complex varying envelope signals such as AM, 8-PSK, QAM and OFDM, and is compared with traditional LINC systems as well as legacy backed-off class AB power amplifiers.