Design and implementation of a π/4-DQPSK transmitter for maritime VHF digital communications

In this paper, a design and implementation of π/4-DQPSK transmitter for the exchange of digital data and e-mail between shore and ship stations and/or between ship stations on the one of the maritime mobile service VHF channels are described. The designed transmitter is composed of a baseband modulator, a baseband-to-RF band up-converter with Cartesian feedback, a power amplifier, and a coupler. Base on the design, the π/4-DQPSK transmitter is implemented. The implemented transmitter is composed two boards. One is implemented for system control, external interface, baseband modulation and mixing with Cartesian feedback. The other is implemented for power amplifier, coupler, and generation of required DC voltages. The emitted power spectrum of implemented transmitter is measured using spectrum analyzer through 30 dB attenuator. The measured results show that the in-band signal power is slightly reduced and the out-of-band signal power is additionally reduced more than 30 dB when the Cartesian feedback loop is used.