The problems associated with single sideband (SSB) radio transmitters are primarily those of achieving both a spectrally clean output and a high DC to RF conversion efficiency. In the majority of applications the linearity of a transmitter is considered to be a more important requirement than efficiency, making the transmitter a poor convertor of DC into RF energy. An approach whereby both good linearity and efficiency may be obtained is known as the Polar-Loop Technique in which a large amount of negative feedback may be applied to a radio transmitter without compromising the stability of the RF amplifiers. This is achieved by resolving the SSB signal into polar coordinate form to produce two signals: one proportional to the instantaneous amplitude of the RF signal and the other to its instantaneous phase. These component are functions of the modulating signal and have bandwidths which are much lower than those of the RF circuits. By applying feedback in each component the excess phase shift introduced by the RF circuits into the feedback loops is small and consequently, a large amount of feedback may be employed without causing instability. This thesis is primarily concerned with the application of the Polar-Loop Technique to high frequency (HF) SSB radio transmitters. The early chapters of this thesis discuss the important aspects of HF transmitter design, practice and constraints together with methods of improving the linearity or efficiency of such transmitters. Subsequently, the Polar-Loop Technique is introduced and the properties and potential sources of spurious emissions from transmitters of this type are described. Several possible transmitter configurations suitable for broadband HF applications are presented and the problems associated with the implementation of HF Polar-Loop Transmitters are outlined. In order to demonstrate the properties and capabilities of transmitters based on the Polar-Loop Technique a 100 W broadband HF unit has been designed and evaluated and the results obtained using this transmitter are presented and described in detail.
|Date of Award||1983|