A high-efficiency RF transmitter using VCO-derived synthesis: CALLUM

The combined analogue locked-loop universal modulator (CALLUM) is an RF transmitter topology which produces a linear output through the use of non-linear but highly efficient RF power amplifiers. This is achieved through careful phasing of two constant-envelope vectors, each derived from voltage-controlled oscillators. The system up-converts and amplifies a baseband signal within a closed-loop feedback scheme. The optimal CALLUM system requires complex baseband processing, but a simplified version, known as CALLUM2 can be implemented using simple analogue circuitry. This paper provides some insight into the behaviour of such systems and gives results from an experimental CALLUM2 system in response to a modulating signal compatible with that used in the TETRA standard.