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

The combined analog locked-loop universal modulator (CALLUM) is a radio fre quency (RF) transmitter topology which produces a linear output through the use of nonlinear, but highly efficient, RF power amplifiers. This is achie ved 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 s ystem requires complex baseband processing, but a simplified version, known as CALLUM2, can be implemented using simple analog circuitry. This paper p rovides some insight into the behavior of such systems and gives results fr om an experimental CALLUM2 system in response to a modulating signal compat ible with that used in the terrestrial trunked radio standard.