Signal processing system

An apparatus for modulating a carrier signal on the basis of an input signal to generate an output signal, said apparatus comprising a control loop having a forward path and a feedback path,

• said forward path comprising:
• a converter operable to convert the input signal into a pair of signal components whose phases are orthogonal to each other;
• first and second combiners, each of the first and second combiners operable to combine a respective one of the pair of orthogonal signal components with a respective different one of a pair of demodulated feedback signals to generate a pair of error signals;
• a phase-shift controller operable to introduce a phase-shift from 0-360° into the said pair of error signals;
• first and second modulators respectively operable to modulate a carrier wave on the basis of the pair of phase-shifted signals output by the phase-shift controller to form a pair of modulated signals;
• and
• a third combiner operable to combine the pair of modulated signals to form the output signal,
• and said feedback path comprising:
• a feedback coupler for coupling a portion of the output signal into the feedback path;
• a splitter operable to split the coupled signal into a pair of orthogonal feedback signals whose phases are orthogonal to each other;
• and
• a demodulator operable to demodulate the pair of orthogonal feedback signals to form the said pair of demodulated feedback signals.