| 摘要 |
<p>This application concerns a modulation unit for generating a modulated output signal 142 from an in-phase modulating input signal 102 and a quadrature modulating input signal 104. A first modulator 110, which operates at a high power, quadrature modulates a carrier signal 130 with the input signals 102, 104. The carrier signal is attenuated 132 and a second modulator 120, which operates at a low power, quadrature modulates the attenuated carrier signal with the input signals to generate a reference signal. A detector 150 compares a signal 148 tapped off from the output signal with the reference signal to generate an amplitude error signal, or an amplitude and phase error signal, which provides a feedback signal to control a correction modulator 166 to reduce the amplitude error. The correction modulator may act on the carrier signal and provide an input to the first modulator, or alternatively it may act on the output of the first modulator (266. Fig. 2). Because the second modulator operates at a lower power than the first modulator, it has better linearity characteristics that the first modulator. The feedback from the detector is used to make the output of the modulation unit adopt the close-in spectral characteristics of the second, lower power modulator, but by operating at the greater power of the first modulator, less amplification is required.</p> |
