Abstract: A Cartesian amplifier is provided with a means of controlling its output power comprising a controllable attenuator in the amplifier forward path and a controllable amplifier in the feedback path. A control signal representing the required power level is applied to the attenuator and controllable amplifier and, in response thereto, the degrees of attenuation and amplification are varied in inverse proportion to one another.

Abstract: A Cartesian amplifier in which an input signal is pre-processed and split into two quadrature components. Both quadrature components are passed, in parallel, through an error amplifier (4), after which they are re-combined and up-converted to RF. The output of the amplifier is used to provide a feedback signal. This feedback is downconverted from RF to baseband and resolved into two quadrature components which are fed to the respective inputs of the error amplifier (4). Periodically the pre-processor (3) is switched into a calibration mode in which test signals are applied to the amplifier instead of the input signal. At these times the signal strength of the output of the power amplifier is measured and used to provide pre-distortion factors in the signal preprocessor (3) to improve amplifier linearity.

Abstract: In a transparent tone-in-band (TTIB) single-sideband-modulated communication system used to communicate digital data; to compensate for variations due, eg, to doppler shift of the pilot tone in the received signal, during the sweep of the frequency of a local oscillator used to tune the receiver, the output of a narrow band filter is monitored for the presence of the expected pilot tone. When the pilot tone appears to be present, the sweep is interrupted and the error rate of the demodulated digital signal is tested, the sweep being resumed if an excessive error rate is encountered.