RF pulse transmitter having incidental phase modulation (IPM) correction

DETAILED DESCRIPTION The present invention relates to an RF pulse transmitter, preferably for use in precision distance measuring equipment (DME/P) of a microwave landing system, having incidental phase modulation (IPM) correction.
In a preferred embodiment, the pulse transmitter produces RF "reply" pulses for the DME/P using a pulse modulator for generating a modulation drive signal, a synthesizer for generating a carrier frequency signal, a phase modulator for receiving the carrier frequency signal and generating a phase-modulated signal, and an RF modulation amplifier for receiving the phase-modulated signal and the modulation drive signal and in response thereto generating the RF pulses.
The RF modulation amplifier has, as an undesirable by-product, incidental phase modulation (IPM), which is then corrected according to the invention by circuitry in the pulse transmitter.
Specifically, a phase detector is provided for receiving a sample of the RF pulse and a reference signal (at the carrier frequency) and in response thereto generating a phase error signal.
A reference phase control circuit adjusts the phase of the reference signal applied to the phase detector to drive the phase error signal to a predetermined value at the pulse peak.
To correct for transmitter IPM, phase modulation equal in magnitude but opposite in polarity to the undesired IPM is then introduced into the RF modulation amplifier.
In particular, a pre-distortion circuit receives the modulation drive signal from the pulse modulator and generates a correction signal for use in correcting incidental phase modulation.
The correction signal, which is pre-modeled to be equal in magnitude but opposite in polarity to the expected IPM, is then applied to the phase modulator to adjust the phase of the carrier frequency signal applied to the RF modulation amplifier.
A phase correction control circuit controls the amount of pre-modeled correction signal applied to the phase modulator to thereby correct for the incidental phase modulation