Radio transmitter

Abstract

A radio transmitter suitable for a frequency hopping communication system which transmits a phase modulated signal with a minimum of spurious signals and phase noise. A direct digital frequency synthesizer (DDS) generates a frequency hopping signal lying in the VHF band. A first phase locked oscillator reduces the spurious components of the frequency hopping signal to produce a corresponding signal lying in the UHF band. This UHF band signal is used as a reference signal for a second phase locked oscillator which sends the phase modulated signal via a voltage controlled oscillator (VCO) built therein. The second phase locked oscillator includes a modulator for further modulating the phase of the phase modulated signal or a frequency divided version thereof by a modulating signal. The output of the modulator is used as a signal to be compared by a phase comparator or a source of signal to be compared. Since the second phase locked oscillator has a phase locked loop band sufficiently broader than the signal band of the modulating signal, it not only generates the phase modulated signal capable of sufficiently following the modulating signal, but also suppresses phase noise of the phase modulated signal ascribable to the VCO.