Signal generator utilizing a combined phase locked and frequency locked loop.

摘要

A programmable low noise frequency modulated signal source including a voltage controlled oscillator (VCO) having a frequency locked loop (FLL) constituting a first feedback path and a phase lock loop (PLL) constituting a second feedback path is provided. The PLL includes a VCO (11), a programmable fractional-N frequency division network (13) for changing the rational number by which the VCO output signal is frequency divided, a phase detector (15) for comparing the phase of the VCO output signal with the phase of a reference signal and for producing an error signal for controllably adjusting the output frequency of the VCO (11). The FLL includes a delay line frequency discriminator (10), a loop amplifier (33) and filter (31) to provide a first feedback signal to the VCO (11) to thereby reduce the phase noise on the VCO output signal. The frequency discriminator (10) includes a first signal path having a frequency sensitive time delay network (27) to provide a phase shift as a function of the VCO output signal frequency and a second signal path which includes a voltage controlled phase shifting network. The VCO tune voltage derived from the PLL phase detector (15) is coupled to an input terminal at the voltage controlled phase shifting network. The tune voltage adjusts the phase difference between the two frequency discriminator signal paths to set the operating point of the FLL phase detector (29) such that the VCO output signal will have a desired frequency. Since the PLL VCO tune signal is coupled to the control input of the voltage control phase shifting network (25) the FLL phase detector operates without an offset voltage at its output. Low distortion frequency modulation of the (FM) VCO output signal is achieved by coupling an FM signal to the voltage controlled variable phase shift network (25).