Patent ID: 8138841

Claim:
An apparatus for controlling the output phase of a VCO, comprising: a phase locked loop having a first input for receiving a reference signal from a low frequency oscillator, a second input for receiving a feedback signal and an output for controlling a VCO; a VCO coupled to the output of the phase locked loop; a feedback path coupling a feedback signal from the VCO to said second input of the phase locked loop; a phase shifter on said feedback path between the VCO and said second input of the phase locked loop, the phase shifter being arranged for shifting the phase of the feedback signal by a controlled amount; the phase shifter being a variable phase shifter comprising a high pass filter, a low pass filter and a switchable by-pass, wherein the phase shifter is capable of shifting the phase of the feedback signal through a range of 0 to 360 degrees; wherein when a desired phase shift is in the range of 0-180 degrees the low pass filter is used and the by-pass is switched to by-pass the high pass filter; and wherein when the desired phase shift is in the range of 180-360 degrees both the high and low pass filters are used and the by-pass is switched off; a reference oscillator buffer coupled between said low frequency oscillator and said phase locked loop; wherein the output phase of the VCO being controlled on the basis of said reference signal and said feedback signal which has been phase shifted by said phase shifter; said output phase of said VCO varying inversely proportional to said shifting of said feedback signal by said phase shifter.