Abstract: A wideband phase locked loop oscillator (PLLO) for tracking the frequency of an altitude signal in an FM/CW type radio altimeter. The phase locked loop oscillator conventionally includes a voltage controlled oscillator (VCO), a phase comparator for comparing the phase of the altitude signal to be tracked with the phase of the VCO output and an error signal amplifier for applying the output of the phase comparator as control voltage to the VCO. The frequency range of the PLLO is extended by changing the natural frequency of the VCO in steps spanning the useful frequency range of the altitude signal. Selection of the proper value of VCO natural frequency is effected through microprocessor directed digital control.