Variable frequency signal generator

A variable frequency signal generator, comprising:

• a phase locked loop having a variable frequency oscillator which has a control port to which a frequency determining signal is applied, the output of the oscillator being fed via a frequency divider to a phase sensitive detector where it is compared with a reference frequency signal, the result of said comparison being arranged to generate a comparison signal which is fed to a loop filter which is coupled to said control port;
• means for applying a frequency control signal to the signal generator, comprising a first path being arranged to adjust the division ratio of the frequency divider, and a second path including means for combining said frequency control signal and said comparison signal to produce said frequency determining signal;
• and calibration means for adjusting the relative characteristics of said first and second paths to compensate for effects arising from tuning sensitivity dependence on frequency of said variable frequency generator, said calibration means including adjustable gain means located in at least one of said paths, a detection means, and coupled to the detection means a control means for adjusting the gain of said adjustable gain means, in a calibration mode of operation of said generator, said means for applying a frequency control signal applying a frequency modulation signal, a frequency instruction signal being provided to said frequency divider, and said detection means comparing said comparison signal with said frequency modulation signal whereby said control means adjusts the gain of the adjustable gain means to a value at which the output of the detection means is at a predetermined level, in normal operation of said generator at the frequency of said frequency instruction signal, the gain of said adjustable gain means being set at said value.