Abstract: A reference signal from a reference signal source is supplied to a frequency transformer and a side band signal generator. The frequency transformer is formed mainly by up-converters and produces a frequency m times as high as the frequency f.sub.s of the reference signal. The side band signal generator outputs signals of base and harmonic frequencies f.sub.s, 2f.sub.s, 3f.sub.s, . . . and nf.sub.s (where n is less than m) and produces less phase noise than does the frequency transformer. The outputs of the frequency transformer and the side band signal generator are frequency mixed by a frequency mixer, and one frequency component in the frequency-mixed output is selected by a variable filter.

Abstract: A sweep voltage from a sweep voltage generator is provided to sweep start and stop adjusting means, the outputs of which are summed by a summing circuit and, by the summed output, the oscillation frequency of a voltage controller oscillator is controlled. The oscillation frequency of the voltage controlled oscillator and the output frequency of a reference signal generator set to a sweep start frequency are compared by comparing means. The output of the sweep voltage generator is put in a sweep start state and sweep start adjusting means is adjusted in accordance with the output of the comparing means, thereby to cause the oscillation frequency of the voltage controlled oscillator to agree with a set sweep start frequency. The output of the sweep voltage generator is put in a sweep stop state and sweep stop adjusting means is adjusted in accordance with the output of the comparing means, thereby to cause the oscillation frequency of the voltage controlled oscillator to agree with a set sweep stop frequency.

Abstract: The output from a first variable frequency oscillator is frequency divided by a variable frequency divider and the frequency-divided output is phase compared by a first phase comparator with a first reference signal. By the phase-compared output is controlled a second variable-frequency oscillator of higher frequency stability than that of the first variable frequency oscillator. The output from the first variable oscillator is frequency divided by another frequency divider and the frequency-divided output is phase compared by a second phase comparator on the basis of the output from the second variable frequency oscillator and, by the phase-compared output, the first variable frequency oscillator is controlled. By changing the frequency dividing ratio of the variable frequency divider, the oscillation frequency of the first variable frequency oscillator is set.

Abstract: First and second frequency signals are frequency converted by a frequency converter and the frequency-converted output and the output from a variable frequency oscillating means are phase compared by a digital phase comparator. The phase-compared output controls the variable frequency oscillating means, constituting a first phase lock loop of a wide capture range. The output from the variable frequency oscillating means is frequency converted by the first frequency signal and the converted output and the second frequency signal are phase compared by an analog phase comparator. By the phase-compared output is controlled the variable frequency oscillating means, constituting a second phase lock loop of a narrow capture range but a large loop gain. The output frequency of the variable frequency oscillating means is varied by changing the setting of the frequencies of the first and second frequency signals.

Abstract: The oscillation frequency of a variable frequency oscillator is swept by a sweep signal from a sweep signal generator and the oscillation frequency is compared with a preset sweep start frequency for detecting coincidence therebetween. Upon detecting the coincidence, the sweep operation of the sweep signal generator is stopped and, at the same time, the oscillation frequency of the variable frequency oscillator is made by phase lock loop means to be phase-synchronized with a reference frequency. When the phase synchronization is established, an error signal in the phase synchronization is held and the loop of the phase lock loop means is cut off and then the error signal is applied to the sweep signal generator to re-start the frequency sweep of the variable freqency oscillator. When coincidence is detected between the oscillation frequency of the variable frequency oscillator and a preset sweep stop frequency, the sweep operation is stopped.