To keep a satisfactory display frequency accuracy when a wide band span is set in the same manner as in the set of a narrow band by receiving a frequency obtained by multiplying the oscillated frequency of a variable frequency oscillator and the oscillated frequency of a voltage control oscillator by a sampler, and phase-comparing the multiplied difference frequency with the oscillated frequency of a fixed frequency oscillator.

Since the oscillated frequency F4 of a variable frequency oscillator 40 is multiplied n-times by the multiplier 62 of a sampler 60, and mixed with the oscillated frequency F1 of an YIG oscillator 20 by a multiplier 61, F1 is expressed by F1=nF4+F3. The difference frequency F3 is phase-compared with the oscillated frequency F2 of a fixed frequency oscillator 40 by a phase comparator 50, and the error voltage of the difference is added to the voltage of a lamp wave generator 10 by an adder 80, and fed back as the control voltage of the oscillator 20, whereby a PLL circuit is constituted. Since the frequency F3 is equal to F2, and F1 is equal to nF4+F2, the oscillated frequency F1 of the oscillator 20 forms a variable frequency of wide range in which the oscillated frequency F4 of the oscillator 40 is extended n-times.