PURPOSE: To only use a simple circuit by inserting a programmable delay line obtaining a stable delay time in response to a digital signal to a positive feedback circuit, and varying the delay time of the programmable delay line, thereby varying the oscillation frequency, and obtaining stably the frequency signal in response to the digital signal.

CONSTITUTION: When an N-bit digital signal corresponding to a signal wave is inputted to the programmable delay line 1, the programmable delay line 1 varies the delay time (τ) of the input signal in response to N-bit binary value. When the number of input bits is N, the delay time τ goes to 2N, and the programmable delay line 1 retards the input signal by the delay time τ of the kinds in response to the number N, thereby deciding the oscillating frequency. That is, the signal of the oscillation frequency is outputted from a 1st amplifier 2 in response to the delay time τ and amplified by an output amplifier 4. Thus, the digital signal is changed in response to the amplitude of the signal wave, and the carrier frequency is changed by the frequency signal oscillated in response to the varied digital signal.