A relaxation oscillator is formed by a Schmitt trigger circuit and an operational integrator circuit interconnected to form an oscillatory feedback loop. A diode switching network is constructed to receive a plural-bit digital input signal and to modify a circuit parameter in the oscillator feedback loop to adjust the frequency of oscillation in accordance with the coding of the digital input signal. An active low-pass filter circuit is connected to the output of the operational integrator circuit and supplies a sinusoidal tone signal to an output circuit. A switching circuit is responsive to the digital input signal for disabling the output circuit except during the occurrence of predetermined digital code values.