Frequency modulation (fm) waveform generator circuit diagram

The circuit in the picture is a frequency modulation waveform generator that generates 220kHz. The internal voltage regulator tube of CD4046B provides a stable power supply voltage for the operational amplifier CA3140. Its rated voltage is 5.4V, and R5 is the current limiting resistor of the voltage regulator tube.

Connect the CA3140 to an inverting AC amplifier with an amplification factor of 20. Resistors R2 and R3 provide static bias for the op amp. Since R2=R3, the static bias voltage added to pin 3 of the non-inverting input of the op amp is about 2.6V. .Therefore, a potential equal to approximately 2.6V is generated at pin 6 at the output end of the op amp, which is the amplitude modulated by the audio input signal amplified 20 times. This signal is applied to the voltage controlled input of the CD4046B VCO, with the C3 and R6 component values ​​shown in the figure, causing the VCO output to produce an average output carrier frequency of 220kHz. This output frequency is still modulated by the original audio input signal.