Voltage control oscillator is designed using CD4046 IC. The signal input can be directly coupled to large voltage signals, or indirectly coupled (with a series capacitor) to small voltage signals. A self-bias input circuit keeps small voltage signals within the linear region of the input amplifiers. With a passive low-pass filter, the “4046A” forms a second-order loop PLL. The excellent VCO linearity is achieved by the use of linear op-amp techniques.
The VCO requires one external capacitor C1 to C5 (at pin no. 6 & 7) and one external resistor R1 (between R1 and GND). Resistor R1 and capacitor C1 determine the frequency range of the VCO. Resistor R2 enables the VCO to have a frequency offset if required.
The VCO output signal has a duty factor of 50% (maximum expected deviation 1%), if the VCO input is held at a constant DC level. A LOW level at the inhibit input (INH) enables the VCO and demodulator, while a HIGH level turns both off to minimize standby power consumption.