Initial Control Voltage Circuit for VCO Locking Speed

The initial control voltage generating circuit is activated during an initialization period before normal PLL operation begins. This preliminary action establishes a predetermined initial control voltage at the VCO input, allowing the oscillator to start at a known frequency state. The circuit includes a resistor connected between a power supply terminal and an output node, and a transmission gate resistor connected between the output node and a ground terminal. During initialization, the transmission gate is in a first state that connects the resistor to the power supply, establishing the initial voltage. After initialization, the transmission gate switches to a second state that connects the resistor to ground, allowing the voltage to be discharged. This preliminary voltage establishment significantly reduces the time required for the PLL to lock onto the target frequency, as the VCO does not need to ramp up from zero or random voltage levels.

The transmission gate operates in periodic cycles rather than continuously. During each initialization period, the transmission gate enters a first state that connects the resistor to the power supply terminal, establishing the initial control voltage for the VCO. After a predetermined time interval, the transmission gate transitions to a second state that connects the resistor to the ground terminal, discharging the voltage. This periodic switching is controlled by an initialization signal that is active only during the initialization period. By limiting the power supply connection to discrete initialization periods rather than maintaining continuous connection, the circuit achieves fast VCO response when needed while minimizing energy consumption during normal operation.