The voltage threshold for triggering a Voltage-Controlled Oscillator (VCO) in a Phase-Locked Loop (PLL) synthesis can vary depending on the specific design and application. Generally, in a PLL, the VCO generates an output frequency that is proportional to its input control voltage. The voltage threshold, in this context, refers to the control voltage level at which the VCO starts oscillating or generating a signal.
The exact voltage threshold for triggering a VCO can be determined by the design specifications of the PLL and the VCO itself. It's influenced by factors such as the VCO's tuning range, the frequency range of operation, the specific application requirements, and the technology used in the PLL circuit.
In some designs, the voltage threshold might be a specific voltage level (e.g., 0.5V or 1.0V) that corresponds to the lower limit of the VCO's tuning range. This ensures that the VCO starts oscillating as soon as the control voltage reaches this level. In other cases, the threshold might be more nuanced and determined by the loop filter and other control circuitry in the PLL.
It's important to note that PLL designs can be highly customized, and the voltage threshold for triggering the VCO can be adjusted according to the desired performance and specifications of the system. If you're working on a specific design or application, it's recommended to consult the datasheets and design guidelines for the components you're using, as well as any relevant literature on PLL design.