The voltage threshold for triggering a Voltage-Controlled Oscillator (VCO) in phase demodulation can vary based on the specific design and application of the circuit. In phase demodulation, a VCO is often used to track changes in the phase of an input signal, and the frequency of the VCO's output is proportional to the phase difference between the input signal and a reference signal.
The voltage threshold for triggering the VCO might refer to the point at which the VCO starts to oscillate or respond to changes in the input signal. This threshold voltage can depend on factors such as the VCO's design parameters, the input signal's frequency, the desired sensitivity of the phase demodulation, and the overall system requirements.
In some cases, the threshold voltage might be set by the biasing and control voltage applied to the VCO. It's common to provide a control voltage that tunes the VCO to a certain frequency, and variations in this control voltage can affect the oscillation behavior of the VCO.
For a more precise answer, you would need to refer to the specific datasheet or technical documentation of the VCO being used in your phase demodulation circuit, or consult with an engineer experienced in RF (radio frequency) and analog circuit design. The threshold voltage can be determined through experimentation and testing in the context of your specific application.