The voltage threshold for triggering a Voltage-Controlled Oscillator (VCO) in a Quadrature Amplitude Modulation (QAM) system can vary depending on the specific design of the system and the characteristics of the VCO being used. However, I can provide you with a general understanding of how VCOs work in QAM systems.
In a QAM system, which is a type of modulation used in communication systems, the information is transmitted by varying both the amplitude and phase of the carrier signal. The carrier signal is generated by a VCO. The VCO's frequency is controlled by an input voltage. When the input voltage crosses a certain threshold, the VCO starts oscillating at a frequency corresponding to the input voltage.
The voltage threshold for triggering the VCO would typically be set based on the desired modulation scheme and the characteristics of the VCO circuit. In most cases, the voltage threshold is set to a level that corresponds to the minimum required input signal strength to reliably generate the carrier signal. This ensures that the VCO doesn't start oscillating due to noise or weak signals but only responds to valid input signals.
The exact threshold voltage can depend on factors like the specific circuit design, the VCO's control voltage range, the desired signal-to-noise ratio, and the modulation order of the QAM system (e.g., 16-QAM, 64-QAM, etc.). It's important to note that the threshold voltage might also need to consider the dynamic range of the VCO and the overall noise characteristics of the system.
Ultimately, the voltage threshold for triggering a VCO in a QAM system is determined through careful design and testing to ensure reliable and accurate modulation of the carrier signal. If you're working on a specific design, you might need to consult the datasheet or technical documentation of the VCO you're using and perform simulations or experiments to determine the appropriate voltage threshold for your application.