The voltage threshold for triggering a Voltage-Controlled Oscillator (VCO) in Quadrature Amplitude Modulation (QAM) demodulation can vary depending on the specific implementation and the requirements of the demodulation scheme. However, I can provide you with a general understanding of how a VCO is used in QAM demodulation and the factors that might influence the voltage threshold.
In QAM demodulation, the received modulated signal is typically down-converted to baseband using a local oscillator (LO) signal that is generated by a VCO. The VCO frequency is controlled by a voltage input, and the VCO output is used to perform the mixing and demodulation process.
The voltage threshold for triggering the VCO in QAM demodulation is often determined by the demodulation scheme's design, the modulation order (e.g., QAM-16, QAM-64), and the characteristics of the received signal. In some cases, the threshold might be set to a specific level to ensure that the VCO locks onto the carrier signal and maintains synchronization with it. This can help in recovering the in-phase (I) and quadrature (Q) components of the modulated signal accurately.
The threshold voltage could be chosen to be above the noise floor of the received signal and below the level where distortion or non-linear effects start to impact the demodulation accuracy. It is also influenced by factors like the signal-to-noise ratio (SNR) of the received signal, the frequency offset between the VCO and the carrier signal, and the characteristics of the VCO itself.
It's important to note that the specific voltage threshold value can vary based on the implementation and system requirements. Designers often determine these threshold values through simulations, testing, and optimization to achieve the best demodulation performance for a given application.
If you are working on a specific QAM demodulation project, I recommend consulting relevant literature, documentation, or experts in the field for more precise guidance on setting the voltage threshold for the VCO in your particular context.