The voltage threshold for triggering a Voltage-Controlled Oscillator (VCO) in Amplitude-Shift Keying (ASK) modulation is not fixed and can vary based on the specific design and implementation of the modulation scheme and the VCO circuitry.
In ASK modulation, the VCO's output frequency is varied based on the amplitude of the input signal. When the input signal is at a high amplitude, the VCO might produce a higher frequency output, and when the input signal is at a low amplitude, the VCO might produce a lower frequency output.
The exact voltage threshold for triggering the VCO can depend on factors like the design of the VCO circuit, the modulation index (the extent of frequency variation), the signal-to-noise ratio, the desired data transmission rate, and the characteristics of the modulating signal.
In some cases, the voltage threshold might be determined empirically through testing and simulation to ensure reliable and accurate modulation. Additionally, other components of the modulation system, such as amplifiers and demodulators, can also affect the overall performance of the ASK modulation scheme.
If you're working on a specific ASK modulation system, it's recommended to consult the circuit's datasheets, application notes, or relevant technical documentation to understand the voltage thresholds and modulation characteristics for that particular implementation.