The voltage threshold for triggering a Voltage-Controlled Oscillator (VCO) in frequency modulation (FM) can vary depending on the specific design of the VCO and the FM modulation scheme being used. In FM, the VCO's output frequency is directly proportional to the input voltage applied to its control port.
In a basic implementation, the voltage threshold to trigger a VCO in FM might be the voltage level at which the VCO starts producing an audible or measurable change in frequency. This threshold voltage can vary widely based on factors such as the VCO's tuning range, its linearity, the modulation index of the FM signal, and the desired frequency deviation.
Typically, the threshold voltage for triggering a VCO in FM will be above the VCO's control voltage offset, which is the voltage level at which the VCO produces its nominal output frequency without any modulation. This is to ensure that the modulation signal has a noticeable effect on the VCO's frequency output.
For a more specific value, you would need to refer to the datasheet or specifications provided by the manufacturer of the particular VCO you are working with, as it can differ from one VCO model to another. If you're designing your own FM system, you might need to experimentally determine the appropriate threshold voltage based on the characteristics of your VCO and modulation scheme.