The voltage threshold for triggering a Voltage-Controlled Oscillator (VCO) in a Frequency Shift Keying (FSK) system can vary based on the specific design and requirements of the system. In FSK, digital data is transmitted by shifting the frequency of the carrier signal between two distinct frequencies, typically referred to as the "mark" frequency and the "space" frequency. The VCO is a key component in generating these frequency shifts.
The voltage threshold for triggering the VCO depends on factors such as the VCO's tuning range, the modulation index of the FSK signal, the desired frequency separation between the mark and space frequencies, and the voltage-to-frequency transfer characteristics of the VCO.
In FSK, the VCO is controlled by varying its input voltage, which in turn changes its output frequency. The input voltage typically corresponds to the digital data being transmitted, where one voltage level represents the "mark" frequency and the other voltage level represents the "space" frequency. The VCO's response to these voltage levels should be designed to achieve the desired frequency separation and modulation characteristics.
The voltage threshold, therefore, will be set in such a way that when the input voltage crosses this threshold, the VCO switches between generating the "mark" and "space" frequencies. The specific value of this voltage threshold can be determined through simulations, measurements, and considering the requirements of the overall FSK communication system.
It's important to note that the voltage threshold is just one part of the design considerations for an FSK system. The design also involves other parameters such as the frequency deviation, data rate, modulation index, and filtering to ensure proper demodulation at the receiver end. The actual threshold voltage value can be calculated or determined experimentally based on the system's requirements and the characteristics of the VCO being used.