In frequency-domain reflectometry (FDR) applications, a voltage-controlled oscillator (VCO) is often used as a signal source for generating a continuous-wave (CW) signal with a frequency that can be controlled by an applied voltage. This technique is commonly used in measurements such as time-domain reflectometry (TDR) and vector network analyzers (VNAs) for impedance or distance measurements.
The voltage threshold for triggering a VCO in FDR is not typically referred to as a specific voltage value. Instead, the VCO's frequency is adjusted based on the voltage applied to its control input. The relationship between the control voltage and the resulting oscillation frequency is determined by the design of the VCO circuit.
In FDR applications, the VCO's frequency is swept over a range of frequencies, and the reflected signals from the device under test are measured at each frequency. By analyzing how the reflected signals change with frequency, information about the impedance, distance, or characteristics of the device under test can be extracted.
So, in summary, there isn't a fixed voltage threshold for triggering a VCO in frequency-domain reflectometry. Instead, the control voltage applied to the VCO determines the frequency at which it operates, and this frequency is swept over a range to perform measurements on the device under test. The specifics of the VCO's behavior and control voltage requirements would depend on the particular design and implementation of the FDR system.