What characterizes the reference sweep signal in radar?

The reference sweep signal in radar is characterized as a trapezoidal waveform whose rise time is proportional to the selected range. This means that as the radar system adjusts to different ranges, the shape and timing of the waveform change accordingly. The trapezoidal form allows for an effective representation of the signal's rise and fall times, which are crucial for properly timing the radar's operation, especially in relation to target distances.

The rise time being proportional to the selected range ensures that the radar can accurately represent the distance to the target by modifying how long the radar signal takes to rise to its peak. This characteristic enables effective signal processing and ensures that the radar can accurately measure and analyze the time it takes for the signal to travel to the target and back, thereby determining the range.

In comparison, the other choices do not accurately describe the nature of the reference sweep signal. Fixed pulse widths may not take into account variable ranges, continuous waveforms do not provide the necessary information for range measurements, and varying voltage levels based on distance do not accurately characterize the trapezoidal waveform approach employed in reference sweep signaling.