What type of microwave oscillator can be tuned by changing the repeller voltage?

The reflex klystron is a type of microwave oscillator that can be tuned by changing the repeller voltage. This tuning mechanism allows for adjustments at specific frequencies, making it particularly useful for applications where fine frequency control is required.

In a reflex klystron, the electron beam is bunched by an input RF signal. By changing the voltage on the repeller, which is located at the output of the device, you can control how the electrons are repelled back towards the resonant cavity. This variation allows for modifications in the output frequency of the oscillator as the repeller voltage changes.

The other types listed do not operate on the same principle regarding tuning through a repeller voltage. Gunn diodes, for example, generate microwave frequencies through a different mechanism, relying on the transit-time effect rather than adjustment via voltage on a repeller. Magnetrons, while also capable of generating microwave frequencies, typically do not allow for precise tuning in the same way as the reflex klystron. BJT oscillators are based on bipolar junction transistors and commonly work in radio frequency ranges rather than the microwave spectrum, lacking the tuning capability provided by a repeller voltage.