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These depend on electron bunching. A beam of electrons is modulated by a field over a short distance. The faster electrons catch up with the slower ones that went by earlier. At a certain distance the electrons are in bunches. These go through a short distance which is a hole through a cavity. These pulses of electrons excite fields in the cavity.
The klystron was invented several times in the decades preceding the Varian brothers, but they were the ones who made commercial products of them.
In the reflex klystron, the electron beam passes through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun. After passing through the resonant cavity they are reflected by a negatively charged reflector electrode for another pass through the cavity, where they are then collected. The electron beam is velocity modulated when it first passes through the cavity.
The formation of electron bunches takes place in the drift space between the reflector and the cavity. The voltage on the reflector must be adjusted so that the bunching is at a maximum as the electron beam re-enters the resonant cavity, thus ensuring a maximum of energy is transferred from the electron beam to the RF oscillations in the cavity. The reflector voltage may be varied slightly from the optimum value, which results in some loss of output power, but also in a variation in frequency. This effect is used to good advantage for automatic frequency control in receivers, and in frequency modulation for transmitters. The level of modulation applied for transmission is small enough that the power output essentially remains constant. At regions far from the optimum voltage, no oscillations are obtained at all.
There are often several regions of reflector voltage where the reflex klystron will oscillate; these are referred to as modes. The electronic tuning range of the reflex klystron is usually referred to as the variation in frequency between half power points—the points in the oscillating mode where the power output is half the maximum output in the mode.
Modern semiconductor technology has effectively replaced the reflex klystron in most applications.
Crystal Oscillator is used for mobile communications.
It provides re-flow compatibility for mobile communications, Bluetooth, and wireless LAN applications. Product resonates at frequencies from 26,000–60,000 MHz and features motional series resistance of 100 ohm @ 26,000–34,999 MHz and 50 ohm @ 35,000–60,000 MHz with load capacitance of 8 pF. Frequency tolerance is ±10 ppm @ 25°C and operating temperature range is -30 to 85°C.