Klystron
A klystron is a specialized vacuum tube (evacuated electron tube) called a linear-beam tube. Klystrons are used as an oscillator or amplifier at microwave and radio frequencies to produce both low power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for linear accelerators. It has the advantage (over the magnetron) of coherently amplifying a reference signal and so its output may be precisely controlled in amplitude, frequency and phase.
Russell and Sigurd Varian of Stanford University are generally considered to be the inventors of the klystron. Their prototype was completed in August 1937. Upon publication in 1939, news of the klystron immediately influenced the work of US and UK researchers working on radar equipment.
In the two-chamber klystron, an electron beam from the cathode of an electron gun is injected into a resonant cavity. The beam is held together by a parallel magnetic field and is attracted through a connecting passage (called a drift tube) to a second resonant chamber containing a positively charged anode. While passing through the connecting chamber the electron beam is velocity modulated (periodically bunched) by the weaker RF signal. The electrons are attracted to a positive anode contained in a second resonant chamber. As the bunched electrons enter the second chamber they induce standing waves at the same frequency as input signal. The signal induced in the second chamber is much stronger than that in the first.
In the reflex klystron, a single toroidal resonant chamber surrounds a tubular chamber. The electrons are fired into one end of the tube by the accelerator grid of an electron gun. After passing by the resonant chamber they are reflected by a negatively charged repeller plate for another pass through the chamber.
In the multicavity klystron, multiple toroidal cavities surround a cylindrical acceleration tube.
These amplifiers are used to produce HF, VHF, UHF, and EHF signals where such high amplitude (power) is required that solid state devices (semi-conductors) remain inadequate. Klystrons can be found at work in radar, satellite and wideband high power communication (very common in television broadcast and EHF satellite terminals), and high-power physics (particle accelerators and experimental reactors).
A klystron is a specialized vacuum tube (evacuated electron tube) called a linear-beam tube. Klystrons are used as an oscillator, amplifier or frequency multiplier at microwave frequencies to produce both low power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for linear accelerators. High power pulsed amplifier klystrons are also used as the transmitting device in advanced radar systems. It has the advantage (over the magnetron) of coherently amplifying a reference signal and so its output may be precisely controlled in amplitude, frequency and phase.
Russell and Sigurd Varian of Stanford University are generally considered to be the inventors of the klystron. Their prototype was completed in August 1937. Upon publication in 1939, news of the klystron immediately influenced the work of US and UK researchers working on radar equipment.
An amplifying klystron requires 2 or more cavities - 1 for input, one for output, and possibly several other intermediate cavities. Devices have been made with between 2 and 7 cavities, but 3 to 5 are the most common number for modern high powered devices. Electrostatic focussing is frequenctly used for 2 or 3 cavity klystrons, but larger multicavity klystrons require magnetic focussing in order to maintain a well focussed electron beam as it must traverse quite a distance. After passing through the cavity resonators, the spent electron beam impacts with the collector electrode. This is a heavily built electrode designed to absorb the electrons and dissipate the excess energy as heat. The electron beam is provided by an electron gun - a cathode and grid assembly designed to emit a finely focussed cylindrical electron beam.
A signal input to the first 'buncher' cavity is impressed across grids on opposite sides of the cavity, resulting in an alternating electric field parallel to the direction of travel of the electron beam in the tube. This impressed electric field serves to alternately increase or decrease the electron velocities in the beam. The electrons leaving the cavity enter an RF field free region known as the drift space. In this space the faster electrons catch up with the slower ones, resulting in bunches of electrons forming at the input frequency. If a second cavity is placed at the point where the bunching is at a maximum, the
bunched electrons cause an oscillating electric field to be produced inside the second 'catcher' cavity. More power may be coupled out of this cavity than was put into driving the first cavity, thus producing a net amplifying effect. A gain of 10dB (10 times) is typical of the 2 cavity amplifier klystron. In a klystron containing more than 2 cavities, the intermediate cavities are also tuned to the operating frequency. As they are of a very low loss design they build up high amplitude oscillating fields at the operating frequency. These in turn remodulate the beam, resulting in higher gain in the tube.
These amplifiers are used to produce UHF, SHF and EHF signals where such high amplitude (power) is required that solid state devices (semi-conductors) remain inadequate. Klystrons can be found at work in radar, satellite and wideband high power communication (very common in television broadcast and EHF satellite terminals), and high-power physics (particle accelerators and experimental reactors).
The 2 cavity amplifier klystron is readily turned into an oscillator klystron by providing a feedback loop between the input and output cavities. 2 cavitiy oscillator klystrons have the advantage of being one of the lowest noise microwave sources available, and for that reason have often been used in the illuminator system of missile targeting radars. The 2 cavity oscillator klystron typically generates more power than the reflex klystron - typically watts of output rather than milliwatts. As there is no reflector, only one high voltage supply is required, but must be adjusted to a particular value for the tube to oscillate. This is because the electron beam must produce the bunched electrons in the second cavity in order to generate output power. as the location of the second cavity is physically fixed with respect to the first, this must be done by varying the velocity of the electron beam to a suitable level. Often several 'modes' can be observed where a given klystron will oscillate.
The floating drift tube klystron. If the wall between the input and output cavities in a 2 cavity klystron is removed, the resulatant tube will have a single cavity with 2 gaps separated by the drift tube. This is effectively similar to the 2 cavity oscillator klystron with a lot of feedback between the 2 cavities. Electrons passing the first gap are velocity modulated by the electric field in the cavity, travel through the drift tube and emerge at the second gap in bunches, delivering power to the oscillation in the cavity. This type of oscillator klystron has the advantage over the 2 cavity klystron on which it is based of only needing one tuning element to effect changes in frequency. The drift tube is electrically insulated from the cavity walls, and DC bias is applied separately. The DC bias on the drift tube may be adjusted to alter the transit time through it, thus allowing some electronic tuning of the oscillating frequency. The amount of tuning in this manner is not large, and is normally used for frequency modulation when transmitting.
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 klysron 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.
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