Virtual-cathode microwave oscillator
For the virtual-cathode microwave oscillator-speed switching arc-discharge tube, see Krytron. For the klystrode, see Inductive output tube. This article needs additional citations for verification.
400 kW klystron used for spacecraft communication at the Canberra Deep Space Communications Complex. This is a spare in storage. 5 kW klystron tube used as power amplifier in UHF television transmitter, 1952. When installed, the tube projects through holes in the center of the cavity resonators, with the sides of the cavities making contact with the metal rings on the tube. In a klystron, an electron beam interacts with radio waves as it passes through resonant cavities, metal boxes along the length of a tube.
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The electron beam first passes through a cavity to which the input signal is applied. A reflex klystron is an obsolete type in which the electron beam was reflected back along its path by a high potential electrode, used as an oscillator. The first prototype klystron, manufactured by Westinghouse in 1940. Part of the tube is cut away to show the internal construction.
On the left are the cathode and accelerating anode, which create the electron beam. In the center between the wooden supports is the drift tube, surrounded by the two donut-shaped cavity resonators, the “buncher” and the “catcher”. The output terminal is visible at top. On the right is the cone shaped collector anode, which absorbs the electrons. Barkhausen-Kurz tube and split anode magnetron, which were limited to very low power. Hansen was instrumental in the development of the klystron and was cited by the Varian brothers in their 1939 paper.
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Allies used the far more powerful but frequency-drifting technology of the cavity magnetron for much shorter-wavelength one centimeter microwave generation. T used 4 watt klystrons in its brand new network of microwave relay links that covered the US continent. Klystrons amplify RF signals by converting the kinetic energy in a DC electron beam into radio frequency power. This beam passes through an input cavity resonator.
RF energy has been fed into the input cavity at, or near, its resonant frequency, creating standing waves, which produce an oscillating voltage, which acts on the electron beam. To reinforce the bunching, a klystron may contain additional “buncher” cavities. The beam then passes through a “drift” tube, in which the faster electrons catch up to the slower ones, creating the “bunches”, then through a “catcher” cavity. In the output “catcher” cavity, each bunch enters the cavity at the time in the cycle when the electric field opposes the electrons’ motion, decelerating them.
Thus the kinetic energy of the electrons is converted to potential energy of the field, increasing the amplitude of the oscillations. The spent electron beam, with reduced energy, is captured by a collector electrode. Positive feedback excites spontaneous oscillations at the resonant frequency of the cavities. The simplest klystron tube is the two-cavity klystron. In this tube there are two microwave cavity resonators, the “catcher” and the “buncher”. When used as an amplifier, the weak microwave signal to be amplified is applied to the buncher cavity through a coaxial cable or waveguide, and the amplified signal is extracted from the catcher cavity.
At one end of the tube is the hot cathode which produces electrons when heated by a filament. The beam first passes through the “buncher” cavity resonator, through grids attached to each side. The buncher grids have an oscillating AC potential across them, produced by standing wave oscillations within the cavity, excited by the input signal at the cavity’s resonant frequency applied by a coaxial cable or waveguide. Beyond the buncher grids is a space called the drift space. This space is long enough so that the accelerated electrons catch up with electrons that were accelerated at an earlier time, forming “bunches” longitudinally along the beam axis. Its length is chosen to allow maximum bunching at the resonant frequency, and may be several feet long. The electron gun is on the right, the collector on the left.
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The two cavity resonators are in center, linked by a short coaxial cable to provide positive feedback. The electrons then pass through a second cavity, called the “catcher”, through a similar pair of grids on each side of the cavity. The function of the catcher grids is to absorb energy from the electron beam. The bunches of electrons passing through excite standing waves in the cavity, which has the same resonant frequency as the buncher cavity. After passing through the catcher and giving up its energy, the lower energy electron beam is absorbed by a “collector” electrode, a second anode which is kept at a small positive voltage. An electronic oscillator can be made from a klystron tube, by providing a feedback path from output to input by connecting the “catcher” and “buncher” cavities with a coaxial cable or waveguide.
In all modern klystrons, the number of cavities exceeds two. Additional “buncher” cavities added between the first “buncher” and the “catcher” may be used to increase the gain of the klystron, or to increase the bandwidth. The residual kinetic energy in the electron beam when it hits the collector electrode represents wasted energy, which is dissipated as heat, which must be removed by a cooling system. Some modern klystrons include depressed collectors, which recover energy from the beam before collecting the electrons, increasing efficiency. Multistage depressed collectors enhance the energy recovery by “sorting” the electrons in energy bins. Low-power Russian reflex klystron from 1963.
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The cavity resonator from which the output is taken, is attached to the electrodes labeled Externer Resonator. Reflex klystrons are almost obsolete now. 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 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. Some klystrons have cavities that are tunable. By adjusting the frequency of individual cavities, the technician can change the operating frequency, gain, output power, or bandwidth of the amplifier.
Each unit has manufacturer-supplied calibration values for its specific performance characteristics. Tuning a klystron is delicate work which, if not done properly, can cause damage to equipment or injury to the technician due to the very high voltages that could be produced. The technician must be careful not to exceed the limits of the graduations, or damage to the klystron can result. Other precautions taken when tuning a klystron include using nonferrous tools. Precautions are routinely taken when transporting klystron devices in aircraft, as the intense magnetic field can interfere with magnetic navigation equipment. Special overpacks are designed to help limit this field “in the field,” and thus allow such devices to be transported safely.
The floating drift tube klystron has a single cylindrical chamber containing an electrically isolated central tube. Electrically, this is similar to the two cavity oscillator klystron with a lot of feedback between the two cavities. Electrons exiting the source cavity are velocity modulated by the electric field as they travel through the drift tube and emerge at the destination chamber in bunches, delivering power to the oscillation in the cavity. Klystrons can produce far higher microwave power outputs than solid state microwave devices such as Gunn diodes. Klystrons, Traveling Wave Tubes, Magnetrons, Cross-Field Amplifiers, and Gyrotrons. A High Frequency Oscillator and Amplifier”.
American Physics Society: Division of Plasma Physics Conference, Pittsburg, PA. This page was last edited on 18 April 2018, at 19:45. Magnetron with section removed to exhibit the cavities. The cathode in the center is not visible. The waveguide emitting microwaves is at the left.
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The magnet producing a field parallel to the long axis of the device is not shown. A similar magnetron with a different section removed. Obsolete 9 GHz magnetron tube and magnets from a Soviet aircraft radar. An early form of magnetron was invented by H. The cavity magnetron was radically improved by John Randall and Harry Boot in 1940 at the University of Birmingham, England.
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In the post-war era the magnetron became less widely used in the radar role. This was because the magnetron’s output changes from pulse to pulse, both in frequency and phase. This makes the signal unsuitable for pulse-to-pulse comparisons, which is widely used for detecting and removing “clutter” from the radar display. The idea of using a grid for control was patented by Lee de Forest, resulting in considerable research into alternate tube designs that would avoid his patents. One concept used a magnetic field instead of an electrical charge to control current flow, leading to the development of the magnetron tube.
With no magnetic field present, the tube operates as a diode, with electrons flowing directly from the cathode to the anode. In the presence of the magnetic field, the electrons will experience a force at right angles to their direction of motion, according to the left-hand rule. In this case, the electrons follow a curved path between the cathode and anode. At very high magnetic field settings the electrons are forced back onto the cathode, preventing current flow. At the opposite extreme, with no field, the electrons are free to flow straight from the cathode to the anode. At fields around this point, the device operates similar to a triode.
It was noticed that when the magnetron was operating at the critical value, it would emit energy in the radio frequency spectrum. This occurs because a few of the electrons, instead of reaching the anode, continue to circle in the space between the cathode and the anode. Early conventional tube systems were limited to the high frequency bands, and although very high frequency systems became widely available in the late 1930s, the ultra high frequency and microwave regions were well beyond the ability of conventional circuits. The bare tube, about 11 cm high. The original magnetron was very difficult to keep operating at the critical value, and even then the number of electrons in the circling state at any time was fairly low. This meant that it produced very low-power signals.
Nevertheless, as one of the few devices known to create microwaves, interest in the device and potential improvements was widespread. The first major improvement was the split-anode magnetron, also known as a negative-resistance magnetron. As the name implies, this design used an anode that was split in two — one at each end of the tube — creating two half-cylinders. When both were charged to the same voltage the system worked like the original model.
At any given instant, the electron will naturally be pushed towards the lower-voltage side of the tube. The electron will then oscillate back and forth as the voltage changes. At the same time, a strong magnetic field is applied, stronger than the critical value in the original design. This would normally cause the electron to circle back to the cathode, but due to the oscillating electrical field, the electron instead follows a looping path that continues toward the anodes. Since all of the electrons in the flow experienced this looping motion, the amount of RF energy being radiated was greatly improved. And as the motion occurred at any field level beyond the critical value, it was no longer necessary to carefully tune the fields and voltages, and the overall stability of the device was greatly improved. Unfortunately, the higher field also meant that electrons often circled back to the cathode, depositing their energy on it and causing it to heat up.
The great advance in magnetron design was the resonant cavity magnetron or electron-resonance magnetron, which works on entirely different principles. In this design the oscillation is created by the physical shaping of the anode, rather than external circuits or fields. A cross-sectional diagram of a resonant cavity magnetron. Magnetic lines of force are parallel to the geometric axis of this structure. Mechanically, the cavity magnetron consists of a large, solid cylinder of metal with a hole drilled through the center of the circular face. A wire acting as the cathode is run down the center of this hole, and the metal block itself forms the anode. The magnetic field is set to a value well below the critical, so the electrons follow arcing paths towards the anode.
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When they strike the anode, they cause it to become negatively charged in that region. As this process is random, some areas will become more or less charged than the areas around them. The anode is constructed of a highly conductive material, almost always copper, so these differences in voltage cause currents to appear to even them out. The oscillating currents flowing around the cavities, and their effect on the electron flow within the tube, causes large amounts of microwave radiofrequency energy to be generated in the cavities. The cavities are open on one end, so the entire mechanism forms a single, larger, microwave oscillator. A “tap”, normally a wire formed into a loop, extracts microwave energy from one of the cavities.
As the oscillation takes some time to set up, and is inherently random at the start, subsequent startups will have different output parameters. Phase is almost never preserved, which makes the magnetron difficult to use in phased array systems. Frequency also drifts from pulse to pulse, a more difficult problem for a wider array of radar systems. Cutaway drawing of a cavity magnetron from 1984. Part of the righthand magnet and copper anode block is cut away to show the cathode and cavities. This older magnetron uses two horseshoe shaped alnico magnets, modern tubes use rare earth magnets. The cathode is placed in the center of an evacuated, lobed, circular chamber.
A magnetic field parallel to the filament is imposed by a permanent magnet. The sizes of the cavities determine the resonant frequency, and thereby the frequency of the emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedance, with changes in the supply current, and with the temperature of the tube. The magnetron is a self-oscillating device requiring no external elements other than a power supply. In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode voltage must be coordinated with the build-up of oscillator output.