Microwave Diodes : Diodes used for signal detection and mixing are the most common microwave semiconductor devices. Two types are available: the point-contact diode and the Schottky barrier or hot carrier diode.
The typical semiconductor diode is a junction formed of P- and N-type semiconductor materials. Because of the relatively large surface area of the junction, diodes exhibit a high capacitance that prevents normal operation at microwave frequencies. For this reason, standard PN junction diodes are not used in the microwave region.
Perhaps the oldest microwave semiconductor device is the point-contact diode, also called a crystal diode. A point-contact diode comprises a piece of semiconductor material and a fine wire that makes contact with the semiconductor material. Because the wire makes contact with the semiconductor over a very small surface area, the capacitance is extremely low. Current flows easily from the cathode, the fine wire, to the anode, the semiconductor material. However, the current does not flow easily in the opposite direction.
Most early point-contact diodes used germanium as the semiconductor material, but today these devices are made of P-type silicon with a fine tungsten wire as the cathode. (See Fig. 16-36.) The forward threshold voltage is extremely low.
Point-contact diodes are ideal for small-signal applications. They are still used in microwave mixers and detectors and in microwave power measurement equipment. They are extremely delicate and cannot withstand high power. They are also easily damaged and therefore must be used in such a way to minimize shock and vibration.
Hot Carrier Diodes
For the most part, point-contact diodes have been replaced by Schottky diodes, sometimes referred to as hot-carrier diodes. Most Schottky diodes are made with N-type silicon on which has been deposited a thin metal layer. Gallium arsenide is also used. The semiconductor forms the cathode, and the metal forms the anode. The structure and schematic symbol for a Schottky diode are shown in Fig. 16-37. Typical anode materials are nickel-chromium and aluminum, although other metals, e.g., gold and platinum, are also used.
Like the point-contact diode, the Schottky diode is extremely small and therefore has a tiny junction capacitance. It also has a low bias threshold voltage. It conducts at a forward bias of 0.2 to 0.3 V, whereas the silicon junction diode conducts at 0.6 V. Higher voltage drops across PN silicon diodes result from bulk resistance effects. Schottky diodes are ideal for mixing, signal detection, and other low-level signal operations. They are widely used in balanced modulators and mixers. Because of their very high-frequency response, Schottky diodes are used as fast switches at microwave frequencies.
The most important use of microwave diodes is as mixers. Microwave diodes are usually installed as part of a waveguide or cavity resonator tuned to the incoming signal frequency. A local-oscillator signal is injected with a probe or loop. The diode mixes the two signals and produces the sum and difference output frequencies. The difference frequency is usually selected with another cavity resonator or with a low-frequency LC-tuned circuit. In a microwave circuit, the mixer is usually the input circuit. This is so because it is desirable to convert the microwave signal down to a lower-frequency level as early as possible, at a point where amplifi cation and demodulation can take place with simpler, more conventional electronic circuits.
Microwave diodes designed primarily for frequency-multiplier service include varactor diodes and step-recovery diodes.
A varactor diode is basically a voltage variable capacitor. When a reverse bias is applied to the diode, it acts as a capacitor. Its capacitance depends upon the value of the reverse bias. Varactor diodes made with gallium arsenide are optimized for use at microwave frequencies. Their main application in microwave circuits is as frequency multipliers.
Fig. 16-38 shows a varactor frequency-multiplier circuit. When an input signal is applied across the diode, it alternately conducts and cuts off. The result is a nonlinear or distorted output containing many harmonics.
When a tuned circuit is used in the output, the desired harmonic is selected and others are rejected. Since the lower harmonics produce the greatest amount of energy, varactor multipliers are usually used only for doubling and tripling operations. In Fig. 16-38, the input tuned circuit L1–C2 resonates at the input frequency fin, and the output tuned circuit L2–C3 resonates at two or three times the input frequency, as desired. In practice, the tuned circuits are not actually made up of individual inductors or capacitors. Instead, they are microstrip, stripline, or cavity resonators. Capacitors C1 and C4 are used for impedance matching.
A varactor frequency multiplier does not have gain like that of a class C amplifier used as a multiplier. In fact, a varactor introduces a signal power loss. However, it is a relatively efficient circuit, and the output can be as high as 80 percent of the input. Typical efficiencies are in the 50 to 80 percent range. No external source of power is required for this circuit; only the RF input power is required for proper operation. Outputs up to 50 W are obtainable with special high-power varactors.
Varactors are used in applications in which it is difficult to generate microwave signals. Usually, it is a lot easier to generate a VHF or UHF signal and then use a series of frequency multipliers to put it into the desired microwave region. Varactor diodes are available for producing relatively high-power outputs at frequencies up to 100 GHz.
Another diode used in microwave frequency-multiplier circuits like that in Fig. 16-38 is the step-recovery diode or snap-off varactor. It is a PN-junction diode made with gallium arsenide or silicon. When it is forward-biased, it conducts as any diode, but a charge is stored in the depletion layer. When a reverse bias is applied, the charge keeps the diode on momentarily. Then the diode turns off abruptly. This snap-off produces an extremely high-intensity reverse current pulse with a duration of about 10 ps (1 ps = 10-12 s) or less. It is extremely rich in harmonics. Even the higher harmonics are of relatively high amplitude.
Step-recovery diodes can also be used circuits like that in Fig. 16-38 to produce multipliers with power ratings up to 5 and 10 W. Power ratings of 50 W can be obtained. Operating frequencies up to 100 GHz are possible with an efficiency of 80 percent or better.
Three types of diodes other than the tunnel diode that can oscillate due to negative- resistance characteristics are the Gunn diodes and IMPATT and TRAPATT diodes.
Gunn diodes, also called transferred-electron devices (TEDs), are not diodes in the usual sense because they do not have junctions. A Gunn diode is a thin piece of N-type gallium arsenide (GaAs) or indium phosphide (InP) semiconductor that forms a special resistor when voltage is applied to it. This device exhibits a negative resistance characteristic. That is, over some voltage range, an increase in voltage results in a decrease in current and vice versa, just the opposite of Ohm’s law. When it is so biased, the time it takes for electrons to flow across the material is such that the current is 180° out of phase with the applied voltage. If a Gunn diode so biased is connected to a cavity resonant near the frequency determined by the electron transit time, the resulting combination will oscillate. The Gunn diode, therefore, is used primarily as a microwave oscillator.
Gunn diodes are available that oscillate at frequencies up to about 150 GHz. In the lower microwave range, power outputs from milliwatts up to several watts are possible. The thickness of the semiconductor determines the frequency of oscillation. However, if the cavity is made variable, the Gunn oscillator frequency can be adjusted over a narrow range.
IMPATT and TRAPATT Diodes
Two other microwave diodes widely used as oscillators are the IMPATT and TRAPATT diodes. Both are PN-junction diodes made of silicon, GaAs, or InP. They are designed to operate with a high reverse bias that causes them to avalanche or break down. A high current flows. Over a narrow range, a negative-resistance characteristic is produced that causes oscillation when the diode is mounted in a cavity and properly biased. IMPATT diodes are available with power ratings up to about 25 W to frequencies as high as about 300 GHz. Pulsed power of several hundred watts is possible.
IMPATT diodes are preferred over Gunn diodes if a higher power is required. Their primary disadvantages are their higher noise level and higher operating voltages.
A PIN diode is a special PN-junction diode with an I (intrinsic) layer between the P and N sections, as shown in Fig. 16-39(a). The P and N layers are usually silicon, although GaAs is sometimes used. In practice, the I layer is a very lightly doped N-type semiconductor.
At frequencies less than about 100 MHz, the PIN diode acts just like any other PN junction diode. At higher frequencies, it acts as a variable resistor or like a switch. When the bias is zero or reverse, the diode acts like a high value of resistance, 5k V and higher.
If a forward bias is applied, the diode resistance drops to a very low level, typically a few ohms or less. When the amount of forwarding bias is varied, the value of the resistance can be varied over a linear range. The characteristic curve for a PIN diode is shown in Fig. 16-39(b). PIN diodes are used as switches in microwave circuits. A typical application is to connect a PIN diode across the output of a microwave transmission line like microstrip or stripline. When the diode is reverse-biased, it acts as very high resistance and has little effect on the normally much lower characteristic impedance of the transmission line. When the diode is forward-biased, it shorts the line, creating an almost total reflection. PIN diodes are widely used to switch sections of quarter half-wavelength transmission lines to provide varying phase shifts in a circuit.
A popular switching circuit is the tee configuration shown in Fig. 16-39(c), which is a combination of two series switches and a shunt switch. It has excellent isolation between input and output.
A simple saturated transistor switch is used for control. When the control input is binary 0, the transistor is off, so positive voltage is applied to the cathodes of diodes D1 and D2. Diode D3 conducts; because of the voltage divider, which is made up of RA and RB, the cathode of D3 is at a lower positive voltage level than its anode. Under this condition, the switch is off, so no signal reaches the load.
When the control input is positive, or binary 1, the transistor conducts, pulling the cathodes of D1 and D2 to ground through resistors R3 and R5. The anodes of D1 and D2 are at positive voltage through resistor R4, so these diodes conduct. Diode D3 is cut off. Thus the signal passes to the load.
PIN diodes are also sometimes used for their variable-resistance characteristics. Varying their bias to change resistance permits variable-voltage attenuator circuits to be created. PIN diodes can also be used as amplitude modulators.
Before transistors were invented, all electronic circuits were implemented with vacuum tubes. Tubes are devices used for controlling a large current with a small voltage to produce amplification, oscillation, switching, and other operations. Today, vacuum tubes are used only for special applications. The cathode-ray tube (CRT), once used in TV sets, computer monitors, oscilloscopes, spectrum analyzers, and other display devices, is a special form of vacuum tube which has been replaced by LCD, LED, and other flat-panel displays.
Vacuum tubes are still found in microwave equipment. This is particularly true in microwave transmitters used for producing high output power. Bipolar field-effect transistors can produce power in the microwave region up to approximately several hundred watts, but many applications require more power. Radio transmitters in the UHF and low microwave bands use standard vacuum tubes designed for power amplification. These can produce power levels of up to several thousand watts. At higher microwave frequencies (above 2 GHz), special tubes are used. In satellites and their earth stations, TV stations, and in some military equipment such as radar, very high output powers are needed. The newer GaN power transistors can produce power levels to about 50 watts but can be paralleled to deliver a power of hundreds and even thousands of watts. However, special microwave tubes developed during World War II—the klystron, the magnetron, and the traveling-wave tube—are still used in some older equipment for microwave power amplification.
A klystron is a microwave vacuum tube using cavity resonators to produce velocity modulation of an electron beam that produces amplification. Fig. 16-40 is a schematic diagram of a two-cavity klystron amplifier. The vacuum tube itself consists of a cathode that is heated by a filament. At a very high temperature, the cathode emits electrons. These negative electrons are attracted by a plate or collector, which is biased with a high positive voltage. Thus current fl ow is established between the cathode and the collector inside the evacuated tube.
The electrons emitted by the cathode are focused into a very narrow stream by using electrostatic and electromagnetic focusing techniques. In electrostatic focusing, special elements called focusing plates to which have been applied high voltages force the electrons into a narrow beam. Electromagnetic focusing makes use of coils around the tube through which current is passed to produce a magnetic field. This magnetic field helps focus the electrons into a narrow beam.
The sharply focused beam of electrons is then forced to pass through the centers of two cavity resonators that surround the open center cavity. The microwave signal to be amplified is applied to the lower cavity through a coupling loop. This sets up electric and magnetic fields in the cavity, which cause the electrons to speed up and slow down as they pass through the cavity. On one half-cycle, the electrons are speeded up; on the next half cycle of the input, they are slowed down. The effect is to create bunches of electrons that are one-half wavelength apart in the drift space between the cavities. This speeding up and slowing down of the electron beam is known as velocity modulation. Since the input cavity produces bunches of electrons, it is commonly referred to as the buncher cavity.
Because the bunched electrons are attracted by the positive collector, they move on through the tube, eventually passing through the center of another cavity known as the catcher cavity. Because the bunched electrons move toward the collector in clouds of alternately dense and sparse areas, the electron beam can be referred to as a density modulated beam.
As the bunches of electrons pass through the catcher cavity, the cavity is excited into oscillation at the resonant frequency. Thus the dc energy in the electron beam is converted to RF energy at the cavity frequency, and amplification occurs. The output is extracted from the catcher cavity with a loop.
Klystrons are also constructed with additional cavities between the buncher and catcher cavities. These intermediate cavities produce further bunching, which causes increased amplification of the signal. If the buncher cavities are tuned off-center frequency from the input and output cavities, the effect is to broaden the bandwidth of the tube. The frequency of operation of a klystron is set by the sizes of the input and output cavities. Because cavities typically have high Qs, their bandwidth is limited. By lowering the Qs of the cavities and by introducing intermediate cavities, the wider-bandwidth operation can be achieved.
Klystrons are no longer widely used in most microwave equipment. Gunn diodes and other circuits have replaced the smaller reflex klystrons in signal-generating applications because they are smaller and lower in cost and do not require high dc supply voltages. The larger multi-cavity klystrons are being replaced by traveling-wave tubes in high-power applications.
Another widely used microwave tube is the magnetron, a combination of a simple diode vacuum tube with built-in cavity resonators and an extremely powerful permanent magnet. The typical magnetron assembly shown in Fig. 16-41 consists of a circular anode into which has been machined with an even number of resonant cavities. The diameter of each cavity is equal to a one-half wavelength at the desired operating frequency. The anode is usually made of copper and is connected to a high- voltage positive direct current.
In the center of the anode, called the interaction chamber, is a circular cathode that emits electrons when heated. In a normal diode vacuum tube, the electrons would flow directly from the cathode straight to the anode, causing a high current to flow. In a magnetron tube, however, the direction of the electrons is modified because the tube is surrounded by a strong magnetic field. The field is usually supplied by a C-shaped permanent magnet centered over the interaction chamber. In the figure, the field is labeled perpendicular to the page; this means that the lines of force could be coming out of the page or going into the page depending upon the construction of the tube.
The magnetic fields of the moving electrons interact with the strong field supplied by the magnet. The result is that the path for electron flow from the cathode is not directed to the anode but instead is curved. By properly adjusting the anode voltage and the strength of the magnetic field, the electrons can be made to bend such that they rarely reach the anode and cause current flow. The path becomes circular loops, as illustrated in Fig. 16-41. Eventually, the electrons do reach the anode and cause current flow. By adjusting the dc anode voltage and the strength of the magnetic field, the electron path is made circular. In making their circular passes in the interaction chamber, electrons excite the resonant cavities into oscillation. A magnetron, therefore, is an oscillator, not an amplifier. A takeoff loop in one cavity provides the output.
Magnetrons are capable of developing extremely high levels of microwave power. Thousands and even millions of watts of power can be produced by a magnetron. When operated in a pulsed mode, magnetrons can generate several megawatts of power in the microwave region. Pulsed magnetrons are commonly used in radar systems. Continuouswave magnetrons are also used and can generate hundreds and even thousands of watts of power. A typical application for a continuous-wave magnetron is for heating purposes in microwave ovens at 2.45 GHz.
One of the most versatile microwave RF power amplifiers is the traveling-wave tube (TWT), which can generate hundreds and even thousands of watts of microwave power. The main virtue of the TWT is an extremely wide bandwidth. It is not resonant at a single frequency.
Fig. 16-42 shows the basic structure of a traveling-wave tube. It consists of a cathode and filament heater plus an anode that is biased positively to accelerate the electron beam forward and to focus it into a narrow beam. The electrons are attracted by a positive plate called the collector to which is applied a very high dc voltage. Traveling wave tubes can be anywhere from 1 ft to several feet. In any case, the length of the tube is usually many wavelengths at the operating frequency. Permanent magnets or electromagnets surround the tube, keeping the electrons tightly focused into a narrow beam.
Encircling the length of a traveling-wave tube is a helix or coil. The electron beam passes through the axis of the helix. The microwave signal to be amplified is applied to the end of the helix near the cathode, and the output is taken from the end of the helix near the collector. The purpose of the helix is to provide a path for the RF signal that will slow down its propagation. The propagation of the RF signal along the helix is made approximately equal to the velocity of the electron beam from cathode to the collector. The helix is configured such that the wave traveling along it is slightly slower than that of the electron beam.
The passage of the microwave signal down the helix produces electric and magnetic fields that interact with the electron beam. The effect on the electron beam is similar to that in a klystron. The electromagnetic field produced by the helix causes the electrons to be speeded up and slowed down. This produces velocity modulation of the beam, which in turn produces density modulation. Density modulation, of course, causes bunches of electrons to group one wavelength apart. These bunches of electrons travel down the length of the tube toward the collector. Since the density-modulated electron beam is essentially in step with the electromagnetic wave traveling down the helix, the electron bunches induce voltages into the helix that reinforce the existing voltage. The result is that the strength of the electromagnetic field on the helix increases as the wave travels down the tube toward the collector. At the end of the helix, the signal is considerably amplified. The coaxial cable of waveguide structures is used to extract energy from the helix.
Traveling-wave tubes can be made to amplify signals in a range from UHF to hundreds of gigahertz. Most TWTs have a frequency range of approximately 2:1 in the desired segment of the microwave region to be amplified. TWTs can be used in both continuous and pulsed modes of operation. One of the most common applications of TWTs is as power amplifiers in satellite transponders. GaN power amplifiers are gradually replacing TWTs in the lower power ranges because they are smaller, more rugged, and less expensive.
Miscellaneous Microwave Tubes
In a backward wave oscillator (BWO), a variation of the TWT, the wave travels from the anode end of the tube back toward the electron gun, where it is extracted. BWOs can generate up to hundreds of watts of microwave power in the 20- to the 80-GHz range. The operating frequency of the BWO can easily be tuned by varying the collector voltage.
Gyrotrons, which are built and operated as klystrons, are used for amplification of microwave frequencies above 30 GHz into the millimeter wave range. They can also be connected to operate as oscillators. Gyrotrons are the only devices currently available for power amplification and signal generation in the millimeter wave range.
A crossed-field amplifier (CFA) is similar to a TWT. Its gain is lower but is somewhat more efficient. For a given power level, the operating voltage of a CFA is usually lower than that of a TWT. The bandwidth is about 20 to 60 percent of the design frequency. Power levels up to several megawatts in a pulse mode can be achieved.
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