3 micro wave devices
TRANSCRIPT
Microwave Devices
In the name of ALLAH, Most Gracious, Most Merciful
Copyright Pakistan International Airlines Training Center Karachi
Open circuit voltage maxima
Open circuit current minima
C I V I L
Current LEADS voltage in a capacitor.
Voltage LEADS Current in an inductor.
Open Ended Transmission Line
Open circuit voltage maxima
Open circuit current minima
C I V I L
Current LEADS voltage in a capacitor.
Voltage LEADS Current in an inductor.
Close Ended Transmission Line
Open circuit voltage maxima
Open circuit current minima
Quarter Wave Close Ended Transmission LineTwo-wire transmission line using ordinary insulators
Quarter-wave section of transmission line shorted at one end
Resonant Cavities
Development of a cylindrical resonant cavity.
Development of a cylindrical resonant cavity.
Factors governing resonant freq.Size.Shape.
Figure 1-60.—Several types of cavities.
Velocity Modulation• The microwave tube uses transit time in the conversion of dc power to radio-frequency
power. The interchange of power is accomplished by using the principle of electron velocity modulation and low-loss resonant cavities in the microwave tube.
• Velocity modulation is then defined as that variation in the velocity of a beam of electrons caused by the alternate speeding up and slowing down of the electrons in the beam.
• This variation is usually caused by a voltage signal applied between the grids through which the beam must pass. The direction of the electron beam and the static electrical field goes to each other parallelly (linearly) into linear beam tubes. Against this the fields influencing the electron beam stand vertically by the electron beam at the cross field tubes.
Resonators• The cavity magnetron is a vacuum tube with a
filament in the center of an evacuated, lobed, circular cavity resonator.
• The klystron, tube waveguide, is a beam tube
including at least two apertured cavity resonators.
• The reflex klystron is a klystron utilizing only a single apertured cavity resonator through which the beam of charged particles passes, first in one direction.
• In a laser, light is amplified in a cavity resonator which is usually composed of two or more mirrors.
The Magnetron• 1920s: American engineer Albert W. Hull
invents the first magnetron while working for General Electric.
• ~1940: Two physicists, John Randall and Harry Boot, working at the University of Birmingham, England develop a much more powerful magnetron that is compact enough to fit into ships, planes, and submarines.
• 1940s: American engineer Percy Spencer accidentally discovers that microwaves produced by a magnetron have enough power to heat and cook food. He patents the microwave oven in 1953.
How Does a Magnetron Make Microwaves?1. There's a heated cathode (a solid metal rod) at the center of the magnetron. Here
it's colored orange. 2. A ring-shaped anode surrounds the cathode (colored red). 3. If you switched on a simple magnetron like this, electrons would boil off from the
cathode and zip across to the anode in straight lines (shown by the black arrow) much like the electron beam in a TV set. But there are two added extra bits in a magnetron that change things completely.
4. First, the anode has holes or slots cut into it called cavities or resonant cavities. Second, a powerful magnet is placed underneath the anode to generate a magnetic field along the length of the tube (parallel to the cathode and, in this diagram, going directly into the computer screen away from you).
5. Now when the electrons try to zip from cathode to anode, they are travelling through an electric field (stretching between the anode and cathode) and a magnetic field (produced by the magnet) at the same time. So, like any electrically charged particles moving in a magnetic field, they feel a force and follow a curved path (blue circle) instead of a straight one, whizzing around the space between the anode and the cathode.
6. As the electrons nip past the cavities, the cavities resonate and emit microwave radiation. Think of the electrons passing energy to the cavities, making then resonate like someone blowing on the open end of a flute—only producing microwaves instead of sound waves.
7. The microwave radiation that the cavities produce is collected up and chanelled by a kind of funnel called a waveguide, either into the cooking compartment of a microwave oven or beamed out into the air by an antenna or satellite dish in radar equipment.
(For animation connect to the Internet & click on Link above to start the Java Applet)
Applications• In radar devices the waveguide is connected to an antenna and works as a high power RF Oscillator.
• The magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of high power microwave energy being radiated.
• Several characteristics of the magnetron's power output conspire to make radar use of the device somewhat problematic.:– The magnetron's inherent instability in its transmitter frequency. This instability is noted not only as a frequency
shift from one pulse to the next, but also a frequency shift within an individual transmitter pulse. – The second factor is that the energy of the transmitted pulse is spread over a wide frequency spectrum, which
makes necessary its receiver to have a corresponding wide selectivity. This wide selectivity permits ambient electrical noise to be accepted into the receiver, thus obscuring somewhat the received radar echoes, thereby reducing overall radar performance.
– The third factor, depending on application, is the radiation hazard caused by the use of high power electromagnetic radiation. In some applications, for example a marine radar mounted on a recreational vessel, a radar with a magnetron output of 2 to 4 kilowatts is often found mounted very near an area occupied by crew or passengers. In practical use, these factors have been overcome, or merely accepted, and there are today thousands of magnetron aviation and marine radar units in service.
– Recent advances in aviation weather avoidance radar and in marine radar have successfully implemented solid-state transmitters that eliminate the magnetron entirely.
Klystron• Applications:– Amplifier.– Oscillator.
Klystron
Applegate Diagram
Reflex Klystron
• Applications:– Automatic Frequency Control.– Frequency Modulation.
Microwave Transistors
• Designed to minimize capacitances and transit time
• NPN bipolar and N channel FETs preferred because free electrons move faster than holes
• Gallium Arsenide has greater electron mobility than silicon
The Varactor Diode
(For animation connect to the Internet & click on Link above to start the Java Applet)
Gunn Device
• Slab of N-type GaAs (gallium arsenide)• Sometimes called Gunn diode but has no
junctions• Has a negative-resistance region where drift
velocity decreases with increased voltage• This causes a concentration of free electrons
called a domain
•The negative differential resistance, combined with the timing properties of the intermediate layer, allows construction of an RF relaxation oscillator simply by applying a suitable direct current through the device.
•The negative differential resistance created by the diode will negate the real and positive resistance of an actual load and thus create a "zero" resistance circuit which will sustain oscillations indefinitely.
•The oscillation frequency is determined partly by the properties of the thin middle layer, but can be tuned by external factors.
•Gunn diodes are therefore used to build oscillators in the 10 GHz and higher (THz) .
•A resonator is usually added to control frequency. This resonator can be take the form of a waveguide, microwave cavity.
•Tuning is done mechanically, by adjusting the parameters of the resonator
Transit-time Mode
• Domains move through the GaAs till they reach the positive terminal
• When domain reaches positive terminal it disappears and a new domain forms
• Pulse of current flows when domain disappears
• Period of pulses = transit time in device
Gunn Oscillator Frequency
• T=d/vT = period of oscillationd = thickness of devicev = drift velocity, about 1 105 m/s
• f = 1/T
IMPATT Diode• IMPATT stands for Impact Avalanche And Transit Time• Operates in reverse-breakdown (avalanche) region• Applied voltage causes momentary breakdown once
per cycle• This starts a pulse of current moving through the
device• Frequency depends on device thickness
The resonant circuit in the schematic diagram of Figure is the lumped circuit equivalent of a waveguide section, where the IMPATT diode is mounted. DC reverse bias is applied through a choke which keeps RF from being lost in the bias supply. This may be a section of waveguide known as a bias Tee. Low power RADAR transmitters may use an IMPATT diode as a power source. They are too noisy for use in the receiver
1. The IMPATT diode is operated under reverse bias conditions. 2. These are set so that avalanche breakdown occurs.3. This occurs in the region very close to the P+ (i.e. heavily doped P region). 4. The electric field at the p-n junction is very high because the voltage appears across a very narrow gap
creating a high potential gradient. 5. Under these circumstances any carriers are accelerated very quickly.6. As a result they collide with the crystal lattice and free other carriers. 7. These newly freed carriers are similarly accelerated and collide with the crystal lattice freeing more
carriers. This process gives rise to what is termed avalanche breakdown as the number of carriers multiplies very quickly.
8. For this type of breakdown only occurs when a certain voltage is applied to the junction. 9. Below this the potential does not accelerate the carriers sufficiently.10. Once the carriers have been generated the device relies on negative resistance to generate and sustain
an oscillation. 11.The effect does not occur in the device at DC, but instead, here it is an AC effect that is brought about by
phase differences that are seen at the frequency of operation. 12. The voltage applied to the IMPATT diode has a mean value that means the diode is on the verge of
avalanche breakdown. 13. When the electrons move across the N+ region an external current is seen, and this occurs in peaks,
resulting in a repetitive waveform.
IMPATT diode operation Intrinsic (N-) NP N+
+
IMPATT diode can be considered to consist of two areas:• Avalanche.• Drift.
The Avalanche region or injection region- creates the carriers which may be either holes of electrons
• Secondly the drift region- is where the carriers move across the diode taking a certain amount of time dependent upon its thickness
Cross section of a SiC IMPATT diode. Hole-electron pairs are created at the point of highest electric field (the "Avalanche Region"). Holes are swept into the cathode, but electrons drift toward the anode, inducing a displacement current in the external circuit as they drift.
Close-up of a single cycle of Figure 2 after oscillations have stabilized. Note that the current and voltage are 180° out of phase, implying a negative ac resistance and the net generation of microwave power.
Buildup and stabilization of microwave oscillations, as predicted by a two-dimensional transient device simulator. In this simulation, the diode is embedded in a resonant circuit, and oscillations are initiated by a small (0.1 V) pulse at t = 0.
PIN Diode
• P-type --- Intrinsic --- N-type• Used as switch and attenuator• Reverse biased - off• Forward biased - partly on to on
depending on the bias
This type of diode is typified by its construction. It has the standard P type and N-type areas, but between them there is an area of Intrinsic semiconductor which has no doping. The area of the intrinsic semiconductor has the effect of increasing the area of the depletion region which can be useful for switching applications as well as for use in photodiodes
PIN Diode
Summary
• Explain the applications of the following devices:– Magnetron.– Klystron.– Reflex klystron.
• What have these devices been replaced by and explain the difference in their mechanism of action/s.
• What is the major application of a PIN diode?