MICROVE DEVICES
To equip students with knowledge on microwaves communication and devices and their applications.
1. Introduction
Topic 1
Introduction
Microwave essentially means very short wave. Microwaves are a form of energy in the electromagnetic (EM) spectrum. The EM spectrum runs from DC voltage to light and beyond as shown in figure 1.0.1.
The microwave frequency spectrum is usually taken to extended form 1 GHZ to 30 GHZ.This corresponds to wavelengths from 1 cm to 30 cm. The main reason why we have to go in for microwave frequency for communication is that lower frequency band (Radio Frequency Spectrum) became congested and demand for point to point communication continued to increase.
Microwaves being EM waves, have two components: the electrical (red) and the mag netic (blue) (Fig.1.0.2). The two components travel perpendicular to each other. The magnetic component allows us to use magnets and ferrite materials to affect wave behavior.
Microwaves have a wide rage of applications depending on their frequencies as shown in 1
Figure 1.0.2: Electromagnetic Wave
figure 1.0.3.
1.1 Microwave Devices
In the late 1930s it became evident that as the wavelength approached the physical di mensions of the vacuum tubes, the electron transit angle, interelectrode capacitance, and lead inductance appeared to limit the operation of vacuum tubes in microwave frequencies.
In 1935 A. A. Heil and 0. Heil suggested that microwave voltages be generated by using transit-time effects together with lumped tuned circuits. In 1939 W. C. Hahn and G.
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F. Metcalf proposed a theory of velocity modulation for microwave tubes. Four months later R. H. Varian and S. F. Varian described a two-cavity klystron amplifier and oscillator by using velocity modulation.
In 1944 R. Kompfner invented the helix-type traveling-wave tube (TWT). Ever since then the concept of microwave tubes has deviated from that of conventional vacuum tubes as a result of the application of new principles in the amplification and generation of microwave energy.
Historically microwave generation and amplification were accomplished by means of velocity-modulation theory. In the past two decades, however, microwave solid-state devices-such as tunnel diodes, Gunn diodes, transferred electron devices (TEDs), and ava lanche transit-time devices have been developed to perform these functions. The concep tion and subsequent development of TEDs and avalanche transit-time devices were among the outstanding technical achievements.
B. K. Ridley and T. B. Watkins in 1961 and C. Hilsum in 1962 independently predicted that the transferred electron effect would occur in GaAs (gallium arsenide). In 1963 J. B. Gunn reported his "Gunn effect." The common characteristic of all microwave solidstate devices is the negative resistance that can be used for microwave oscillation and amplific ation.
The progress of TEDs and avalanche transit-time devices has been so swift that today they are firmly established as one of the most important classes of microwave solid-state devices.
1.2 Microwave Systems
A microwave system normally consists of a transmitter subsystem, including a microwave oscillator, waveguides, and a transmitting antenna, and a receiver subsystem that includes a receiving antenna, transmission line or waveguide, a microwave amplifier, and a receiver. Figure 1.2.1 shows a typical microwave system.
In order to design a microwave system and conduct a proper test of it, an adequate knowledge of the components involved is essential. Besides microwave devices, the text therefore describes microwave components, such as resonators, cavities, microstrip lines, hybrids, and microwave integrated circuits.
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Figure 1.2.1: Microwave System
1.3 Microwaves Unit of Measure
Microwave measures can be expressed in different units, such as the CGS (centimeter gram-second) unit, MKS (meter-kilogram-second) unit, or another unit. The meter-kilogram second units (the International System of Units) are used throughout unless otherwise in dicated.
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