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Tunnel Diode Devices - Continued - 14183_122

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2-42 mechanical action called TUNNELING. Tunneling causes the negative-resistance action and is so fast that no transit-time effects occur even at microwave frequencies. The lack of a transit-time effect permits the use of tunnel diodes in a wide variety of microwave circuits, such as amplifiers, oscillators, and switching devices. The detailed theory of tunnel-diode operation and the negative-resistance property exhibited by the tunnel diode was discussed in NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, Chapter 3. Figure 2-39.—Tunnel-diode characteristic curve. TUNNEL-DIODE OSCILLATORS.—A tunnel diode, biased at the center point of the negative- resistance range (point B in figure 2-39) and coupled to a tuned circuit or cavity, produces a very stable oscillator. The oscillation frequency is the same as the tuned circuit or cavity frequency. Microwave tunnel-diode oscillators are useful in applications that require microwatts or, at most, a few milliwatts of power, such as local oscillators for microwave superheterodyne receivers. Tunnel-diode oscillators can be mechanically or electronically tuned over frequency ranges of about one octave and have a top-end frequency limit of approximately 10 gigahertz. Tunnel-diode oscillators that are designed to operate at microwave frequencies generally use some form of transmission line as a tuned circuit. Suitable tuned circuits can be built from coaxial lines, transmission lines, and waveguides. An example of a highly stable tunnel-diode oscillator is shown in figure 2-40. A tunnel-diode is loosely coupled to a high-Q tunable cavity. Loose coupling is achieved by using a short, antenna feed probe placed off-center in the cavity. Loose coupling is used to increase the stability of the oscillations and the output power over a wider bandwidth.


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