2-28 Figure 2-24.—Pierce oscillator, common-emitter configuration. Q-18. What is the impedance of a crystal at its resonant frequency when it is used in the parallel mode? Q-19. What is the impedance of a crystal at its resonant frequency when it is used in the series mode? PULSED OSCILLATORS A sinusoidal (sine-wave) oscillator is one that will produce output pulses at a predetermined frequency for an indefinite period of time; that is, it operates continuously. Many electronic circuits in equipment such as radar require that an oscillator be turned on for a specific period of time and that it remain in an off condition until required at a later time. These circuits are referred to as PULSED OSCILLATORS or RINGING OSCILLATORS. They are nothing more than sine-wave oscillators that are turned on and off at specific times. Figure 2-25, view (A), shows a pulsed oscillator with the resonant tank in the emitter circuit. A positive input makes Q1 conduct heavily and current flow through L1; therefore no oscillations can take place. A negative-going input pulse (referred to as a gate) cuts off Q1, and the tank oscillates until the gate ends or until the ringing stops, whichever comes first. Figure 2-25A.—Pulsed oscillator.
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