Sinusoidal Oscillators
Communication transceivers require oscillators that generate pure electrical sinusoidal signals (i.e. stable time reference signals) for further use in modulators, mixers, and other circuits. Although oscillators may be designed to deliver other waveforms
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Communication transceivers require oscillators that generate pure electrical sinusoidal signals (i.e. stable time reference signals) for further use in modulators, mixers, and other circuits. Although oscillators may be designed to deliver other waveforms as well, e.g. square, triangle, and sawtooth waveforms, if intended for applications in wireless radio communications, the sinusoidal and square waveforms are the most important ones. A good sinusoidal oscillator is expected to deliver either a voltage or a current signal that is stable both in amplitude and frequency. Because a variety of oscillator structures are available that are suitable for generation of periodic waveforms, circuit designers make the choice mostly based on their personal preference for one particular type of oscillator. In this chapter, we study several oscillator circuits, with emphasis on understanding the underlying principles, rather than very detailed analysis of any special oscillator type.
13.1
Closed-Loop Principle
Literally every modern communication system relies on the master timing reference, similar to a symphony orchestra musicians who must follow the rhythm given by the conductor. Synchronization of time among all blocks in a communication system is one of the main principles used in the communication theory.1 The time synchronization is implemented with the help of a circuit known as a voltage-controlled “oscillator” (VCO) that produces a periodic waveforms whose frequencies are made as stable and precise as possible in the given technology. In addition, various precisely controlled periodic waveforms are needed to perform the mathematical multiplication operation, which is the key operation in RF systems. For example, an RF receiver in Fig. 13.1 contains one “local” oscillator (LO) whose output signal is used by the multiplying circuit (i.e. mixer) to shift frequency of the incoming RF carrier. Here, the term “local” simply means that the oscillator circuit itself is part of the receiver, not the external circuit. Inherently the amplitude of the signal inside an oscillator circuit may (theoretically) infinitely increase, that is, in reality its output signal amplitude becomes large. Consequently, the conclusion is that small signal circuit analysis is not an applicable method anymore. Large signals imply nonlinear operation, which means that we have to apply numerical methods in order to estimate the circuit’s internal states. The good news, however, is that all oscillator circuits belong to the general group of 1 The
alternative, i.e. asynchronous, systems are still the subject of research.
© Springer Nature Switzerland AG 2021 R. Sobot, Wireless Communication Electronics, https://doi.org/10.1007/978-3-030-48630-3_13
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13 Oscillators
RF amp
mixer
demodulator
RF matching network
IF amp local oscillator
LO
peak detector
audio amplifier
Fig. 13.1 Heterodyne AM radio receiver architecture—local oscillator (LO)
closed-loop feedback circuits (e.g. PLL, adaptive equalizer, ΣΔ modulator, etc.) whose fun
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