RF Oscillators, PLLs and Frequency Synthesizers

Chapter Chapter 2

Electronic Communication – Theory and Problems – Second Edition Book
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Electronic Communication – Theory and Problems – Second Edition Book

  • Chapter 2RF Oscillators, PLLs, and FrequencySynthesizersINTRODUCTIONRadio-frequency (RF)oscillators play a veryimportantrole in every electronic communication system. Bydefinition, anoscillatoris acircuit capable of continuously generating arepetitive waveform ofthe desiredfrequency. Depending on theapplication, therepetitive waveform may be either sinusoidal orrectangularinnature.If anoscillator requires no external input signal ortrigger toproduce the desired output waveform,it issaid tobe self-sustaining orfree-running.Those oscillators requiring anexternalinput signal arecalled triggeredorone-shot oscillators. In this chapter, wewill deal only with self-sustaining orfree-runningoscillators. Alsoincludedin this chapter is the coverage of frequency synthesizers which arespecial frequency-generatingcircuitsencountered in the mostmodernelectronic communication systems. Like the oscillator, afrequency synthesizerproduces arepetitive waveform ofthedesired frequency. Because afrequency synthesizer uses aphase-lockedloop (PLL) as abasic building block,complete coverage ofphase-locked loops isprovided prior tothe coverageof frequency synthesizers.2.1 OSCILLATOR FUNDAMENTALSFigure 2-1(a) shows the block diagram of anoscillator containing anamplifier anda feedback network.The output voltage Vout fromthe amplifier is calculated as:Fout =Fin XAvwhere Vin represents the totalinput voltageapplied tothe amplifier.Aportion ofthe output voltageis fedback totheinput ofthe amplifier through the feedback network.The fractional part ofthe output voltagewhich is fedback totheinput is called the feedbackvoltage andis designated IV The ratio of feedback voltage VQ, tooutput voltageVout is called the feedback fractionand is designated B. Die feedback fraction canbe expressed as:(2.1)Vfl,B = -ÿ-(2.2)VoutRearranging Eq. (2.2)gives us:Fa, =BVOM(2.3)Equation (2.5)clearly statesthat the feedbackvoltage is afractional part ofthe output voltage. As youcan seeinFig. 2-1(a),the feedback voltage Vn, isin series withthe sourcevoltage Vs. Ifthephase ofthefeedback voltageis negative, then aportion of Vs will be cancelled,in which casethe output voltage willbereduced inamplitude. If thephase ofthe feedback voltage ispositive, however, then theinput voltageVin is largerthanwith Vs alone.All oscillators mustusepositive feedback tosustainoscillations. Assumingthe feedback voltageispositive in Fig. 2-l(a),the actual input voltage tothe amplifier is:Vin =V.+ WSubstituting Vs+for Vin in Eq. (2.1)gives us:Fout =(Vs+ VfbMv(2.4)(2.5)Replacingwith BVout gives us:V0Ut = (Vs+ BF0„t)Av(2.6)37