The PLL frequency synthesizer

Chapter The PLL frequency synthesizer

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Why control the frequency of an oscillator in this way? It is commonly done in mod-ern communications equipment; there must be a reason. In fact there are several goodreasons why varactor control is better than the use of mechanically variable capacitorsor inductors. But it all comes down to basically one thing: Varactors are cheaper.They’re also less bulky than mechanically variable capacitors and inductors.Nowadays, many frequency readouts are digital. You look at a numeric display in-stead of interpolating a dial scale. Digital control is often done by a microcomputer. Youprogram the operating frequency by pressing a sequence of buttons, rather than by ro-tating a knob. The microcomputer might set the frequency via a synchro on the shaft ofa variable capacitor or inductor. But that would be unwieldy. It would also be ridiculous,a “Rube Goldberg” contraption. A varactor can control the frequency without all thatnonsense.The PLL frequency synthesizerOne type of oscillator that combines the flexibility of a VFO with the stability of a crys-tal oscillator is known as a PLL frequency synthesizer. This scheme is extensively usedin modern digital radio transmitters and receivers.The output of a VCO is passed through a programmable divider, a digital circuitthat divides the VCO frequency by any of hundreds or even thousands of numerical val-ues chosen by the operator. The output frequency of the programmable divider islocked, by means of a phase comparator, to the signal from a crystal-controlled refer-ence oscillator.As long as the output from the programmable divider is exactly on the referenceoscillator frequency, the two signals are in phase, and the output of the phase com-parator is zero volts dc. If the VCO frequency begins to drift, the output frequency of theprogrammable divider will drift, too (although at a different rate). But even the tiniestfrequency change—a fraction of 1 Hz—causes the phase comparator to produce a dcerror voltage. This error voltage is either positive or negative, depending on whetherthe VCO has drifted higher or lower in frequency. The error voltage is applied to a var-actor in the VCO, causing the VCO frequency to change in a direction opposite to thatof the drift. This forms a dc feedback circuit that maintains the VCO frequency at a pre-cise multiple of the reference-oscillator frequency, that multiple having been chosen bythe programmable divider. It is a loop circuit that locks the VCO onto a precise fre-quency, by means of phase sensing, hence the term phase-locked loop (PLL).The key to the stability of the PLL frequency synthesizer lies in the fact that the refer-ence oscillator is crystal-controlled. A block diagram of such a synthesizer is shown in Fig.25-8. When you hear that a radio receiver, transmitter, or transceiver is “synthesized,” itusually means that the frequency is determined by a PLL frequency synthesizer.The stability of a synthesizer can be enhanced by using an amplified signal from theNational Bureau of Standards, transmitted on shortwave by WWV at 5, 10, or 15 MHz,directly as the reference oscillator. These signals are frequency-exact to a minusculefraction of 1 Hz, because they are controlled by atomic clocks. Most people don’t needprecision of this caliber, so you won’t see consumer devices like ham radios and short-wave receivers with primary-standard PLL frequency synthesis. But it is employed bysome corporations and government agencies, such as the military.466 Oscillators