TABLE 76.1 PLL Noise Source Noise source Filter Function Reference oscillator phase nois Phase detector noise Active loop filter input noise www Active loop filter output noise High pass vCO free-running phase noise High pass A PLL is frequently used to enhance the noise performance of an oscillator by taking advantage of these noise-filtering properties. For example, a crystal oscillator typically has very good low-frequency noise charac teristics, and a free-running LC oscillator can be designed with very good high-frequency noise performance but will exhibit poor low-frequency noise characteristics By phase-locking an LC oscillator to a crystal oscillator When designing frequency synthesizers using PLLs, care must be taken to prevent noise from the PLL components from introducing excessive noise. The divider ratio(N) used in the feedback of the loop has the effect of multiplying any noise that appears at the input or output of the phase detector by this factor. Frequently, a large value of N is required to achieve the desired output frequencies. This can cause excessive output noise All these effects must be taken into account to achieve a pll design with optimum noise performan 76.4 PLL Design procedures The specific steps used to design a Pll depend on the intended application. Typically the architecture of the loop will be determined by the output frequency agility required (frequency synthesizer) and the reference ources available. Other requirements such as size and cost play important factors, as well as available standard components. Once the topology has been determined, then the desired loop transfer function must be synthe sized. This may be dictated by noise requirements as discussed above or other factors such as loop lock-up time or input signal tracking ability. The design Eqs.(76. 11)through(76. 15) may then be used to determine the component values required in the loop filter. Frequently several of these factors must be balanced or traded off to obtain an acceptable design. a design that requires high performance in several of these areas usually can be realized at the expense of design complexity or increased component cost. 76.5 Components The development of large-scale integrated circuits over the past several years has made the design and implementation of PLLs and frequency synthesizers much cheaper and easier. Several major manufacturers (Motorola, Signetics, National, Plessey, etc. )currently supply a wide range of components for PLL impleme tation. The most complex of these are the synthesizer circuits that provide a programmable reference divider, programmable divide by N, and a phase detector. Several configurations of these circuits are available to suit lost applications Integrated circuits are also available to implement most of the individual blocks shown in Fig.76.1 A wide variety of phase detector circuits are available, and the optimum type will depend on the circuit requirements. An analog multiplier (or mixer) may be used and is most common in applications where the comparison frequency must be very high. This type of phase detector produces an output that is the multipli- cation of the two input signals. If the inputs are sine waves, the output will consist of a double-frequency component as well as a dc component that is proportional to the cosine of the input phase difference. The double-frequency component can be removed with a low-pass filter, leaving only the dc component. The analog multiplier has a somewhat limited phase range of #90 degrees. The remainder of the phase detector types discussed here are digital in nature and operate using digital edges or transitions of the signals to be compared e 2000 by CRC Press LLC© 2000 by CRC Press LLC A PLL is frequently used to enhance the noise performance of an oscillator by taking advantage of these noise-filtering properties. For example, a crystal oscillator typically has very good low-frequency noise charac￾teristics, and a free-running LC oscillator can be designed with very good high-frequency noise performance but will exhibit poor low-frequency noise characteristics. By phase-locking an LC oscillator to a crystal oscillator and setting the loop response corner frequency to the noise crossover point between the two oscillators, the desirable characteristics of both oscillators are realized. When designing frequency synthesizers using PLLs, care must be taken to prevent noise from the PLL components from introducing excessive noise. The divider ratio (N) used in the feedback of the loop has the effect of multiplying any noise that appears at the input or output of the phase detector by this factor. Frequently, a large value of N is required to achieve the desired output frequencies. This can cause excessive output noise. All these effects must be taken into account to achieve a PLL design with optimum noise performance. 76.4 PLL Design Procedures The specific steps used to design a PLL depend on the intended application. Typically the architecture of the loop will be determined by the output frequency agility required (frequency synthesizer) and the reference sources available. Other requirements such as size and cost play important factors, as well as available standard components. Once the topology has been determined, then the desired loop transfer function must be synthe￾sized. This may be dictated by noise requirements as discussed above or other factors such as loop lock-up time or input signal tracking ability. The design Eqs. (76.11) through (76.15) may then be used to determine the component values required in the loop filter. Frequently several of these factors must be balanced or traded off to obtain an acceptable design. A design that requires high performance in several of these areas usually can be realized at the expense of design complexity or increased component cost. 76.5 Components The development of large-scale integrated circuits over the past several years has made the design and implementation of PLLs and frequency synthesizers much cheaper and easier. Several major manufacturers (Motorola, Signetics, National, Plessey, etc.) currently supply a wide range of components for PLL implemen￾tation. The most complex of these are the synthesizer circuits that provide a programmable reference divider, programmable divide by N, and a phase detector. Several configurations of these circuits are available to suit most applications. Integrated circuits are also available to implement most of the individual blocks shown in Fig. 76.1. A wide variety of phase detector circuits are available, and the optimum type will depend on the circuit requirements. An analog multiplier (or mixer) may be used and is most common in applications where the comparison frequency must be very high. This type of phase detector produces an output that is the multipli￾cation of the two input signals. If the inputs are sine waves, the output will consist of a double-frequency component as well as a dc component that is proportional to the cosine of the input phase difference. The double-frequency component can be removed with a low-pass filter, leaving only the dc component. The analog multiplier has a somewhat limited phase range of ±90 degrees. The remainder of the phase detector types discussed here are digital in nature and operate using digital edges or transitions of the signals to be compared. TABLE 76.1 PLL Noise Sources Noise Source Filter Function Reference oscillator phase noise Low pass Phase detector noise Low pass Active loop filter input noise Low pass Digital divider noise Low pass Active loop filter output noise High pass VCO free-running phase noise High pass