Phase Locked Loop, PLL, DPLL, ADPLL, VCO, Phase Detector, Loop Filter, Closed Loop, and Synthesizer<\/em>.<\/p>\n Loop stability and bandwidth configuration are baseline aspects of PLL design, directly impacting both the dynamic response and noise performance of the system. The loop bandwidth of a PLL dictates its responsiveness to changes in the input reference or other system disturbances. A wider bandwidth generally leads to these attributes:<\/p>\n However, these come with trade-offs. A wider bandwidth also allows more of the VCO\u2019s intrinsic noise to reach the output, and this heightens the risk of loop instability. Conversely, a narrower bandwidth often used in low-jitter designs offers several benefits:<\/p>\n The drawbacks of a narrower bandwidth include sacrificing settling speed and tracking capability.<\/p>\n Not surprisingly, as digital process technology and designs took over the implementation of many functions previously done using analog designs, there was an incentive to migrate the analog PLL to a mostly digital design or even an all-digital design called the digital PLL (DPLL).<\/p>\n In general, the DPLL is more robust with respect to noise and process variations compared to analog PLLs, and is obviously easier to integrate with digital circuits in VLSI design. However, it has slightly lower performance in phase noise compared to analog PLLs, and increased design complexity for high-frequency applications.<\/p>\n In the DPLL, the conventional PFD is replaced by a time-to-digital converter (TDC), which directly quantizes the phase difference between the reference ( ) and feedback ( ) signals into a digital word. This eliminates the analog charge pump, making the loop mostly digital while preserving the core function of precise phase and frequency comparison, shown in Figure 1<\/strong>.<\/p>\n The resulting error signal is then processed by a digital filter. This filter approach allows for precise mathematical manipulation of control parameters, effectively eliminating issues commonly associated with analog component tolerances and environmental variations.<\/p>\n The filtered control signal adjusts a numerically controlled oscillator (NCO), also called a digitally controlled oscillator (DCO), which generates the output frequency. Note that the digital\/analog converter (DAC) of the DCO is partially analog, while the VCO is all analog. The term DPLL is thus somewhat of a misnomer, but still used as standard terminology in the industry.<\/p>\n There are several ways to build the TDC. These include the up\/down counter, flash, vernier, two-step, interpolating, and the oscillator-based TDC, among others. As usual, the \u201cright\u201d one for a given DPLL application is a function of the priorities, tradeoffs, operating frequency, and IC process capabilities.<\/p>\n The DPLL has advantages compared to analog PLLs:<\/p>\n That last point is especially important, as one of the standout advantages of DPLLs is that flexibility. Since their loop parameters are controlled by digital filters and algorithms, they can be adjusted to accommodate different frequency ranges and application requirements without hardware modifications.<\/p>\n They can be re-programmed dynamically to meet different signal conditions or environments, and employ advanced algorithms to enhance performance, things which cannot be done with fixed-function analog filters. This inherent flexibility makes DPLLs exceptionally well-suited for applications demanding dynamic frequency synthesis, adaptive clock recovery, and phase synchronization in digital communication networks where SNR and other input-signal perturbations are continuously varying during use.<\/p>\n However, the discrete-time nature of DPLLs introduces some trade-offs:<\/p>\n As noted, the so-called DPLL is really a \u201cmostly\u201d DPLL. The next step in PLL evolution is to replace the DAC and VCO with an all-digital equivalent to the VCO, called a digital VCO or a Digitally Controlled Oscillator (DCO), as seen in Figure 4<\/strong>. This is done by using a specialized direct digital synthesis \u00a0(DDS) arrangement similar to the ones used for frequency tuning in wireless communication systems, or a topology scheme using capacitors that are switched in and out of the circuitry to change its resonant frequency.<\/p>\n Thus far, we have only looked at the basics of PLLs of all types. Each implementation \u2014 analog, digital, or all digital \u2014 offers tradeoffs in performance, integration, cost, size, power consumption, and more, summarized in Figure 3<\/strong>.<\/p>\n As a general guide, with today\u2019s process technology, the general \u201cwhich way to go\u201d guidelines are:<\/p>\n However, the rapid shifts in user needs, properties, process technology, packaging, and VLSI technology mean that these guidelines have many exceptions. For example, high-performance PLLs are now often embedded in RF ICs that work up to a gigahertz and beyond, which was unimaginable just a few years ago.<\/p>\n To overcome subtle performance issues and improve performance in critical areas needed in a specific application, PLL designers have enhanced the basic designs with many clever \u201ctwists\u201d. They have developed numerous add-ons and adjunct circuitry schemes that enhance performance along one or more parameters or suppress undesired attributes, while accepting tradeoffs in other parameters in order to meet system priorities.<\/p>\n For example, Injection-Locked PLLs (ILPLLs) are often used in high-speed communications design. They use a feedforward scheme to inject a corrective signal that is derived ahead of the filter, resulting in superior low-jitter performance and rapid locking times that are often needed in high-speed communications. Similarly, Delay-Locked Loops (DLLs) use a variation of ILPPLs to synchronize phase and delay rather than frequency, and are targeted at clocking applications in digital systems to reduce clock skew.<\/p>\n PLLS in all their variations and topologies are essential building blocks for modern design across a wide range of applications. They are available in pure analog, hybrid analog\/digital, and all-digital arrangements, offering benefits and drawbacks with respect to different performance parameters. Clever designs overcome many of these shortcomings with an assessment of cost, size, and power considerations. The versatility, flexibility, and applicability of the PLL make this a viable approach.<\/p>\n There is much more that can be said about PLLs of all three types. Much of this information is available on the web, of course, but there are also textbooks and longer academic and practical works that explore their capabilities, application issues, and subtleties.<\/p>\n TUTORIAL: Phase Locked Loops<\/a>, Integrated Device Technology, Inc. What is Proportional (PID) Control and why is it used? (Part 1)<\/a> Filed Under: Communications<\/a>, Featured<\/a><\/span><\/p>\n \u00a0<\/p>\nPLL Dynamics<\/strong><\/h3>\n
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The digital PLL<\/strong><\/h3>\n
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Going all-digital<\/strong><\/h3>\n
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\u00a0What\u2019s next for PLLs?<\/strong><\/h3>\n
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Conclusion<\/strong><\/h3>\n
References<\/strong><\/h3>\n
ECEN620: Network Theory, Broadband Circuit Design<\/a>, Texas A&M University
A Design Procedure for All-Digital Phase-Locked Loops Based on a Charge-Pump Phase-Locked-Loop Analogy<\/a>, IEEE Transaction on Circuits and Systems
An Overview of Phase-Locked Loop: From Fundamentals to the Frontier<\/a>, National Library of Medicine
Types of PLL in VLSI: Analog, Digital, and All-Digital PLLs<\/a>, <\/strong>Success Bridge
Digitally controlled oscillator<\/a>, Wikipedia
Three Major PLL Implementations<\/a>, Movellus
Phase-Locked Loop (PLL) Fundamentals<\/a>, Analog Devices
Signetics SE\/NE 565 PLL Overview<\/a>, Scribd
The Phase Locked Loop IC as a Communication System Building Block<\/a>, Texas Instruments
Lecture 080 \u2013 All Digital Phase Lock Loops (ADPLL)<\/a>, Georgia Institute of Technology<\/p>\n\u00a0Related EEWorld Online content<\/strong><\/h3>\n
What is Proportional (PID) Control and why is it used? (Part 2)<\/a>
RCA & Color TV: A dominant company and standard, both now gone \u2013 Part 2<\/a>
Does 32 kHz or 3.58 MHz mean anything to you? part 2<\/a>
Synthesized tuning, Part 1: Basic frequency-synthesizer principles<\/a>
Synthesized tuning, Part 2: Advanced synthesizers and performance<\/a><\/p>\n