Basics of phase-locked loops, from analog-only to all-digital: part 1

Each of the three topologies of this versatile system building block has its place in modern systems.

The phase-locked loop (PLL) is crucial in various electronic systems and circuits, with a history dating back to the 1930s. It serves functions related to timing, clocking, frequency synthesis, synchronization, and signal recovery.

Despite being named a phase-locked loop, many applications focus on frequency rather than phase. The relationship between phase and frequency is intimate, as frequency is the time derivative of phase.

From an operational standpoint, the PLL functions as a phase-driven device, with its operation being driven by a phase difference. The term “phase” accurately reflects its operation compared to frequency.

This article will briefly discuss the original all-analog PLL, its successor, the digital PLL with analog elements, and the latest implementation, the all-digital PLL (ADPLL).

The article will not delve into operational details, as those are extensively covered in various sources. Instead, it will touch on issues associated with all-analog and hybrid PLLs.

For evidence of the PLL’s importance, the book “Phaselock Techniques” by Floyd M. Gardner, first published in 1966, remains a seminal work referenced in PLL analyses even today.

Table of Contents

A brief history of PLLs

The first PLLs emerged in the 1930s using vacuum-tube technology to synchronize analog carriers. The transition to solid-state transistors reduced costs and enabled mass applications like color TV in the 1950s.

Figure 1: The Signetics NE565 PLL, introduced in 1969

The NE565 PLL by Signetics Corp., introduced in 1969, was a landmark integrated PLL IC widely adopted for its performance and popularity.

Basic PLL operation (without equations)

The PLL functions as a closed-loop feedback-control system, typically employing a proportional-integral (PI) loop strategy. Various setup and performance issues impact PLL performance, necessitating detailed analysis.

The analog PLL comprises five key components, including the Phase/Frequency Detector, Charge Pump, Loop Filter, Voltage-Controlled Oscillator, and Frequency Divider.

The loop filter, often a second-order passive-RC network, plays a crucial role in determining PLL dynamic performance to meet application requirements.

While the PLL’s other components aim for optimal performance, filter selection is critical due to the diverse choices available impacting PLL loop performance.

The article will explore digital PLLs and all-digital versions in the subsequent sections.