Mastering Clock Generator Parameters

Table of Contents

1. Introduction
2. The Importance of Clock Generators
3. VCO Frequency Range
4. Overcoming High VCO Frequency
5. Clock Generators with Embedded Fractional Dividers
6. PLL Loop Filter Bandwidth
7. Phase Detector Frequency
8. Reducing Radiated Emissions with Spread Spectrum Clocking
9. Loop Filter Components
10. Clock Generators with Non-Volatile Memory

Introduction

The Importance of Clock Generators

VCO Frequency Range

Overcoming High VCO Frequency

Clock Generators with Embedded Fractional Dividers

PLL Loop Filter Bandwidth

Phase Detector Frequency

Reducing Radiated Emissions with Spread Spectrum Clocking

Loop Filter Components

Clock Generators with Non-Volatile Memory

A Comprehensive Guide to Clock Generators: Enhancing System Design with Versatile and Reliable Timing Solutions

Clock generators play a pivotal role in complex system designs, especially when multiple clock sources with different frequencies are required. Traditionally, the use of multiple oscillators and crystals to generate various clock frequencies leads to increased points of failure and the burden of managing the bill of materials (BOM). However, with the advent of clock generators, designers can overcome these challenges and improve system reliability.

Introduction

In this era of advanced technology, systems such as networking, factory automation, and medical imaging rely on disparate clock frequencies. The conventional approach of mounting multiple oscillators and crystals all over the circuit board creates complexity and increases the potential for failure points. Clock generators offer an efficient solution to these issues by consolidating and generating multiple clock sources simultaneously. By replacing discrete clock sources, clock generators simplify BOM management and enhance system reliability.

The Importance of Clock Generators

Clock generators provide a convenient and versatile solution to system designers by offering a single device capable of generating multiple clock sources with varying frequencies. By leveraging a clock generator, designers can eliminate the need for multiple oscillators and crystals scattered throughout the circuit board. This consolidation not only reduces the complexity of the system but also improves overall reliability. Furthermore, clock generators simplify BOM management and reduce costs by replacing multiple discrete clock sources with a single integrated solution.

VCO Frequency Range

The voltage-controlled oscillator (VCO) frequency range is a crucial parameter to consider when selecting a clock generator for a specific system design. The VCO frequency determines whether all system clocks can be generated simultaneously. To determine the necessary VCO frequency, designers can calculate the least common multiple of all the required clock frequencies. In some cases, the resulting VCO frequency may be too high to be realistic, posing a challenge in clock generation.

Overcoming High VCO Frequency

When faced with a high VCO frequency that surpasses practical limits, there are two effective approaches to overcome this limitation. One option is to select a clock generator that incorporates multiple phase-locked loop (PLL) and VCO units. Each PLL/VCO unit can then handle clocks within the same frequency domain, enabling the generation of multiple system clocks. Another approach is to choose a clock generator equipped with embedded fractional dividers. The resolution of these fractional dividers determines the frequency accuracy of the output clocks.

Both of these solutions have their advantages and considerations. Clock generators with multiple PLL/VCO units offer flexibility and simplicity in generating clocks within the same frequency domain. However, fractional dividers, while providing frequency accuracy, may introduce additional spurs that require careful design considerations to ensure minimal disruption to system performance.

PLL Loop Filter Bandwidth

To achieve clean output clocks, it is crucial to configure the phase-locked loop (PLL) loop filter bandwidth appropriately. A wider loop bandwidth is generally desirable and should exceed 100 kilohertz. By selecting a clock generator with a wide loop bandwidth filter, designers can generate output clocks with reduced noise and improved performance.

Phase Detector Frequency

The phase detector frequency is another significant factor to consider when choosing a clock generator. A higher phase detector frequency offers benefits such as a smaller N-divider, resulting in reduced PLL noise. Determining the maximum possible phase detector frequency involves calculating the greatest common denominator between the reference clock and the VCO. This calculation helps identify the potential limitations of the clock generator in generating specific system clocks.

Reducing Radiated Emissions with Spread Spectrum Clocking

For applications requiring reduced electromagnetic interference (EMI), clock generators supporting Spread Spectrum Clocking (SSC) can be advantageous. SSC is a common technique employed to minimize radiated emissions. The amount of spread determines the reduction in the fundamental frequency tone, which can decrease by at least 6 dB. The modulation depth and profile of SSC are typically configurable, allowing users to select the optimal settings that comply with system spectral mask limits.

Loop Filter Components

The integration of loop filter components within clock generators adds to their versatility and convenience. Loop filters are configurable components that play a crucial role in controlling the performance of the PLL. The flexibility of clock generators in configuring loop filters enables system designers to optimize the loop bandwidth according to the specific requirements of their applications. Additionally, clock generators often feature integrated non-volatile memory, such as EEPROM, which proves helpful in scenarios where no microcontroller or FPGA is available for pre-configuring the clock generator before system startup.

Clock Generators with Non-Volatile Memory

Clock generators equipped with non-volatile memory, such as EEPROM, offer added convenience for system designers. With integrated non-volatile memory, clock generators retain their configurations even during power cycles or system resets. This feature proves valuable in applications where clock configurations need to be preserved without relying on external microcontrollers or FPGAs for reprogramming.

Highlight:

• Clock generators simplify system design and enhance reliability.
• VCO frequency range determines the feasibility of generating multiple system clocks.
• Overcoming high VCO frequency through multiple PLL/VCO units or embedded fractional dividers.
• Proper PLL loop filter bandwidth is crucial for generating clean output clocks.
• Spread Spectrum Clocking helps reduce radiated emissions and comply with spectral mask limits.
• Clock generators with non-volatile memory offer convenience and flexibility in configuration.

FAQ

Q: What is the advantage of using clock generators over multiple oscillators and crystals? A: Clock generators consolidate multiple clock sources into a single device, simplifying system design, improving reliability, and reducing BOM complexity.

Q: How do clock generators overcome high VCO frequency limitations? A: Clock generators can incorporate multiple PLL/VCO units or embedded fractional dividers to handle clocks within the same frequency domain, enabling the generation of multiple system clocks.

Q: Can clock generators reduce radiated emissions? A: Clock generators supporting Spread Spectrum Clocking (SSC) can help reduce electromagnetic interference (EMI) and comply with spectral mask limits.

Q: What is the benefit of clock generators with non-volatile memory? A: Clock generators with integrated non-volatile memory retain their configurations even during power cycles or system resets, eliminating the need for external programming devices.