Inside Frequency Control


A Beginner’s Guide to Understanding CMOS Clocks

[fa icon="calendar"] Jun 13, 2017 6:30:00 AM / by Bliley Technologies

Bliley Technologies

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There are so many different devices and products for frequency generation and timing, but between all the acronyms and complex terminology of the RF industry, it can be confusing trying to understand what each type of clock or oscillator does, and what the best applications for each of them are. In this article, we’re going to help clear up some of the confusion, with a specific emphasis on CMOS clocks. We’ll cover what these devices are, why they are important, and how to know if one of these clocks is the best fit for your application.

What are Clocks? Are They Different from Oscillators?

Part of the reason why some people are confused about the distinction between clocks and oscillators is that these terms are sometimes used interchangeably by mistake. An oscillator is composed of a crystal to which an electrical charge is applied. This charge causes the crystal to vibrate and oscillate at a particular frequency, producing a signal in the process.

The term “clock” is a little more abstract…  In the most basic sense, the purpose of a clock is to regulate a process or device in some way. The clock’s job is to maintain and regulate the timing of cycles and instructions, to make sure that the larger system functions properly and stays in sync.  Some specific application examples of clocks range anywhere from the clocks in your car to more advanced timing devices in communications equipment.  

A crystal oscillator can be used to produce a clock signal, which is probably the source of the confusion in some people’s minds.  Essentially, oscillators can refer to simple clocks that output a timing signal or more complex modules used for accurate signal references.  Clocks just refer to the former.

There’s a chance we just confused you more there…

Hold on to your horse there, cowboy (or cowgirl)! We’re about to clear things up for you. Hang in there!

A clock is a oscillator that does not make use of any form of temperature compensation or control (TCXO), or oven-controll (OCXO). One way of thinking of clocks would be an OCXO without the “oven controlled”, or “OC."

Remember this: Clocks are just XOs

CMOS Clocks

CMOS clocks are a type of clock discussed above. Clocks output logic level timing references.  CMOS clocks ouput, well... CMOS logic.  CMOS clocks are used to help timing devices function in many different systems and products, just like general clocks. They play a role somewhat analogous to that of a conductor of an orchestra, making sure everything in a circuit happens at the right time and order.

Some of the simplest examples of products that make use of CMOS technology would be digital clocks like a wristwatch or those found in a car, microwave or home oven. But this same technology is also used to provide advanced timing capabilities in far more complex systems in more extreme environments including:

  • Military applications
  • Communication systems
  • Spacecrafts and Satellites
  • Anywhere where strong external vibrations or g-forces may be a threat

The traces on a printed circuit board act almost like highways, carrying the clock signal where it needs to go. The higher the frequency of a clock signal, the more vulnerable it is to phase noise, distortion, and attenuation. One of the most basic steps to minimizing this risk is to select the right clock output type for your application. The outputs of CMOS clocks are especially well suited for circuits with shorter trace lengths, and lower frequency clock sources, defined here as under 200 Mhz. CMOS outputs are primarily used for digital circuits, whereas analog circuits are usually better served by sinewave outputs.

One of the biggest benefits of CMOS clocks is low power consumption. Compared to other solutions, they are also relatively inexpensive to implement. CMOS clocks also offer good jitter performance and generally low phase noise. These clocks come in different variations, including low-voltage (LVCMOS) and high-speed (HCMOS) designs.

To determine if a CMOS solution is the right fit for your application, check out our free guide to picking the right output signal.


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Topics: CMOS, Clocks

Bliley Technologies

Written by Bliley Technologies

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