There are so many different devices and products for frequency generation and timing, and with 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.
In this article, we’re going to help clear up some of the confusion around CMOS clocks. We’ll cover what these devices are, why they're important, and how to know if they're the best fit for your application.
The Difference Between Clocks & 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 the timing of cycles and instructions to ensure the larger system functions properly and stays in sync. Specific application examples range anywhere from the clocks in your car to the 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 between clocks and oscillators. Essentially, oscillators can refer to either simple clocks that output a timing signal or to more complex modules used for accurate signal references. Clocks just refer to the former.
In other words, a clock is a oscillator that does not use any form of temperature compensation (TCXOs) or oven control (OCXOs). One way of thinking of clocks would be as an OCXO without the “oven controlled,” or “OC."
Remember this: Clocks are just XOs.
Related: Crystal Oscillators: The Beginner's Guide (OCXO, TCXO, VCXO, & Clocks)
CMOS Clocks
One type of clock is the complementary metal-oxide-semiconductor (CMOS) clock. Clocks output logic level timing references, and CMOS clocks output 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 the conductor of an orchestra, making sure everything in a circuit happens at the right time and in the right 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
Related: CMOS Clocks: Common & Advanced Applications
The traces on a printed circuit board (PCB) 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.
Related: CMOS vs. LVCMOS: Which Is the Best Output Signal for Your Application?
Browse Our Clocks & Oscillators
At Bliley, we've been making industry-leading frequency control devices for almost 100 years. Browse our full line of clocks (XOs), including CMOS, LVCMOS, and HCMOS.
