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# What Are Clock Signals in Digital Circuits, and How Are They Produced?

## Mariel Galarza in Blogs on July 12, 2023

Mariel Galarza is an Applications Engineer at Symmetry Electronics. She has a bachelor’s degree in Engineering in Renewable Energy from CETYS Universidad with focused studies in biomass, wind, and solar energy. In her current role, Mariel provides technical support regarding timing, power and microcontrollers. Mariel’s professional background in sales, training, and customer service fortifies her skills in helping customers find optimal solutions for their desired applications. Mariel has a passion for learning and the fast-paced environment of the electrical engineering industry.

Timing components are one of the most ubiquitous components in electronics. They are needed in nearly every complex design and all our electronics wouldn’t work without them. We previously discussed how to design a clock tree, but understanding how a clock works is crucial to understanding how your design operates. Let’s examine the different kinds of timing components available and the importance of clocks in digital circuits.

# What is a clock signal?

A clock signal (Figure 1) is a particular type of signal that oscillates between a high and low state. With the signal acting as a metronome, the digital circuit follows in time to coordinate its sequence of actions. Digital circuits rely on clock signals to know when and how to execute the functions that are programmed.

If the clock in a design is like the heart of an animal, then clock signals are the heartbeats that keep the system in motion.

Figure 1: Clock signals oscillate between a high and low state.
Source: Steemit

# How are clock signals produced?

There are different ways a clock signal can be produced, but they all start off with a crystal resonator. A crystal resonator is commonly referred to as a crystal. In order to operate, crystals are combined with an amplifier circuit to apply voltage to an electrode near or on the crystal.

The quartz crystal is a tiny slit of quartz with each of the two surfaces metalized and attached with an electrical connection. It’s important the physical size and shape of the quartz crystal are precisely cut because this determines the frequency of oscillations produced from the crystal. Once the crystal is cut and shaped, it cannot be used at any other frequency. Quartz crystals are more commonly used since the frequency generated from quartz crystals are more resistant to changes in temperature. If an internal RC resonator was used instead, changes in temperature would affect the behavior of the oscillator, leading to changes in the output frequency.

A popular quartz crystal from Abracon is the ABS05 Tuning Fork Crystal, its used in a wide variety of applications, including industrial and medical. The ABS05 family provides low power consumption without compromising performance.

# 5 Common Types of Oscillators

In a clock, an oscillator (Figure 2) is responsible for generating a precise and regular timekeeping signal. It provides a reference frequency that determines the speed at which the clock measures time. Let’s take a look at the 5 most common types of oscillators.

Figure 2: An oscillator generates a precise and regular timekeeping signal.

## Crystal Oscillator

Crystals have a sinusoidal output and are typically used if the target IC has an integrated oscillator and on-chip phase-locked loops (PLLs) for internal timing. When a crystal and oscillation circuit are combined in the same package, it is commonly referred to as a crystal oscillator (XO). This quartz piezo-electric oscillator outputs a usable oscillating signal, most commonly a square wave with 50% duty cycle. Usually, this clock signal is fixed at a constant frequency and synchronization may become active at either the rising or falling edge of each clock cycle.

The ASAKMP family is part of Abracon’s Tiny but Mighty 2.0 products. The ASAKM is a miniature crystal oscillator family that comes in a compact package size of 1.6 x 1.2 x 0.6 mm.

## Temperature Compensated Crystal Oscillators (TCXO)

Temperature-compensated crystal oscillators are XOs that implement a temperature compensation technique through temperature-sensitive reactance circuit in its oscillation loop. TCXOs are mean for applications where high stability is required through a wide temperature range, especially high temperature environments. some examples are general electronic circuit designs, aerospace systems and industrial equipment.

Abracon’s ATXK-H product family is designed for applications such as IoT and network timing synchronization. It was developed for precise timing in applications where space saving is a priority.

## Oven Controlled Crystal Oscillator (OCXO)

Oven-controlled crystal oscillators, accurately described by their name, are quartz-based timing devices controlled by a very small internal oven, OCXOs are the most improved accuracy among the other types of crystal oscillators. This timing device works by heating up the crystal to higher temperatures that it would normally face and keeping its temperature constant, due to this they consume more power and have a higher price point compared to other crystal oscillators. OCXOs are normally used in applications that require temperature stabilities around ±1 x 10-8 or better.

Common applications for OCXOs include military communication equipment, GPS steered applications, cellular towers, etc.

The AOC2012 family in the Stratum 3/3E OCXOs are oven controlled SC-Cut crystal oscillators with very low long-term aging characteristics over 20 year whether it be in fixed or voltage controlled LVCMOS clock output configurations.

## Voltage Controlled Crystal Oscillator (VCXO)

Voltage controlled crystal oscillators generate frequencies determined by the variation of its control voltage on the voltage input pin. A VCXO utilizes diodes to tune the signals frequency, this is a process known as pulling and it’s measured in ppm. How much the frequency changes by the modification of voltage is known as deviation, and the deviation vs the control voltage can be measured and shown on a graph known as transfer function or slope polarity.

Voltage controlled crystal oscillators are commonly found in phase lock loop applications and modulation schemes.

The ABLJO VCXO crystal family has a wide frequency range and operating temperature, as well as an Ultra Low Jitter performance.

## Microelectromechanical System Oscillators (MEMS Oscillators)

MEMS oscillators are timing devices that can generate highly stable reference frequencies that can be used to measure time. A MEMS oscillator is formed by a MEMS resonator, this is a microelectromechanical structure that vibrates in response to piezoelectric or electrostatic stimulation from the analog driver IC and by doing this helps define stable frequencies.

MEMS oscillators can be used as alternatives to older, more established quartz crystal oscillators, this due to their reliability relative to temperature variation as well as their better resilience against vibration and mechanical shock. They can be found in different applications such as data transfer management, radio frequency definition. They are also present in aerospace, precision GNSS, avionics, etc.

Abracon's ASEMB clock oscillators provide a low power solution for your application. That paired with exceptional stability (+/- 10 ppm over a -40C to +105C temperature range), makes it a great component to include in your upcoming project.

# Real Time Clocks (RTC)

A real time clock (RTC) is an electronic device, most of the time found in the form of an integrated circuit, that has the purpose of measuring the passage of time. An RTC counts seconds, minutes, hours, days and even years. They are found in almost any electronic device that needs to keep an accurate track of the time of day. This device has to keep time even when its encompassing application is powered down, this IC is normally operating on an alternative power source, permitting it to work under low power.

The AB08X5 Real-Time Clock Family from Abracon carries advances timekeeping and power features. It is well suited for applications such as smart cards, medical electronics, data loggers, etc.

# Ceramic Resonators

A ceramic resonator is an electronic component composed of piezoelectric ceramic material, which functions as a mechanical resonator. When a voltage is applied to this type of resonator its piezoelectric "vibration behavior" causes an oscillating signal. In this component the resonant frequency is determined by the thickness of the ceramic substrate.

Ceramic resonators have a similar function as a quartz crystal in that they are used to generate a clock signal in applications where high accuracy isn’t required. They are commonly found in telephones, cameras, communication equipment, copiers, etc.

The AWSCR-CELA family encompasses miniature ceramic resonators with an industry standard footprint and comes with built in capacitance options and wide operating temperatures.

# SAW Resonators

A surface acoustic wave or SAW resonator is a device that works at a higher frequency compared to other crystal or ceramic resonators. This higher frequency capabilities are due to the mechanical wave motion produced by the surface acoustic wave, making it possible for it to couple with the surface, the characteristics of the coupling allow the resonator to sense mechanical and mass properties allowing these higher frequencies.

Common uses for SAW resonators include radio transmitters and radio devices where channelization is not necessary, this would include remote controls, garage door openers, access control, among other things.

Abracon has many SAW Resonator solutions to choose from. Among them there’s the ASR315E, ASR390E, and ASR423.22E.

# What are Synchronous and Free-Running Designs?

Systems and their combination of various subsystems may require a timing architecture that is free-running or synchronous.

If a system is free-running, independent clocks can be used without any special phase-lock or synchronization requirements. Examples include standard processors, memory controllers, SoCs and peripheral components (e.g., USB, PCI Express switches).

An example of a complex IC everyone is familiar with would be the microcontroller. Microcontrollers rely on a clock from a crystal oscillator to function with an exception for when used in asynchronous circuits, like in the case of asynchronous CPUs. Most common microcontrollers contain an internal R-C oscillator that is good enough for things like UART communication, although external crystal oscillators are necessary for other types of communication like USB or ethernet.

Conversely, synchronous timing systems require continuous communication and network-level synchronization across all associated systems. In these applications, low-bandwidth PLL-based clocks provide jitter filtering to ensure that network-level synchronization is maintained. For example, synchronizing all SerDes (serialization-deserialization) reference clocks to a highly accurate network reference clock (e.g., Stratum 3 or GPS) guarantees synchronization across all system nodes.

Examples of synchronous clock trees include Optical Transport Networking (OTN), SONET/SDH, Mobile backhaul, Synchronous Ethernet and HD SDI video transmission. There are various applications that require accurate frequency or timing other than communications, though. Some applications require long-term synchronization between two subsystems that are not connected to one another. If an oscillator used as the basis for a real-time clock was off just 0.1%, a week later the clock would be almost 10 minutes off. Long-term accuracy might also be needed without having to know real time.

For example, suppose you want several Bluetooth modules to wake up once every hour to exchange data for a few seconds and then go back to sleep, in order to preserve battery power. A standard 20ppm oscillator would be off just fractions of a second per hour, whereas a 1% RC resonator could be off by half a minute. If the RC resonator is used, the Bluetooth modules would have to remain on for longer periods of time in order to communicate with one another, thus wasting battery power.

# Internal vs External Oscillators

Internal oscillators are commonly used to provide timing for MCUs that don’t require accurate timing. Internal oscillators are good enough for low-baud UART communication, although external crystals and oscillators are required for communication protocols such as CAN, USB or Ethernet which have stricter timing accuracy requirements.

Using an external oscillator allows a wider range of frequencies where the internal oscillator(s) are typically one frequency with a handful of clock prescaler options. In electronics, time is a property that can be measured accurately and cheaply, so often times a problem is transformed into one of measuring time or producing pulses with accurate timing.

Some advantages of external clocks and oscillators include:

• Precision - internal clocks are not precise and can be affected by noise
• Temperature independence - oscillators and clocks (especially temperature compensating oscillators) can be used in low or high-temperature applications or where the temperature varies greatly. With changes in temperature, the oscillation frequency can remain relatively the same.
• Speed - internal oscillators might not reach the highest speed of the IC, in which case an external oscillator is required
• Voltage - The speed of an internal oscillator may be dependent on the voltage that it is being run at. If an oscillator drives equipment that may generate radio-frequency interference, adding a varying voltage to its control input can disperse the interference spectrum making it closer to ideal. In this example, only an external, voltage-controller oscillator could provide that capability
• Multiple clocks required - if many subsystems need to operate in sync and are connected to one another, a single external clock generator can be used to replace each subsystem’s free-running timing component

# Timing Solutions Available at Symmetry Electronics

Abracon is a leading global manufacturer of frequency control, signal conditioning, clocking, and magnetic components, providing innovative solutions for diverse industries. As an authorized distributor of their next generation portfolio, we are able to offer a wide range of the industries latest timing solutions like quartz crystals, oscillators, resonators, and real time clocks to include in your clock tree design. Abracon timing solutions are known for their:

• High precision
• Reliability
• Rigorous testing and quality control measures
• Industry expertise
• Application versatility

To learn more about which Abracon timing solution is ideal for you device’s design, contact Symmetry Electronics today!

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