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

Timing components are one of the most ubiquitous components in electronics. They are needed in nearly every complex design, and electronics wouldn’t function properly 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. This makes timing components and clocks foundational to how digital circuits operate.

A clock signal is a timing signal that repeatedly oscillates between a high and low states (figure 1). Acting like a metronome for digital circuits, it tells the circuit when to perform each operation to ensure a coordinated sequence of actions.

If the clock is the “heart” of a design, then clock signals are the steady heartbeat that keeps the entire system moving in sync.

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, commonly called a crystal, operates when combined with an amplifier circuit that applies voltage to an electrode on or near the crystal.

A quartz crystal is a tiny piece of quartz with each of the two surfaces metalized and attached with an electrical connection. It’s important that 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 commonly used because their output frequency stays stable even when temperatures fluctuate. In contrast, internal RC resonators are more sensitive to temperature change which could affect the behavior of the oscillator and lead to frequency drift.

5 Common Types of Oscillators

In a clock, an oscillator 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.

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 meant for applications where high stability is required across a wide temperature range, especially in high-temperature environments. Some examples include general electronic circuit designs, aerospace systems and industrial equipment.

Microelectromechanical System Oscillator (MEMS Oscillator)

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.

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 measures 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.

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.

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. These higher frequency capabilities are due to the mechanical wave motion produced by the surface acoustic wave, which allows it to couple with the surface. The characteristics of the coupling allow the resonator to sense mechanical and mass properties, enabling these higher frequencies

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.

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

A familiar example is the microcontroller. Microcontrollers rely on a clock from a crystal oscillator to function, except in asynchronous circuits, such as asynchronous CPUs. Most common microcontrollers contain an internal R-C oscillator, which is sufficient for tasks 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) ensures 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. For instance, if an oscillator used as the basis for a real-time clock was off by just 0.1%, a week later the clock would be almost 10 minutes off. Long-term accuracy might also be needed without knowing the exact real time.

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, to preserve battery power. A standard 20ppm oscillator would be off by 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 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 sufficient 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, whereas internal oscillator(s) are typically limited to one frequency with a handful of clock prescaler options. In electronics, time is a property that can be measured accurately and cheaply, so often 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 stable.
• 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 at which it is operated. 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-controlled 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 Now Available at Braemac Americas

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 offer a wide range of the industry’s latest timing solutions like quartz crystals, oscillators, resonators, and real time clocks to include in your clock tree design.

Leverage Braemac Americas comprehensive suite of value-added services for expert guidance, clock tree support, and design optimization every step of the way. Our team can help you select and integrate the ideal timing solution for your design.

• XO (Crystal Oscillator) : fixed-frequency, stable, widely used
• TCXO (Temperature-Compensated Crystal Oscillator): maintains stability across wide temperature ranges
• OCXO (Oven-Controlled Crystal Oscillator): highest stability, uses an internal oven
• VCXO (Voltage-Controlled Crystal Oscillator): tunable frequency via control voltage
• MEMS Oscillator: uses microelectromechanical resonators for high shock/vibration resistance

• required frequency accuracy
• operating temperature range
• jitter requirements
• footprint constraints
• power consumption
• whether the system needs synchronization