Expert technical article: It's time to upgrade your clock.

Almost every electronic device requires a clock source. For example, a microcontroller (MCU) uses an oscillator to advance to the next instruction, and radios require a precise oscillator to mix the radio frequency signal into the baseband for processing.

The emergence of smart, connected devices has placed higher demands on clock performance. This article explains how designers can address these challenges while mitigating technical risks, shortening design time, and reducing bills of materials. We focus on quartz crystals, quartz oscillators (XOs), and highly integrated clock solutions utilizing quartz and MEMS-based technologies.

MCUs typically include internal RC phase-shift oscillators for non-precision computing applications. These oscillators use integrated resistor-capacitor pairs to create the time constant controlling the oscillator frequency. Such oscillators have approximately 1% accuracy and exhibit high jitter (unexpected random fluctuations in the timing of clock transitions). They are suitable for applications where transition timing is not critical, such as clocking a computing MCU and driving a simple seven-segment digital liquid crystal display (LCD). The display requires multiple clock waveforms, but the transition timing tolerance is only a few milliseconds. Furthermore, UART communication up to several Mbps can be achieved, in which case the timing tolerance is a few hundred nanoseconds, but this also represents the limitations of a simple RC oscillator.

Smart connected products communicate with the cloud via Bluetooth®, wired Ethernet, Wi-Fi®, or other connectivity protocols. Due to the involvement of radio waves and/or high-speed data, low-jitter precision clocks with an accuracy of parts per million (ppm) are required.

A key factor in generating a precision clock is a stable reference frequency, which requires the use of a resonator. A resonator is a passive electronic device that naturally oscillates at certain frequencies with a higher amplitude than at others—a violin string is a simple example. Quartz crystals and MEMS resonators are commonly used in electronic devices. The requirements for a resonator are as follows:

1. The resonant frequency remains stable over time and temperature. This helps prevent clock frequency drift.

2. A high quality factor (Q) ensures that the resonator responds only to a very narrow frequency band.

3. It can operate at high signal levels, thus achieving a good signal-to-noise ratio at the output.

The second and third items are crucial for ensuring low jitter clock signals, enabling stable timing transitions.

Since resonators are passive devices, controlled energy is required to maintain oscillation and generate a reference frequency. This stable oscillation can be achieved by coupling the resonator to a sustaining amplifier in a feedback configuration. If a quartz crystal or MEMS resonator is paired with a suitable amplifier, it would be well-suited as a frequency reference for data transmission in domains of 10 Mbps and above.

Quartz resonators offer high Q values ​​and high output capability, making them suitable for applications where extremely low jitter is required. Phase noise of 100 femtoseconds can be achieved (measured in a conventional 12 kHz to 20 MHz bandwidth). MEMS resonators operate at very stable frequencies under extended-range temperatures, offering extremely high reliability, shock and vibration resistance, and enabling ultra-small clock solutions (approaching 1 square millimeter). MEMS resonators have higher Q values ​​and lower output; phase noise of 500 femtoseconds can be achieved, and recent resonator designs continue to reduce this value. For example, many modern networking applications (such as PCIe) support smaller integrated bandwidths, making both technologies well-suited.

In embedded systems, clock signals can be generated using three common types of resonators.

• Connect the quartz crystal directly to the target SoC (which will be clock-driven).

Figure 1: Two crystals are directly connected to the MCU, showing the load capacitance and series resistance.

• A clock output is created for the entire system via a quartz crystal oscillator (XO) .

Figure 2: Crystal oscillators consist of quartz crystal wafers, traditionally packaged in ceramic and fitted with a metal cap.

• Quartz or MEMS-based clock generators (creating one or more clock outputs at low and high frequencies [>50 MHz] )

Figure 3: The integrated clock generator combines a MEMS (or crystal) resonator with an oscillator and extends functionality through a programmable PLL and a buffered output stage.