Almost every electronic device needs a clock source. For example, a microcontroller (MCU) uses an oscillator to advance the next instruction, and a radio needs an accurate oscillator to mix radio-frequency signals to baseband for processing.
The advent of smart, connected appliances has created higher demands on clock performance. This article explains how the designer can meet these challenges while reducing technical risk, design time and bill of materials. We look at the options of quartz crystals, quartz crystal oscillators (XOs) and highly-integrated clock solutions using quartz and MEMS-based technology.
Smart, connected devices need sophisticated clock trees
MCUs often include internal RC phase-shift oscillators for non-precision computing applications. These oscillators use an integrated resistor-capacitor pair to create the time constant that governs the oscillator frequency. Such oscillators have an accuracy of about 1% and exhibit high jitter (unwanted random fluctuations in the timing of the clock transitions). They are suitable for applications in which transition timing is not critical, such as clocking an MCU for computation and driving a simple numeric seven-segment liquid crystal display (LCD). The display requires multiple clock waveforms; however, transition timing tolerance is a few milliseconds. UART communication up to a few Mbit/second is also possible, with timing tolerances of hundreds of nanoseconds, but this represents the limit of a simple RC oscillator.
Smart, connected products communicate over a network to the cloud, via Bluetooth, wired Ethernet, Wi-Fi or other connectivity protocols. The inclusion of radio and/or high-speed data drives the need for precision clocks with an accuracy of a few parts per million (ppm) and low jitter.
A key component required in the generation of a precision clock is a stable reference frequency, and this requires a resonator. A resonator is an electrically passive device that naturally oscillates with larger amplitudes at certain (resonant) frequencies than others – the strings of a violin are a simple example. Common choices for electronic devices are quartz crystals and MEMS resonators. The requirements of a resonator are:
- Stability of the resonant frequency over time and temperature. This avoids clock frequency drift.
- A high quality factor, or Q, that ensures resonator response to only a very narrow band of frequencies.
- Ability to operate at a high signal level, which supports a good signal to noise ratio at the output.
Items two and three are key to ensuring a clock signal with low jitter, enabling stable timing transitions.
Since a resonator is a passive device, it requires controlled energy to oscillate and create the reference frequency. Coupling a resonator to a sustaining amplifier in a feedback configuration achieves this stable oscillation. Quartz crystals or MEMS resonators with the appropriate amplifier are very suitable as frequency references for data transmission in the 10 Mbit/s and above domain.
Quartz resonators have a high Q and high-output capability, and are suited to applications where jitter must be extremely low. 100 femtoseconds of phase noise (measured in the traditional 12 kHz to 20 MHz bandwidth) is achievable. MEMS resonators operate over extended temperatures with very stable frequency, offer very high reliability, are resistant to shock and vibration and enable very small clock solutions, close to 1 mm square. MEMS resonators have high Q with lower output; 500 femtoseconds phase noise is possible, and new resonator designs are pushing this lower. For many modern networking applications, such as PCIe, for example, a smaller integration bandwidth applies, and both technologies are very suitable.
Implementing a clock in an embedded system
In an embedded system, there are three common implementations of resonators to produce a clock signal.
- A quartz crystal connected directly to a “target SoC,” (which is to be clocked)
- A quartz crystal oscillator or XO, creating one clock output for the system as a whole
- A quartz or MEMS-based clock generator (creating one or multiple clock outputs, at both low and high [>50 MHz] frequencies)
Implementation 1: Crystals connected to the target SoC
In this case, the System on Chip (SoC) is designed with a sustaining amplifier and the crystal can be connected directly, usually with capacitors to arrange the right feedback and tune the frequency. The SoC amplifies and may frequency-translate the reference clock, according to its needs. Diagram 1 shows the schematic of both a high-frequency and a low-frequency crystal connected directly to an MCU.
Two crystals connected directly to an MCU, showing loading capacitors and series resistors.
A system requiring only one or two crystals connected in this way is cost-effective; PC board layout is easy with the crystal placed immediately next to the SoC, generally avoiding issues with signal integrity and electromagnetic interference (EMI).
However, there are some caveats to consider:
- The crystal must be carefully chosen to be compatible with the SoC’s internal sustaining amplifier circuit. If the equivalent series resistance of the crystal is too high compared to the negative resistance of the amplifier, the oscillator may fail to start.
- The crystal will likely require loading capacitors to ensure that the feedback is correctly phased, as well as setting the frequency accurately.
- Quartz crystals have a relatively large temperature coefficient. Applications requiring operation outside -40°C to 70°C may need to use a temperature-compensated crystal oscillator (TCXO) or an integrated MEMS-based clock.
- Standard quartz crystals operating in a fundamental mode have resonant frequencies at or below 50 MHz. Quartz crystals operating above 50 MHz in an overtone mode tend to be more expensive.
Implementation 2: Crystal Oscillators (XO)
An integrated crystal and sustaining amplifier in a single package is known as a crystal oscillator or XO. Diagram 2 shows how the crystal wafer blank is combined with an oscillator Application-Specific Integrated Circuit (ASIC) in a hermetic assembly. This pre-packaged solution, although more expensive than a single crystal, avoids the interfacing issues of caveats one and two above, so that reliable start-up and correct output frequency is assured.
Crystal oscillators consist of a quartz crystal blank, traditionally inside a ceramic package with a metal lid.
Again, a system requiring one or two XO’s can be cost effective. If multiple frequencies, additional buffered outputs or frequencies above about 50 MHz are required, the system can benefit from an integrated clock generator.