FEATURE EMBEDDED SYSTEMS NEGOTIATING THE CRYSTAL MAZE
Andy Treble at Euroquartz discusses what you need to know for selecting the right quartz crystal and oscillator for embedded systems and how careful consideration can have a significant performance impact
I
n order to discuss the use of crystals and oscillators within embedded systems it is useful to understand the types of embedded systems that are being designed. Embedded systems may be categorised into three types as follows: Standalone Embedded Systems – these are embedded systems that are self- sufficient and can work by themselves. They do not require a host system or computer to function. Examples include videogame consoles, music players and microwave ovens. Real-time Embedded Systems – An
embedded system that gives an output within a specified amount of time is called a real-time embedded system. Examples include vehicle systems, process control, building management etc. Networked Embedded Systems –
Embedded systems that are connected to a network and depend on it for their functioning are called networked embedded systems. Examples include home security systems and heat sensor systems, not forgetting the world of the Internet of Things (IoT). They all have one thing in common in that they use microprocessors of some sort and therefore require accurate timing control to synchronise processing activities. As systems have become more compact over the last 10 years, so too have the crystals and oscillators used for such applications. Many modern embedded systems are complex in operation and place more stringent requirements on the oscillators.
As processing speeds increase oscillator frequencies have also had to rise to offer correct timing. The difficulty with this is that, in general, to increase oscillator frequency, a phase-locked loop (PLL) circuit is required. The introduction of a PLL increases the phase noise and jitter of the oscillator which can cause data integrity problems due to introduction of waveform errors. This primarily results from amplification of initial errors which compounds the phase noise problem. This is especially the case as frequencies rise above the 70MHz region. However, the latest oscillators offer phase jitter figures of 150fsecs which is
26 FEBRUARY 2018 | ELECTRONICS
sizes have been reduced significantly in the last few years. The trade-off here is that frequency range is restricted, with maximum frequencies up to 48MHz available. It should also be noted that due to the physics of quartz the Equivalent Series Resistance (ESR) will be higher (see table 1). The higher the ESR, the more drive current will be required to produce oscillation in the crystal, as the ESR will be exhibited as a spread of values for each crystal type. Therefore, it is essential to assess the circuit gain to ensure that the maximum case ESR is considered. Failure to do this may result in circumstances where a varying percentage of circuit failures occurs in production batches. This means the production line will effectively be screening out the higher band of ESR product which obviously is undesirable. It is worth noting that the ESR will
also increase when third overtone crystals are used to obtain higher frequencies.
an order better than previous devices. This new generation of parts offers frequencies up to approximately 2.1GHz enabling much faster processing speeds to be considered in designs. Use of embedded systems generally implies smaller sizes so crystal package
Table 1: Typical oscillators for embedded systems !"#$%#&'() "!#!"$%&'!
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Figure 1: Euroquartz latest
oscillators for embedded applications offer ultra- low phase jitter and frequencies from 50 to 2100MHz with choice of outputs
ALL SHAPES AND SIZES Crystal oscillators come in many variants from simple clock oscillators to complex TCXOs, but most embedded systems will utilise basic clock oscillators. The criteria for clock selection will obviously depend on the system into which it is being designed, however some basic requirements can be identified. There are six common characteristics,
namely: frequency, input voltage, current consumption, phase noise/jitter, output type and package size. As the complexity of embedded system designs increases so too do the timing requirements. Not only are quartz oscillators becoming more sophisticated, but the integration of timing circuits within silicon devices is also continuing apace. The next few years should be an exciting and interesting time as IoT and other embedded developments gather momentum.
Euroquartz
www.euroquartz.co.uk T: 01460 230000
/ ELECTRONICS
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