PARTNER FOCUS
Embedded buffer crystal
he need to control numerous gates with a single crystal os- cillator occurs in many control systems and the more gates that are being timed the higher the output drive capability required from the oscillator.
The distribution of clock signals has traditionally been performed by having a high drive level oscillator, usually 50pF to sustain several IC gates. Using a 5V oscillator can also increase the drive capability of the circuit and many defence applications are still based on 5V rails established for many decades in TTL technology. Design trends in the latest
semiconductors will phase out 5V versions with 50pF drive capability completely, in favour of lower power variants. This creates design issues for systems based on 5V operation and engineers should be considering the problem before 5V chipsets become completely obsolete. One alternative is to use several discrete oscillators that may be designed into a circuit so that each drives a different section of the system. The difficulty of this approach is that the
synchronisation of the outputs can be problematic, particularly if the oscillator outputs turn on at varying rates.
It should also be noted that driving an oscillator to its limits will have a detrimental effect on the ageing of the crystal due to mechanical fatigue and induced thermal conditions.
In addition, each individual oscillator will experience slight temperature variance which can lead to issues of skew, defined as the frequency variance between the master oscillator and other oscillators in the system. Another phenomenon to
consider is that of start-up time.
oscillators T
This is especially acute when oscillators have been powered down for some time as each device may differ slightly in its start-up time performance. Varying start-up performance
can also be problematical if the system is designed to activate processors in a fixed sequence of operations with processors timed to switch at different rates causing a sequencing problem. Deploying multiple discrete 15pF oscillators with additional buffer ICs on the circuit board to provide the multiple drive capability required also uses up valuable real estate on the board. A neat solution to these
problems is to incorporate a buffer within the CMOS oscillator design and provide multiple outputs to drive several 15pF loads in parallel. Thus, for such a device with four outputs, the drive capability from one single oscillator becomes 60pF instead of 15pF which reduces board space, offers the high drive capability desired and a single small package outline. The other advantage of this
approach is that as the outputs are derived within a single package from a single crystal the thermal variance is less of an issue. The issue of skew is also effectively eliminated as the outputs are buffered in the single oscillator package. The current consumption
would be dependent of the number of outputs used. For instance, with a 25MHz oscillator running at 3.3V, the current consumption would be around 6mA if two outputs are used. For four outputs it would be in the order of 9mA.
When the drive electric current is high, the ageing property and long-term frequency stability will
20 JUNE 2021 | ELECTRONICS TODAY Figure 1: RMS Jitter : 90fsec typ. @3.3V , 25MHz
performance from the oscillator and so there may be a concern that these important characteristics could be compromised by incorporating a buffer within the oscillator. In fact, the latest chipsets offer
be degraded. But when the drive level is too small it should be noted that electric current generated by noise may be relatively large in comparison to the crystal electric current, and this will impair short-term frequency stability.
This type of Embedded Buffer Crystal Oscillator is available in various package sizes and numerous input voltages. Digital circuits require good phase noise and jitter
extremely good performance and the close proximity of the buffer to the oscillator is actually a benefit from a noise generation perspective. The curve shown in figure 1
indicates the phase noise and jitter that may be expected. With the reduced availability of
50pF drive oscillators and 5V variants, these new embedded buffer oscillators will begin to play a larger role in circuit design especially in embedded systems and industrial control networks.
Euroquartz
www.computercomponents.com
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