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(typically 50ppm), aging (typically 5ppm / year), and loading (up to 50ppm). In some cases this can mean that a part with a specifi ed frequency tolerance of 50ppm does in practice, have an effective tolerance of up to 200ppm. CMOS oscillators are not affected by the factors described above, so long as they are operated within their designated operating temperature range. Therefore a 100ppm part should exhibit this frequency tolerance in operation, in an application.
Durability The non-mechanical nature of CMOS oscillators means that they can be housed in low-cost conventional IC plastic packaging rather than more costly hermetically sealed ceramic packages that are needed for crystal-based devices. The proliferation of portable electronics equipment has placed increased importance on the durability of components contained within. As would be expected, CMOS oscillators offer high levels of durability and have semiconductor grade levels of shock and vibration resistance. Conversely, the electro- mechanical nature of crystal oscillators means that they can be susceptible to failure caused by shock and vibration experienced in normal use.
Noise and start-up In applications such as high-end datacom, telecom and RF equipment there is often a requirement for very low phase jitter. In such cases, the performance of current generation of CMOS oscillators may not be good enough to allow replacement of crystal devices. However, because of the potential power and cost savings of switching to a CMOS part, designers should carefully consider whether their application could accept the alternative device. It is to be noted that next generation of CrystalFree oscillators are expected to break the 1ps phase jitter performance that is required to break into the high end Datacom and telecom applications. Susceptibility to increased jitter should also be considered carefully, with the metal can enclosures
22 EIU
utilized by some crystals and crystal oscillators attracting noise and the associated performance and reliability issues. In certain applications this may necessitate the need for large ground planes to mitigate the problem. CMOS parts in their plastic enclosures do not suffer this problem and can therefore be used confi dently in close proximity to areas of the circuit that may radiate EMI.
As the speed of modern
electronic equipment increases and products spend much of their life in standby or sleep modes to conserve power, the issue of start-up time has gained greater importance. CMOS oscillators are able to move from an ultra- low power standby mode to a fully operational state in around just 100 µs; this is compared to
approximately 10 ms for crystal oscillators.
Conclusion With data rates increasing and a proliferation of portable, battery powered products with high levels of functionality continually coming to market, the need for high speed oscillators that are robust, have exceptional power effi ciency, and are small in size, is signifi cant. While crystal oscillators have served the electronics industry well for decades, aside from a limited number of high-end applications, the reasons for designers to consider switching to ‘crystal-free’ CMOS based timing solutions are compelling.
About IDT Integrated Device Technology, Inc., the Analog and Digital Company™, develops system-level solutions that optimize its customers’ applications. IDT uses its market leadership in timing, serial switching and interfaces, and adds analog and system expertise to provide complete application- optimized, mixed-signal solutions for the communications, computing and consumer segments. Headquartered in San Jose, Calif., IDT has design, manufacturing and sales facilities throughout the world. IDT stock is traded on the NASDAQ Global Select Stock Market® under the symbol “IDTI.” Additional information about IDT is accessible at
www.IDT.com.
www.electronicscomponentworld.com /
www.electronicproductionworld.com September 2011
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