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PCI Express. By using a widely available and proven CMOS process, the cost, yield and reliability of the new devices is acceptable and can be well controlled. A programmable architecture allows fl exible confi guration to suit specifi c applications, the most critical of these options is the frequency of operation which is factory programmed and can be set at the exact desired level – this includes uncommon and application unique frequencies. In the case of the IDT 3C Series, parts can be chosen with operating frequencies ranging from 4 MHz to 200 MHz; this covers a wide range of applications in both the consumer, and computing and communications markets. Before designers can confi dently embrace such a fundamentally different approach to timing, they must of course compare and understand the relative performance of the two approaches for a number of key criteria.

Power consumption At lower frequencies, the power consumption of crystal oscillators is acceptable to designers. However, higher frequency devices –typically required for high data applications that are now commonplace – consume considerably larger, often prohibitive, amounts of power. For example, a typical crystal oscillator may draw several tens of mA. By comparison, a CMOS-based oscillator such as the IDT 3Cseries consumes only about2 mA (unloaded, typical) and just 200 nA in standby mode. In active mode, this level of performance represents a power saving versus crystal oscillators that of up to 90% From both environmental and running cost

standpoints, power consumption has become a critical specifi cation that designers consider when selecting a component for their application, and that end users refer to when deciding which product to purchase. In the case of portable, battery powered equipment, this is heightened because overall low power consumption translates directly into longer periods between charging.

Size The fact that a crystal oscillator requires a specifi c size and cut of crystal to give the required oscillation frequency means that there is a barrier when it comes to the potential to shrink the device package in terms of both footprint on the PCB and overall component height. Furthermore, the need for an additional circuit inside the package to achieve the desired frequency multiplication impacts overall package dimensions. Finally, in many designs, external

September 2011

capacitors and other passive components may be required in order to achieve stable performance of the oscillator. Typical package sizes for crystal oscillators are in the region of 5mm x 3mm with component height ranging from one to 1.5mm. Consideration must also be given to the PCB real estate required to accommodate any external passive components. All-silicon CMOS oscillators are completely self-contained requiring no external components; this helps to simplify design work for engineers and reduce both costs and the bill of materials. Device footprints are the same as those of standard crystal oscillators, however overall height is less at 0.5mm. This can be important to designers of small, portable pieces of equipment where there is competition for space with batteries, displays and user interfaces. Crucially, there is plenty of scope for further shrinking of package sizes for CMOS oscillators; indeed IDT is planning to offer a part measuring just 2mm x 1.6mm in the near future. As designers of portable equipment are continually

pressured to include more functionality in products with incredibly small overall dimensions, the potential to integrate CMOS oscillators into multi-chip modules (MCMs) can prove interesting where production volumes are signifi cantly high. Including the oscillator in an MCM with the microprocessor and blocks such as fl ash memory can save space, cost (component and assembly) and power and enhance the reliability of the overall design.

Frequency error Frequency error is an important specifi cation for designers to consider. In some applications such as telecoms equipment only small frequency errors can be tolerated. For these applications specialized crystal oscillators with frequency errors of as low as 2ppm offer the only viable solution. However, for many applications, and in particular those in the consumer market, frequency errors of 100ppm are perfectly acceptable. This is the performance level of standard crystal oscillators and of currently available CMOS oscillators. Continued development of CMOS based parts looks set to see 50ppm parts on the market in the very near future that would make a wider range of end applications accessible to the technology. When considering and comparing frequency error

specifi cations it is important not to just look at the initial frequency tolerance. This is because in the case conventional crystals, additional errors need to be considered due to factors such as operating temperature / EIU 21

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