How to select the right timing device - Part II

April 20, 2012 //By Steve Pratt and Mehdi Behnami
How to select the right timing device - Part II
Steve Pratt and Mehdi Behnami of SiTime focus on several important factors for high performance oscillators: temperature response, frequency control and addressing EMI reduction. They also consider some of the practical design considerations such as board design, product lead times and cost.

Part 1 of this article described basic requirements for timing devices and listed classes of oscillators for different applications. Table 1 compares the performance of silicon MEMS and quartz oscillators, considering many of the parameters discussed in Part 1. In Part II, we go into more detail on several important concerns for high performance oscillators: temperature response, frequency control and addressing EMI reduction. We also cover practical considerations such as board design, product lead times and cost.

Table 1. Comparison of silicon MEMS and quartz oscillators.

TEMPERATURE COMPENSATION

Resonators expand and contract due to changes in temperature, affecting their resonating frequency and making temperature compensation critical to oscillator performance for demanding applications. Although quartz has a very low coefficient of thermal expansion, variation due to temperature change is still a large component in the frequency stability of quartz oscillators. Overall frequency stability of fixed-frequency quartz oscillators is ±20 to ±50 ppm without temperature compensation (See Figure 1).

Figure 1. Frequency stability for a SiTime ±25ppm rated MEMS XO and a ±25ppm rated quartz XO over a standard industrial temperature range.

The coefficient of thermal expansion of silicon is an order of magnitude higher than that of quartz, so temperature compensation is built into the oscillator circuitry of MEMS oscillators. MEMS oscillators incorporate a temperature-to-digital converter (TDC) to automatically correct for frequency variations of the oscillator due to temperature. Since all temperature compensation functions are integrated within the existing oscillator circuit, no additional components are needed.

As seen in Figure 1, SiTime's ±25ppm rated MEMS oscillators have better margin at both low and high temperature compared to ±25ppm rated quartz oscillators. MEMS TCXOs are available for applications demanding more precise frequency stability. Typically, MEMS TCXOs have more complex and higher performance compensating circuits, and also require higher order calibration algorithms as well as more extensive testing than MEMS XOs.

STABILITY FOR TELECOM AND NETWORKING APPLICATIONS

Telecom applications require an extremely stable

Design category: 

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