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Choosing the right timing device (Part 2)

Posted: 07 Nov 2012 ?? ?Print Version ?Bookmark and Share

Keywords:EMI reduction? quartz oscillators? MEMS?

Part 1 of this series looks into the basic requirements for timing devices and the classes of oscillators for different applications. The table below compares the performance of silicon MEMS and quartz oscillators, considering many of the parameters discussed in Part 1. In Part 2, 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.

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Table: Comparison of silicon MEMS and quartz oscillators.

Temp 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 (figure 1).

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.

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

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 telco, networking apps
Telecom applications require an extremely stable local clock to keep time in case the reference clock is temporarily unavailable. The internal timing circuit needs to provide holdover for 24 to 48 hours when the system cannot communicate with a GPS satellite, for example.

Keeping the clock at a constant temperature is one way to achieve the precise frequency stability required for these applications. One way that a quartz-based TCXO accomplishes this is to add a local temperature environment called an ovenized compensation. The oven temperature is chosen at a flat point in the frequency versus temperature curve of the crystal, further improving frequency stability. This approach is very effective, enabling frequency stability on the order of 0.05 to 0.5 ppm, but the heating chamber occupies valuable board space and consumes additional power. Another drawback is the time lag for the heater to reach steady state temperature after the device is powered on.

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