Five things to consider when choosing a crystal oscillator

October 03, 2016 // By Steve Fry, Greenray Industries, Inc.
Most electronic systems require some sort of oscillator as a critical functional block in their design. Some typical uses would include: a clock for a digital system that synchronizes the operation, a stable RF signal for a receiver or transmitter, an accurate frequency reference for precision measurements or a real time clock for accurate timekeeping. The specifications for the system and how the oscillator needs to function will determine most of the parameters of the device.

The key component in any oscillator is the resonator which will control the frequency and determine what stability specifications may be achieved. While it is possible to implement a simple oscillator with an LC or RC resonator that may suffice for some applications, the addition of a quartz crystal will greatly improve the frequency stability of the device by several orders of magnitude, often with a minimal cost impact.

 

1. Output Frequency

The most fundamental attribute of any oscillator is the frequency that it will produce. By definition, an oscillator is a device that accepts an input voltage (usually a DC voltage) and produces a repetitive AC output at some frequency. The frequency that is needed is dictated by the type of system and how it will be used.

Some applications call for low frequency crystals in the kHz range. A common example would be a watch crystal at 32.768 kHz. But most current applications need higher frequency crystals ranging from less than 10 MHz to greater than 100 MHz.

 

2. Frequency Stability and Temperature Range

The required frequency stability is determined from the system requirements. The stability of an oscillator is simply given as the change in frequency due to some phenomenon divided by the center frequency.

That is: Stability = Change in Frequency ÷ Center Frequency

For example, if the oscillator output frequency is 10 MHz and it changed 10 Hz over temperature, it’s temperature stability would be: 10/10,000,000 = 1x10-6 = 1ppm. Typical stabilities for a crystal oscillator could range from 100ppm to 0.001ppm.

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