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.