5 elements

1. Output frequency

The most basic property of any oscillator is the frequency it generates. By definition, an oscillator is a device that accepts an input voltage (usually a DC voltage) and produces a repetitive AC output at a certain frequency. The required frequency is determined by the type of system and how the oscillator is used.

Some applications require low frequency crystals in the kHz range. A common example is a 32.768 kHz hand (clock) crystal. But most current applications require higher frequency crystals, ranging from less than 10MHz to greater than 100MHz.

2. Frequency stability and temperature range

The required frequency stability is determined by the system requirements. The stability of an oscillator can be simply expressed as: the frequency change caused by some reason divided by the center frequency. (Ie: stability = frequency change ÷ center frequency)

3. Input voltage and power

Any type of crystal oscillator can usually be designed to operate with the DC input power voltage already in the system. In a digital system, it is usually desirable to drive the crystal oscillator with a voltage that matches the voltage used by the logic device in the system that the oscillator will drive, so that the logic levels are directly compatible. +3.3V or +5V is the typical input of these digital units. Other devices with higher power output can use higher voltages, such as +12V or +15V. Another consideration is the amount of current required to power the device. XO or TCXO may only need a few mA, so in a low-voltage system, its power consumption can be less than 0.01W. On the other hand, at power-up, some OCXOs may require 5W or 6W.

4. Output waveform

Then choose the output waveform to match the load that the oscillator will drive in the system. One of the most common outputs is CMOS-to drive logic level inputs. The CMOS output will be a square wave swinging between ground potential and the Vdd rail of the system. For higher frequencies above about 100 MHz, a differential square wave is usually used. These oscillators have two outputs that are 180° out of phase, have fast rise and fall times, and very little jitter. The most common types are LVPECL and LVDS. If the oscillator is used to drive RF components, such as mixers or other devices with 50Ω input impedance, a sine wave output at a certain power level is usually specified. The output power produced is usually between 0dBm and +13dBm (1mW to 20mW), although higher power can be output if required.

5. Package size and shape

Based on the oscillator type and specifications, the requirements for crystal packaging will vary greatly. Simple clock oscillators and some TCXOs can be installed in packages as small as 1.2×2.5mm2; while some OCXOs can be as large as 50×50mm2, or even larger for certain specific designs. Although some through-hole packages such as dual in-line 4 or 14-pin types are still used for larger components (such as OCXO or dedicated TCXO), most current designs use surface mount packages. These surface mount configurations can be hermetically sealed ceramic packages, or FR-4 based components with I/O construction.

6. Summary

Generally, some margin should be left during device selection to ensure product reliability. Choosing higher-end devices can further reduce the probability of failure and bring potential benefits. This should also be considered when comparing product prices. To make the "overall performance" of the oscillator tend to be balanced and reasonable, it is necessary to weigh many factors such as stability, operating temperature range, crystal aging effect, phase noise, cost, etc. The cost here does not only include the price of the device, It also includes the cost of using the product throughout its life.