3. Input Voltage and Power
Crystal oscillators of any type can usually be designed to operate with a DC input supply voltage that is already available in the system. In a digital system it is normally desirable to use a voltage which matches the voltage used by the logic devices in the system that the oscillator will be driving so that the logic levels will be directly compatible. +3.3 V or +5 V are common inputs for these digital units. Other devices with higher power outputs may use higher voltages such as +12 V or +15 V. Another consideration is the amount of current that is needed to power the device. An XO or TCXO may only need a couple of milli-amps so in a low voltage system they could operate on less than 0.01 W. On the other hand, some OCXO’s could draw 5 W or 6 W at turn on.
4. Output Waveform
The output waveform would then be selected to match the load that the oscillator will be driving in the system. One of the most common outputs would be CMOS to drive logic level inputs. A CMOS output would be a square wave swinging between ground and the Vdd rail for the system. For higher frequencies greater than about 100 MHz a differential square wave is often used. These oscillators have two outputs 180° out of phase with fast rise and fall times and very little jitter. The most popular types are LVPECL and LVDS. If the oscillator is used to drive RF components such as a mixer or other devices with a 50 Ω input, a sinewave output at some power level is usually specified. The output power produced would typically fall between 0 dBm and +13 dBm (1 mW to 20 mW) although higher power may be possible if needed.
5. Package Size and Outline
The package required for a crystal oscillator will vary widely depending on the type of oscillator and the specifications. Simple clock oscillators and some TCXO’s can be housed in packages as small as 1.2- x 2.5-mm while some OCXO’s may be as large as 50- x 50-mm or even larger for some particular designs. Although some through-hole packages such as dual-in-line 4 or 14 pin types are still used for larger parts such as the OCXO’s or specialized TCXO’s, the majority of current designs use surface mount packages. These surface mount configurations may be either a hermetically sealed ceramic package or an FR-4 based assembly with castellations for the I/O’s.
As has been shown, there are many different options to be considered when specifying a crystal oscillator. However, by examining the system in which the unit will be used, the most convenient selections will become obvious such as the input voltages that are available to power the unit and the type of device that the output of the oscillator will be driving. Other constraints of the application such as physical size and the operating environment must also be taken into account. In addition to these basic parameters there is a multitude of other specifications that may be invoked for specific applications. But when all things are considered, it is likely that a crystal oscillator can be found to satisfy the requirements of your system.
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