Crystal oscillators come in many different types, the most basic is the Clock Oscillator. Here we are not talking about discrete circuits but a single component that is actually a module containing a crystal and a chip that provides the oscillator circuit, including the load capacitors for the crystal and the output buffers.
Below are the minimum set of requirements we would like from a customer to specify a clock oscillator.
Frequency
Usually self evident and is set by the application.
Case Size
Specifying too large a case size can lead to designing in a part that may be close to going obsolete. Specifying too small a case size can increase the cost of the part. The smallest parts can also raise the PCB cost by requiring small pads, and gaps between the pads, that are too demanding for the cheapest PCB production lines. This is especially true for the smallest 6 pad packages for oscillators with balanced outputs.
Supply Voltage
This depends on the supply rails available in your design. For CMOS output types this also sets the output levels.
Output Type
The most common format is CMOS. This is a square wave that goes close to rail to rail. The other options are differential types. The differential types are used for higher frequencies and / or driving long PCB tracks where signal reflections can cause problems for CMOS signals. The output options for differential types include LV-PECL, LVDS, HCSL, and CML, the first two being the most popular.
Stability
Just to confuse matters this is not defined the same way as stability in crystals. Here it is inclusive of manufacturing tolerance, variation with supply, variation with output load, one year’s aging at room temperature and temperature variation.
Operating Temperature Range
This is taken from the temperature range of the equipment the part is designed into, with a little extra at the hot end to allow for local heating on the PCB.
Optional Specifications
The above will give a supplier the basics to select a suitable clock oscillator for an application where the specifications are not that tight for instance USB 1 and 2, or a microcontroller where the timing is not critical. Where power consumption is critical, or lower jitter/phase noise is required, or less usual operating conditions apply then the parameters below should be considered.
Supply Current
This is important for applications that are battery powered. When comparing specifications for CMOS output types from different sources you should be aware that some parts will be specified for no load, while others will be specified at maximum load capacitance. This makes a significant addition to the current drawn. The usual for parts from TechPoint Golledge is to specify the current with the maximum load capacitance.
Startup Time
Time should be allowed between applying power or enabling an oscillator and using the clock output. This is usually a few ms. The startup time specified is the time between the supply rising above half the nominal supply voltage and the clock output beginning to toggle. Note that the oscillator may not have settled to its final frequency at this point. If you need frequency verses time plots, please ask our Application Support Team
Rise & Fall Times
Some chips specify a minimum rise and fall time for their digital inputs. If this is the case for the clock input of the chip you are driving, you need to ensure that the oscillator specification is equal to or shorter than this.
Phase Noise and Jitter
This is an important consideration for clocks driving high performance ADCs and DACs. Refer to the data sheets and application notes for these parts for guidance. Jitter is usually calculated from the phase noise. When specifying RMS phase jitter you need to specify the frequency offset range over which jitter is calculated.
Ageing
For tight tolerance oscillators and oscillators running at elevated temperatures this may be significant beyond the aging included in the overall stability specification. Talk to us if this is the case, custom specifications are available.
Driving Ability
This is only applicable to CMOS output types. For other types, the load is defined by the standard for that output format. The usual specification is 15pF. High drive level devices are available that can drive 50pF. Some very low current consumption types may be specified at a lower load capacitance.
Storage Temperature Range
This is as the name suggests. On rare occasions, for extreme operating conditions, this will be less than the operating temperature range.