VCXOs are Clock Oscillators that come with the addition of a tuning function. They allow for fine-tuning of the output frequency by varying the control voltage, making them useful in applications that require precise frequency adjustments.
Below are the minimum set of requirements we would like from a customer to specify a VCXO.
Centre Frequency
Usually self-evident and is set by the application.
Tuning Range
VCXOs have a small tuning range compared to other types of tunable oscillator, but still larger than VCTCXOs. Tuning range is limited to a few 100ppm. In most applications the VCXO is being locked to another frequency source. In this case the minimum tuning range required is the maximum frequency error of the reference source plus the maximum frequency error of the VCXO in question. Tuning range is usually specified as just a minimum. In some cases, specifying a maximum may be necessary if too great a tuning slope becomes a problem.
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 lowest cost PCB production lines. This is especially true for the smallest 6-pad packages for oscillators with balanced outputs or an enable input.
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. This is specified for the tune voltage at its centre point, or reference point if this is different.
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 VCXO for an application where the specifications are not that tight. 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 Applications Support Team.
Tune Linearity
Ideally the graph of frequency verses tune voltage would be a straight line. In practice this is not the case. The tune linearity specification quantifies how much the actual tune curve deviates from a straight line. This is done by calculating the best straight line fit to the tune data using the least squares method. Then find the data point with the largest deviation from the straight line. The tune linearity is that largest deviation expressed as a percentage of the whole tuning range. The default is 10%.
Modulation Bandwidth
Usually, the response time to a change in tune voltage is not a problem. Where a rapid change in output frequency or a flat modulation frequency response is required then this parameter should be requested.
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 and in high speed communication where accurate sampling of an incoming data stream is required. 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.