OCXOs are Crystal Oscillators situated inside an oven that keeps them at a constant temperature. This relies on the oven temperature being hotter than the maximum ambient temperature. Because temperature variation of the crystal oscillator is almost eliminated, OCXOs can be more accurate than TCXOs. The drawback is the oven requires considerable extra current consumption, particularly while the oven is warming up. The oven also adds bulk to the component.
Below are the minimum set of requirements we would like from a customer to specify an OCXO.
Centre Frequency
Usually self-evident, and is set by the application.
Calibration
This is the manufacturing frequency tolerance at 25°C as delivered. For OCXOs with tuning, the tuning is set to the centre of the voltage control range.
Temperature Stability
Stability defines by how much the frequency can vary with ambient temperature. There are two common ways of specifying stability. Referenced to the measured frequency at 25°C, or the peak to peak variation over temperature divided by two. The latter is more common for tighter stability parts.
Ageing
For OCXOs, this is the dominant part of the overall error budget when considered over the lifetime of the product the OCXO is going into. Ageing is usually specified as a daily rate in Parts Per Billion (ppb) per day and the first year assuming the part is powered 24/7. There is usually a specification of the ageing over 10 years or over the lifetime of the product. The rate of ageing decreases with time, so the second year will be significantly less than the first year and so on. The daily ageing is very small so can only be measured after other sources of variation are eliminated including the warm-up after the device is switched on. Sometimes the daily ageing is specified after 30 days of continuous operation.
Case Size
OCXOs are much larger than other types of crystal oscillator. For leaded types there are a few standard packages. Aim to use these if you need the option to be able to switch between suppliers. The industry standards are European 36.1 x 27.2 mm, US 51 x 51 mm, 25.4 x 25.4 mm and 14 DIL. There are now smaller surface mount options down to 9.7 x 7.5 mm, a substantial space saving. Standardisation for these is still emerging.
Supply Voltage
This depends on the supply rails available in your design. There are three common supplies to choose from 12V, 5V and 3.3V. Not all options are available for all types. Something that needs to be considered is that the oven needs a certain wattage so a lower supply voltage leads to a higher supply current. For 5V and 3.3V supply CMOS output types, the supply sets the output level. For 12V supply, CMOS output types have a 5V CMOS output.
Output Type
The most common format is sinewave, designed to drive a 50 ohm load. The level is usually expressed in dBm with a ± tolerance or just the minimum level specified. The other common option is CMOS. The smallest surface mount parts have a clipped sine output.
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.
Phase Noise and Jitter
OCXOs are usually used as the reference for RF chips that include a PLL to multiply the frequency. Unfortunately, this also multiplies the phase noise of the OCXO used as the reference within the PLL loop bandwidth. It is therefore important to calculate the allowable phase noise for the OCXO based on the system requirements. OCXOs can have superior phase noise to TCXOs and sometimes much better than the best clock oscillators.
Optional Specifications
The above will give a supplier the basics to select a suitable OCXO for an application where the specifications are not that tight. In more demanding applications the following should be considered.
OCXO Tuning Range
Most OCXOs are voltage tuneable. OCXOs have a small tuning range compared to other types. In most applications this is used to remove the manufacturing tolerance and ageing, assuming the equipment is available for periodic calibration. Tuning range is often specified as just a minimum. In contrast to other types of crystal oscillator, tuning range is specified relative to the nominal frequency, not the measured frequency at the centre of the tuning range.
Control Voltage Range
This is the range of the tuning voltage required to achieve the tuning range specified.
Control Input Impedance
Usually, 100kΩ or more. This needs to be taken into consideration when designing the tuning voltage source.
Reference Voltage Output
Some OCXOs provide a precision voltage output that can be used by the circuit that generates the tuning voltage to reduce variation in tune voltage due to temperature and supply variation.
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%.
Supply Current
This comes in two parts, Warmup and Steady State. Warmup current is much higher.
Warmup Time
This is much slower than other types of crystal oscillator as it takes time for the oven to reach its working temperature. There is some variation between suppliers in the way this is specified. A format preferred by TechPoint Golledge is error in ppb 5 minutes after switch on referred to the frequency settled on 1 hour after switch-on.
Retrace
When an OCXO is switched off, allowed to cool and then switched back on again and allowed to settle, it will not return to exactly the same frequency as before; this is known as Retrace error. It should be considered as part of the overall error budget if this is tight and the OCXO is not going to be in continuous operation.
Short Term Stability
Also known as Allan Deviation or Root Allan Variance. See the Wikipedia entry for an explanation.
Voltage Stability
Usually defined for a ±5% variation in supply voltage and is generally expressed in ppb.
Load Stability
Usually defined for a ±5% change in the impedance of the load presented to the output.
Rise & Fall Times
Only valid for CMOS outputs, 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. This parameter is not applicable to clipped sine outputs.
Storage Temperature Range
This is as the name suggests.