TCXOs are Crystal Oscillators that come with the addition of a tuning function that is designed to cancel out the drift in the output frequency caused by variation in the temperature of the device. Voltage Controlled Temperature Compensated Crystal Oscillators (VCTCXOs) have a tuning voltage input which allows for external tuning in addition to the tuning required for temperature compensation.
Below are the minimum set of requirements we would like from a customer to specify a TCXO.
Centre Frequency
Usually self evident and is set by the application.
Calibration
The is the manufacturing frequency tolerance at 25°C as delivered. This may or may not include the effects of reflow soldering. You need to check the detail of the spec.
Stability
Stability defines by how much the frequency can vary with 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. For VCTCXOs this is specified for the tune voltage at its centre point, or reference point if this is different.
Ageing
For TCXOs this is a significant part of the overall error budget, especially when considered over the lifetime of the product the TCXO is going into. Aging is usually specified for the first year at 25°C. Storing or using the part at elevated temperatures will accelerate the aging process. The rate of aging decreases with time, so the second year will be significantly less than the first year and so on.
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 clipped sine. The clipped sine output is not suitable for driving long PCB traces but does have the advantage of lower phase noise floor and it makes it easier to achieve the tightest stability due to lower currents in the supply and output pins. Many RF chips are designed to accept clipped sine input. The other options are CMOS and 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.
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
TCXOs 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 TCXO used as the reference within the PLL loop bandwidth. It is therefore important to calculate the allowable phase noise for the TCXO based on the system requirements.
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.
VCTCXO Tuning Range
VCTCXOs have a small tuning range compared to VCXOs. In most applications this is used to remove the manufacturing tolerance and reflow induced frequency shift. If the equipment is available for periodic calibration the tuning can be used to calibrate out the aging. Tuning range is often specified as just a minimum. In some cases, specifying a maximum may be necessary if the application is the reference for an FM (or FSK) transmitter where the modulation is applied to the tune input of the VCTCXO.
Supply Current
This is important for applications that are battery powered. Clipped sine output types will have significantly lower current consumption that other output types.
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 for help.
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.
Frequency v Temperature slope
If the equipment the TCXO is in is subject to fairly rapid temperature rises this can create a frequency wobble within the allowed range of the stability. This can cause problems with signals using complex modulation schemes. In this instance a maximum frequency V temperature slope. In ppm/°C should be calculated based on reference stability requirements and the expected rate of temperature change of the equipment.
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. This parameter is not applicable to clipped sine outputs.
Enable Function
The most common 4-pad package types lose the enable input of a clock oscillator as the clock oscillators enable pad is used for the tune input instead. There are some VCXOs in a 6-pad package that include an enable function, please ask for further information.
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.