Like most oscillators, VCXOs have many specifications. APR – Absolute Pull Range – is a single specification that combines many of them to make selection easy. The term “Pull” is unique to the oscillator industry and refers to the small frequency tuning available via the voltage control pin. Pull is measured as df/fnom in ppm.
VCXOs are typically used in a Phase Locked Loop (PLL) that synchronizes the output frequency to a reference frequency. APR ensures that the VCXO will have sufficient tuning range to lock to the reference frequency over all conditions for the lifetime of oscillator. In the majority of applications APR is the only specification needed to ensure the system works.
The required APR spec is the same as the frequency tolerance of the reference clock input. Frequency tolerances are specified as a +/- ppm. Often the +/- is dropped but it’s important to remember it is still there. A typical source clock might have a 50ppm tolerance, really +/-50ppm. For the PLL to work with this reference clock the VCXO simply needs to be specified for 50ppm APR.
This is how APR actually works; the APR specification combines worst case frequency tolerance specifications for temperature, supply voltage, output load, and aging. These numbers are used to create test specification used in production. Here’s an example:
|+/- 50ppm||=||APR||+/- 50ppm|
|Temp Tolerance||+/- 20ppm|
|Supply Tolerance||+/- 5ppm|
|Load Tolerance||+/- 4ppm|
For this example the production room temperature test would require each part to pull at least -84ppm from nominal frequency at low control voltage; typically 0.3V for a 3.3V part. And pull +84 ppm at 3.0V control voltage. We would also say that this part requires at least 168ppm total pull.
Today there are hundreds of millions of VCXOs happily humming away in systems specified solely by this method. But there are other VCXO specifications and APR does not tell the whole story. That’s a subject for the next post.