Wider ranges increase cost, so designers should order exactly what they require and no more. The second method considers variations from nominal. Here, the design engineer must calculate the absolute pull range by starting with the total pull range of the VCXO, then subtract the sum of all variations and tolerances such as calibration, temperature, power supply and load. The APR method is preferred because the design engineer can easily narrow the requirements to the bottom line without having to deal with the individual factors leading up to that figure. For example, a VCXO with an APR of 50ppm will track a 50ppm source oscillator under all specified operating conditions.
Many designers overspecify, attempting to build margin into their design with high VCXO pullability.
Ordering greater pullability than needed is an excellent way to expand a project's budget. The application's circuit board form factor and production process determine package type and footprint. Customers need to specify any differences from these shipping methods. A website can be especially useful if the manufacturer includes a phone number. Originally published in the January 1999 issue of Medical Equipment Designer.