Absolute Pulling Range (APR) A VCXO is a crystal oscillator where its output frequency changes in direct proportion to the application of an input control voltage. There are many names given or assigned to this output frequency being changed with the control voltage, which can be confusing. All the following terms are used and basically mean the same thing: Deviation Frequency Deviation Modulation Frequency Modulation Pulling Pull Pullability Absolute Pulling Range (APR) Warping You will find these terms and others in different VCXO manufacturer data sheets. MIL-0-55310 uses the term “Deviation” but most suppliers like to use “Pullability” in their data sheet. 1 Ramon M. Cerda Crystek Corporation August 18, 2008 A typical PLL application using a VCXO is depicted in Figure 1. In this application the designer must make sure the PLL remains locked under all conditions including the life time of the product. Absolute Pulling Range (APR) has become a very common way to specify a VCXO with simply one parameter which will guarantee the PLL remains locked under all conditions. A VCXO has its own errors that if not accounted for can lead to not having enough frequency pulling range under certain conditions. These conditions can include: 1) 2) 3) 4) Frequency vs. temperature Aging vs. time Center frequency calibration Load and Supply change variation The minimum pulling range of a VCXO is determined by: Minimum VCXO Pull = Total VCXO errors + APR Desired How does one determined what a APR is desired? The APR desired is a direct function of the overall frequency accuracy of the input reference in Figure 1. Hence if the input reference frequency is +/20ppm (overall), then the APR required is +/-20ppm minimum. If effect, the APR number is the tracking range of the PLL in ppm. The VCXO manufacturer will determine the minimum pull required for a +/-50ppm APR unit as shown in the example below: 2 Ramon M. Cerda Crystek Corporation August 18, 2008 3 Ramon M. Cerda Crystek Corporation August 18, 2008