Absolute Pulling Range (APR) By Crystek

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:
Frequency Deviation
Frequency Modulation
Absolute Pulling Range (APR)
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.
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:
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:
Ramon M. Cerda
Crystek Corporation
August 18, 2008
Ramon M. Cerda
Crystek Corporation
August 18, 2008