What is Frequency Pulling?

Frequency Pulling is a variation in the frequency of the VCO/oscillator due to a change in the load impedance. All oscillators are designed to operate at a particular frequency for a specific load impedance. So frequency pulling or pulling factor in a VCO quantifies its susceptibility to output load variations.

So frequency pulling basically specifies the change in output frequency caused by operating the oscillator into all phases of a specified mismatch. 

Frequency pull can be specified as less than 10 kHz at VSWR of 1.80:1, or less than 2 kHz at 11 dB return loss. This value is usually specified for oscillators in their datasheet.