Crystal pulling is a method of very slightly changing the load capacitance on a crystal so that there is small frequency variation. As a result it provides a method to add analogue frequency modulation to any transmitter, allowing a smaller shift than would be provided by a PLL based method.
The circuit presented on this page is simple, requiring easy to obtain components, and has been flight tested, and is known to work on a wide range of transmitters
The component which provides the varaible capacitance is a varicap (D1) (also called varactor). It is just a diode, all diodes exhibit variable capacitance when in reverse bias, however in a varicap this capacitance is controlled and the relationship printed in the datasheet. The difference in different varicaps is how much the capacitance changes with reverse bias, with varicaps that have a smaller capacitance change being more expensive - however the sensitivity of the circuit can be controlled, as explained below.
R1 and C1 filter the PWM signal, with R2 providing a high impedance between the two sides of the circuit to prevent the capacitance of C1 from affecting the crystal. A DC voltage proportional to the PWM duty cycle then reverse biases the varicap, providing a variable capacitance which is AC coupled to the oscillator through C2. As the capacitance seen by the crystal is D1 and C2 in series, the sensitivity of the circuit can be reduced by reducing the value of C2. A low sensitivity (being defined as change in frequency per input voltage) is required for small shift schemes such as DominoEX.
While not important for all transmitters, it is generally best to attach the crystal pulling network to the side of the crystal that is 'driven' by the IC. This pin is usually marked as XOUT or similar.