Method and apparatus for generating second-order predistortion without third-order distortion

An apparatus for providing predistortion to a nonlinear device along an in-line electrical path between a signal source and the nonlinear device, the apparatus comprising a shunt element electrically connected between the in-line electrical path and a ground, the shunt element being operative to generate second-order distortion as the predistortion and to generate substantially no third-order distortion, wherein:

• the shunt element has an adjustable resistance value and comprises a shunt impedance in series with a shunt diode;
• the apparatus further comprises a bias source connected to, and providing a bias current to, the shunt diode, wherein the bias source is capable of varying a magnitude of the bias current;
• and
• the shunt impedance and the shunt diode are collectively operative to generate the second-order distortion and to generate third-order distortion as the predistortion when the adjustable resistance value of the shunt impedance is a second value and the magnitude of the bias current provided to the shunt diode is a second current level.

An apparatus for providing predistortion to a nonlinear device along an in-line electrical path between a signal source and the nonlinear device, the apparatus comprising a shunt element electrically connected between the in-line electrical path and a ground, the shunt element being operative to generate second-order distortion as the predistortion and to generate substantially no third-order distortion, wherein:

• the shunt element has an adjustable resistance value and comprises a shunt impedance in series with a shunt diode;
• and
• the apparatus further comprises:
• a bias source connected to, and providing a bias current to, the shunt diode, wherein the bias source is capable of varying a magnitude of the bias current;
• and
• a varactor circuit electrically connected between the in-line electrical path and the ground, the varactor circuit being operative to generate additional second-order distortion in an offset-phase relationship when compared to the second-order distortion generated by the shunt element.

A predistortion circuit comprising:

• an input node for receiving an electrical signal;
• a shunt impedance electrically connected to the input node, wherein the shunt impedance comprises a shunt capacitor and a shunt resistance having an adjustable resistance value, the shunt resistance being coupled to the input node;
• at least one shunt diode connected in series with the shunt impedance, wherein the shunt capacitor is coupled between the shunt resistance and the at least one shunt diode;
• and
• a bias source connected to a bias node between the shunt capacitor and the at least one shunt diode, the bias source providing a bias current to the bias node for forward biasing the at least one shunt diode, and the bias source comprising an inductive element connected to the bias node and coupled in series with a voltage source, the voltage source capable of supplying the bias current to the at least one shunt diode through the inductive element,
• wherein the shunt impedance and the at least one shunt diode are collectively operative to generate second-order distortion out of the input node and to generate substantially no third-order distortion when the adjustable resistance value of the shunt resistance is a first predefined value and the magnitude of the bias current provided to the at least one shunt diode is a first predetermined level.

A predistortion circuit comprising:

• an input node for receiving an electrical signal;
• a shunt impedance electrically connected to the input node;
• at least one shunt diode connected in series with the shunt impedance, the shunt impedance and the at least one shunt diode being operative to generate second-order distortion out of the input node as the predistortion and to generate substantially no third-order distortion;
• and
• a varactor circuit electrically connected in between the input node and the ground node, the varactor circuit comprising an adjustable impedance element in series with a varactor diode element, the varactor circuit being operative to generate additional second-order distortion in an offset phase relationship when compared to the second-order distortion generated by the shunt element.

A method for generating predistortion having second-order distortion and substantially no third-order distortion from a static predistortion circuit, comprising the steps of:

• generating an electrical signal;
• providing the electrical signal to the static predistortion circuit;
• applying a bias current to a static diode within the static predistortion circuit to generate the predistortion, the predistortion having at least the second-order distortion and the third-order distortion;
• adjusting a level of the bias current to substantially eliminate generation of the third-order distortion;
• and
• adjusting a value of a static impedance within the static predistortion circuit to optimize generation of the second-order distortion in the predistortion so that the second-order distortion in the predistortion substantially cancels inherent distortion generated by a nonlinear device.