Second order predistortion circuit

Abstract

A predistortion generator for coupling in-line with a non-linear device produces an output predistorion signal of useful amplitude, but with low composite triple beat and cross modulation distortions. The predistortion circuit comprises a distortion circuit which utilizes the non-linear current flowing through at least one diode, to provide a desired amount of signal attenuation bandwidth. The distortion generator circuitry is always matched to the non-linear device, thereby ensuring a frequency response that is predictable and predefined.

What is claimed is:  
     
         1 . A second order predistorter for coupling with a non-linear device comprising: 
 a predistortion circuit having a forward DC bias; and    a transmission line transformer having first and second windings; the transmission line transformer receiving a linear RF signal on the first and second windings; the first winding being coupled to the non-linear device and the second winding being coupled to the predistortion circuit.    
     
     
         2 . The predistorter of  claim 1  whereby said predistortion circuit further comprises: 
 a first signal path having at least one diode; and  
 a second signal path having a DC bias current input.  
 
     
     
         3 . The predistorter of  claim 2 , whereby said second signal path further includes a resistor.  
     
     
         4 . The predistorter of  claim 2  whereby the RF current from the RF signal flows through both said first signal path and said second signal path, and whereby between approximately one-third to one-half of the RF current flows said first signal path.  
     
     
         5 . The predistorter of  claim 2  whereby the DC bias current from said DC bias current input forward biases said at least one diode.  
     
     
         6 . The predistorter of  claim 5 , whereby the RF current from the RF signal flows through both said first signal path and said second signal path, and whereby between approximately one-third to one-half of the RF current flows said first signal path.  
     
     
         7 . The predistorter of  claim 6  wherein the DC bias current through said first signal path is approximately between two to three times said RF current through said first signal path.  
     
     
         8 . The predistorter of  claim 5 , wherein the DC bias current through said first signal path is at least two times said RF current through said first signal path.  
     
     
         9 . The predistorter of  claim 1  whereby said predistortion circuit generates RF distortion which is primarily second order distortion.  
     
     
         10 . The predistorter of  claim 9  wherein said non-linear device is a laser transmitter.  
     
     
         11 . A method for providing second order predistortion to a non-linear device, comprising the steps of: 
 providing a differential linear RF signal through first and second windings of a transmission line transformer;    coupling the first winding to a non-linear device;    coupling the second winding to a predistortion circuit, having at least a resistance and a diode;    controlling the current through the predistortion circuit to generate a distortion signal; and    transferring the distortion signal from the second winding to the first winding.    
     
     
         12 . The method of  claim 11  whereby the flow of RF current from the RF signal is split between said diode and said resistance; and said controlling step further comprises biasing said diode such that approximately one-third to one-half of the RF current flows through said diode.  
     
     
         13 . The method of  claim 12 , whereby said biasing step controls the amount of DC bias current through said diode.  
     
     
         14 . The method of  claim 13 , whereby the amount of said DC bias current through said diode is approximately between two to three times the amount of said RF current through said diode.  
     
     
         15 . A method for generating second order predistortion, comprising the steps of: 
 providing a differential linear RF signal through first and second windings of a transmission line transformer;    coupling the first winding to a non-linear device;    coupling the second winding to a predistortion circuit having a diode in parallel with a resistance;    forward DC biasing the predistortion circuit to control current flow through the diode to distort the RF signal in the second winding;    modulating said linear RF signal in said first winding with the distorted RF signal in the second winding; and    providing a predistorted RF signal to the non-linear device.    
     
     
         16 . A distortion amplifier for generating a selectively distorted RF signal, including an RF amplifier and a distortion circuit; the distortion circuit comprising: 
 a first signal path, for conducting both RF and DC bias current, including at least one nonlinear device;    a second signal path, for conducting both RF and DC bias current, coupled in parallel to said first signal path with respect to RF current, coupled in series to said first signal path with respect to DC bias current, and including at least one resistance means; and    a DC bias current input for providing said DC bias current to said first and second paths;    whereby approximately one-third to one-half of the current output from the RF amplifier flows through said first path.    
     
     
         17 . The distortion amplifier of  claim 16  whereby the amount of DC bias current which flows through said first path is approximately between two to three times the amount of said RF current.  
     
     
         18 . A distortion control circuit coupled between an RF amplifier and a laser transmitter for selective modulation of the RF signal output from the RF amplifier comprising: 
 a first signal path comprising a non-linear circuit having at least one diode;    a second signal path comprising a DC bias current input coupled with a capacitor;    whereby said first and second signal paths are coupled in parallel between said RF amplifier and ground.    
     
     
         19 . The distortion control circuit of  claim 18  whereby the RF current from the RF signal flows through both said first signal path and said second signal path, and whereby between approximately one-third to one-half of the RF current flows said first signal path.  
     
     
         20 . The distortion control circuit of  claim 19  wherein the DC bias current through said first signal path is approximately between two to three times said RF current through said first signal path.  
     
     
         21 . A method for generating second order predistortion, comprising the steps of: 
 providing a differential linear RF signal through first and second windings of a flex-coupled device;    coupling the first winding to a non-linear device;    coupling the second winding to a predistortion circuit having a diode in parallel with a resistance;    forward DC biasing the predistortion circuit to control current flow through the diode to distort the RF signal in the second winding; and    coupling the distorted RF signal in the second winding to said linear RF signal in said first winding to provide said second order predistortion.