High frequency common mode choke
 A choke for reducing common mode signals in high frequency circuits. A microstrip transmission line includes a pair of conductors spaced above a ground plane conductor. In input and output regions the conductors are spaced relatively far apart from each other such that each conductor presents a characteristic impedance that is similar for common mode and for differential mode signals. Between the input and output regions the conductors are located much closes together such that they present different characteristic impedances to common mode and to differential mode signals so as to reflect the common mode signal but not the differential mode signal. In another version a hole in the ground plane beneath the conductors is partially occupied by a conductive island that is coupled to the ground plane through a resistance to absorb the common mode signal rather than reflecting it. Other versions are realized in a pair of parallel coaxial cables and in a coplanar transmission line. 
Hewlett-Packard Company
H
H01P
inventional
William J. Richard C. John
McFarland Walker Domokos