a dumbbell shaped resonator efficiently couples if signals from short transmission lines to an output transmission line at a radio frequency mixer &# 39 ; s if output port . the rf mixer uses short transmission lines and terminating resistors to match impedances within the mixer and to minimize the mixer &# 39 ; s distortion . the rf mixer , including the dumbbell resonator , is constructed on a printed circuit board and has a low manufacturing cost . alternatively , the output transmission line is center - tapped to provide a pair of if signals having opposite phases .