The hybrid 90 degree combiner is a passive device, built around ¼ wavelength transmission lines inside the device. For this reason it has limited bandwidth, and so is used to its best advantage in an image reject signal chain where the IF is constant. In simplest terms, the signal at the 90 degree input port is shifted by 90 degrees and the signal at the 180 degree port is shifted by 180 degrees. The two signals are then combined at the output port.