  Passive and linear nonreciprocal networks at microwave frequencies hold great
promises in enabling new front-end architectures for wireless communication
systems. Their nonreciprocity has been achieved by disrupting the time-reversal
symmetry using various forms of biasing schemes, but only over a limited
frequency range. Here we demonstrate a framework for synthesizing theoretically
frequency-independent multi-port nonreciprocal networks. The framework is
highly expandable, and can have an arbitrary number of ports while
simultaneously sustaining balanced performance and providing unprecedented
programmability of non-reciprocity. A 4-port circulator based on such a
framework is implemented and tested to produce broadband nonreciprocal
performance from 10 MHz to 900 MHz with a temporal switching effort at 23.8
MHz. With the combination of broad bandwidth, low temporal effort, and high
programmability, the framework could inspire new ways of implementing multiple
input multiple output (MIMO) communication systems for 5G.
