  We introduce a method for breaking Lorentz reciprocity based upon the
non-commutation of frequency conversion and delay. The method requires no
magnetic materials or resonant physics, allowing for the design of scalable and
broadband non-reciprocal circuits. With this approach, two types of gyrators
--- universal building blocks for linear, non-reciprocal circuits --- are
constructed. Using one of these gyrators, we create a circulator with > 15 dB
of isolation across the 5 -- 9 GHz band. Our designs may be readily extended to
any platform with suitable frequency conversion elements, including
semiconducting devices for telecommunication or an on-chip superconducting
implementation for quantum information processing.
