  In this paper, we discuss the design rationale and guidelines to build
magnet-less circulators based on spatio-temporal modulation of resonant
junctions consisting of first-order bandstop filters connected in a delta
topology. Without modulation, the junction does not allow transmission between
its ports, however, when the natural oscillation frequencies of the constituent
LC filters are modulated in time with a suitable phase pattern, a synthetic
angular-momentum bias can be effectively imparted to the junction and a
transmission window opens at one of the output ports, thus realizing a
circulator. We develop a rigorous small-signal linear model and find analytical
expressions for the harmonic S-parameters of the proposed circuit, which
significantly facilitate the design process. We validate the theory with
simulations and further discuss the large signal response, including power
handling and non-linearity, and the noise performance. Finally, we present
measured results with unprecedented performance in all metrics for a PCB
prototype using a Rogers board and off-the-shelf discrete components.
