  Non-reciprocal components, such as isolators and circulators, are critical to
wireless communication and radar applications. Traditionally, non-reciprocal
components have been implemented using ferrite materials, which exhibit
non-reciprocity under the influence of an external magnetic field. However,
ferrite materials cannot be integrated into IC fabrication processes, and
consequently are bulky and expensive. In the recent past, there has been strong
interest in achieving non-reciprocity in a non-magnetic IC-compatible fashion
using spatio-temporal modulation. In this paper, we present a general approach
to non-reciprocity based on switched transmission lines. Switched transmission
lines enable broadband, lossless and compact non-reciprocity, and a wide range
of non-reciprocal functionalities, including non-reciprocal phase shifters,
ultra-broadband gyrators and isolators, frequency-conversion isolators, and
high-linearity/high-frequency/ultra-broadband circulators. We present a
detailed theoretical analysis of the various non-idealities that impact
insertion loss and provide design guidelines. The theory is validated by
experimental results from discrete-component-based gyrators and isolators, and
a 25GHz circulator fabricated in 45nm SOI CMOS technology.
