A Novel Microwave Nonreciprocal Isolator Based on Frequency Mixers

We propose a nonreciprocal microwave isolator realized using an electrical circuit with two frequency mixers. A phase shift between the mixers at the same frequency as the local oscillator (LO) frequency is
$\pi /4 + m\pi /2$
, and the powers of LO applied to each mixer have a phase difference of
$\pi /4+ n\pi /2$
(where
$n$
and
$m$
are the arbitrary integers). The up-converted signals in the upper and lower sidebands at the first mixer are combined at the subsequent mixer, even if they have phase differences. Because each sideband is in-phase for one direction and out-of-phase for the other direction, the circuit principally functions as an isolator with broadband from near dc to LO frequency. An isolator using commercially available microwave components successfully demonstrated more than 17 dB of isolation across the 0.01–1.50-GHz band at an LO frequency of 2.30 GHz. An on-chip broadband isolator with positive gain can be realized using superconductor–insulator–superconductor mixers.