In literature about the instrumentation of radio astronomy, I frequently come across a type of mixer described as "2SB". I'm familiar with single-sideband (SSB) and double-sideband (DSB) mixers, but I have never studied a 2SB mixer.
What is a 2SB mixer and how does it compare to a DSB mixer? Ideally, I would also like to know of formal references that directly address the topic for further reading.