A while back, there was a discussion on how best to use double balanced diode ring mixers for delay measurements. I came upon a very useful paper by Enrico Rubiola: "Tutorial on the double balanced mixer", Enrico Rubiola, 21 Aug 2006 , 52 pages, arXiv:physics/0608211v1 [physics.ins-det]. <http://arxiv.org/abs/physics/0608211> Joe