John Phillips wrote: "I would think that a lot of the patents would be running out soon as if that would make any difference." I think that John Phillips is right. Here are some patents related to the HP3458A. It looks like all of these patents have expired. A new instrument based on these concepts and schematics can be marked, "Protected by the following Patents". US4357600 US4951053 US5148171 US5689260 etc ... I do not own a HP3458A, but I admire the instrument and the people that made it. The best meter I own is a HP34401A. It is good enough for the measurements I usually make. Ivan Cousins A time-nut since before 1974, the year I started working at Tektronix.