FYI, when a US patent expires the product is no longer patented and the maker cannot use the term patented, or its number, in products or literature. Pequignot v. Solo Cup, 09-1547, June 2010, the Federal Circuit confirmed that "articles marked with expired patent numbers are falsely marked." The case also discusses the use of the term patented. Speaking of the 34401a, it was extensively redesigned as the 34461a and a few variants of it. You can use it as a case study of what needs to be done to redesign a defacto standard. At 07:02 PM 5/17/2015, wrote: >"I would think that a lot of the patents would be running out soon as if >that would make any difference." > >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. > >_______________________________________________