Although there doesn't seem to be anything similar in the GPS4 manual, the Brandywine GPS8 manual does carry a table of various oscillator options on page 33. The OCXOs listed there are either from from Milliren or HCD Research. Nigel, GM8PZR Hi, Does someone happen to know what OCXO sits in the Brandywine GPS-4 GPSDO? It's pretty good sized OCXO, possibly double-oven. It has a screw on the side which I presume opens up to the trimmer position. If someone could share info it would be useful. Cheers, Magnus