Back in 1961 HP put out a seminal application note AN52, Frequency and Time Standards. In the mid-70s, they published four new app notes meant to update AN52 given the designations AN52-1, AN52-2, AN52-3, and AN52-4. I have three of the four, but AN52-3, which "deals with the stability of frequency and time standards and methods of measuring stability" seems not to exist in the wilds of the internet. It's not at any of the usual sources, and a discussion on the HP-Agilent-Keysight list a few years ago also turned up nothing. If anyone has a copy of AN52-3 to share, I'd love to get it. If you have a paper copy I'd be happy to scan and return it to you, and will make sure the scanned version is accessible on the web. NOTE -- I *don't* need any of the other AN52 documents, just AN52-3. Please contact me directly at jra at febo dot com if you can help me out. Thanks, John