I've been talking to a recent purchaser of the OH300 GPSDO and he's got some expertise in RF and has made some very helpful suggestions for making the output a little bit cleaner. I'm going to try his suggestions and the result should be cleaner output. Note that by 'cleaner' we're really referring to the waveforms themselves. The frequency stability is unchanged.

• Add additional bulk capacitance near the clock fanout chip
• Add some capacitance to the CMOS output
• Add a common mode choke to the twisted pair output leads

The bulk capacitance adds additional bypassing to the clock chip, which results in improved stability of the voltage on the 5v digital rail. The extra capacitance on the CMOS output increases the rise time slightly, but tames the overshoot that's present, making for a more stable output. The common mode choke reduces ground loop noise that can happen between the two outputs. That common mode noise was the reason for the rising amplitude of the higher order harmonics on the sine output. I haven't duplicated his results just yet, but the other fellow tried all of that and got all of the harmonics on the sine output to under 50 dB down from the fundamental, which is good enough to score.

As soon as I can duplicate these results, look for those additions to become a standard part of the design.

Note that the 56x0 discipline board doesn't actually come with output wiring, so users of those boards will need to do the common mode choke themselves if they wish. Of course, if you just connect one output up to the board, that makes ground loops between the two outputs a non-issue.