I finally had an opportunity to build the GPSDO with the 5 pole output filter for the sine output instead of the 3 pole. It was also an excuse to dig out my old spectrum analyzer.

Here's the old circuit:

You can see that the first harmonic is only about 35 dB down from the fundamental and the third is not quite 50 dB down.

Here's the new circuit:

Now that second harmonic is knocked right down under 50 dB. I'm not sure what's up with the higher harmonics starting to drift up, but I did have to compromise a little on one of the capacitor values. I'm going to buy a reel of a better fit and maybe that will help a little bit. But even that I think is an improvement. I believe the higher order harmonics are less distressing than the low ones for a 10 MHz reference output.

Of course, on my scope it looks like a sine wave, but the old circuit did too since my scope only has 50 MHz bandwidth.