I've drawn a block diagram (see attached picture and yEd-File).
The Filters for the ADF4355, which I have chosen over the 4351 because of its much finer frequency control (16bit Integer + 24 bit Fractional instead of 16bit INT + 12bit Fractional) - the 6 GHz is basically a bonus that comes with it, but can be left out by chosing the 10€ cheaper ADF4355-2BCPZ that can only go up to 4.4GHz, are:
- 750 MHz: Johanson 0500LP15A500E
- 1200 MHz: Johanson 0915LP15B026
- 2 GHz: TDK DEA202025LT-5052C1
- 3 GHz: Johanson 2450LP14A100T
- 6 GHz: Johanson 5515LP15A730
The Filter for the AD9957 is a Johanson 0400LP15A0122.
The current setup (AD9910 Module from ebay with lumped element LC-Filter) has basically no harmonics and just a few fixed spurs around 200 to 300 Mhz, which are most likely caused by some deficiencies on the module.
Approaching 400 MHz, a tiny amount of image frequency can be seen approaching 600 MHz, but its almost down in the noise. With the Johanson-Filter I expect this image frequency to be absent.
The signal of the ADF4351 has a harmonic at twice the fundamental frequency that is pretty consistent 33 dB weaker than the fundamental. Not quite the level of THD the HP 8648 is capable of, but keep in mind, this is the result of Microstrip-Filters on 0.1mm 2113 Prepreg FR4-Board with no shields over the filters. So adding shields and using the SMD-Filters, I'm guessing the result will be better, maybe even reaching -40dB.
The 3rd harmonic is mostly gone, except for frequencies around the switch-threshold of the filters (800 MHz, 1.5 GHz) - The finer granularity of the filters on the new Mainboard should suppress this third harmonic better.
Output-Power was about 0dBm each time I measured, which equates to an output-power of about +1 dBm at the output of the Filter-Board.
Once I'm done taking the measurements (and I found my USB Floppy-Drive) I'll post the measurements, comparing ADF4351 "RAW"-Output and the filtered one.