I finally got around to making a proper case for my 4m transverter
Body is milled from solid aluminium bar, the lid is milled from 12mm plate and has a lip and recess. After rechecking the alignment and bias, the two-tone output looks sort-of OK with 5th order IMD about -36dB relative to each tone.
For comparison, here is the same span for the 28MHz Elad FDM-DUO SDR radio driving the IF. Very clean indeed, as you’d expect.
Just to confirm how good the SDR is, here is a spectrum measured on my Agilent E4406A 4GHz transmitter tester at 0.5Hz resolution bandwidth.