The way that the 8920 generates the low frequency is somewhat counter intuitive.
There are three paths in the Outpur Assembly
The synthesizer runs from 500-1ghz? ?so that range is just straight amplification for path 1 and the 500 -1G range
Path 2 uses a? /2 divider on the Synthesizer so you get 250 -500 Mhz range?
Path 3 which is what you are having problems with? mixes the synthesizer with 1Ghz? to get the .4 to 250 range
For a 1 Mhz? output you have 999 Mhz mixing with 1Ghz.
The higher the output frequency needed the lower the Synthesizer frequency needed.
This is the more complex of the three but at least at this point you know your 1Ghz and Synthesizer? signals are correct going into the assembly??
You can use a 9 Pin D extender which gets you everything but the RF signals? ...usually use some small coax and RS-232?Male and female pins to extend those as needed for the inputs.
You should have an external Spectrum Analyzer? ....without that I wouldn't bother.
It is better to fix the original module to maintain the calibration
