Thanks.
I wonder if it could be converted.
Update:
From the service manual:
"The First Local Oscillator Assembly is a yttrium-iron-garnet (YIG) tuned oscillator (YTO). The YIG, a ferro-magnetic material, is polished into a small sphere precisely oriented in a magnetic field. Changes in this magnetic field alter the frequency of the signal generated by the YTO. Voltage control of the magnetic field surrounding the YIG sphere allows the analyzer to be swept or tuned within the frequency range of 2.05 to 3.55 GHz used in the first LO."
So I'm guessing a mod for a 10MHz reference input would not be possible. Pretty cool technology though!