Keysight (formerly Agilent T&M) 86290B Sweep Oscillator Plug-In

I have an HP 86290B with option H08. The frequency of this sweep oscillator plug goes from 2 GHz to 22 GHz. However, the frequency reference from the BNC connector on the back of the unit only goes up to a maximum of 18.8 volts when measured using my DVM. I thought it was supposed to produce an output of about 1 volt/GHz so that the voltage should go up to 22.0 volts? I checked the output frequency, however, and it does go to 22 GHz at the end of the scale.