HP 8560E Service Manual page 555

Synthesizer Section
Unlocked Fractional N PLL
c. The integrator op amp (U106) output (TP13) should try to go very
positive (about +12 V) if its average input (TP12) is positive. If its
average input is negative, it should try to go very negative (about
−4 V). If its average input is zero and it is functioning correctly, it
may take on any output voltage between −4 V and +12 V (between
−12 V and +12 V for 08560-60062 and below).
554 Chapter 10