Agilent Technologies 8564EC Service Manual page 433

Synthesizer Section
Unlocked Fractional N PLL
The polarity of the output of the loop gain (block AP, TP12) should be
the same as the polarity of the input (TP11).
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
Chapter 11 567