Greetings, Although the ac frequency response is not hard spec'd, it is very flat in this range. You could expect flatness from 0-600MHz to be about 0.1dB-0.2dB. Frequency response data is supplied when the unit is new (Option UK6) and when it is re-calibrated at an Agilent Service Center. Alternatively, it would be relatively easy to verify yourself using a swept sine wave technique.
e.g. Setup a signal generator to output 0dBm at frequencies from 50MHz-600MHz (or whatever range your sig gen will cover). Verify "0dBm" output power using an accurate power meter (you need to ensure the sig gen output power is not changing by tenths of a dB with a change in frequency). Move the sig gen signal to the 86112A input and record the amplitude. Given the same input signal (e.g. 0dBm), the measured amplitude (Vpp or Vrms) should remain relatively constant (e.g. Vpp within ~15mv, Vrms within ~6mv). Hope this helps!