HP 8560E, Service Manual

Page: 451 / 665

450 Chapter 8
IF Section
Cal Oscillator Assembly (P/O A4)
Cal Oscillator Assembly (P/O A4)
The cal oscillator on the A4 assembly supplies the stimulus signal for
automatic IF adjustments. Normally, the oscillator operates only during
retrace (for a few milliseconds) to adjust part of the IF. (All IF
parameters are to be readjusted about every 5 minutes.) With
continuous IF adjust ON, a group of IF parameters are adjusted during
each retrace period (non-disruptive). If continuous IF adjust is OFF, the
most recent IF calibration data is used.
The IF parameters adjusted include step gains, log amplifier gain and
offset, bandwidth centering, 3 dB bandwidth, bandwidth amplitude,
and crystal-filter symmetry.
The cal oscillator provides three types of output signals (all
−
35 dBm):
• 10.7 MHz
• 9.9 to 11.5 MHz in 100 kHz steps
• Frequency sweeps from 20 kHz to 2 kHz centered at 10.7 MHz
(lasting 5 to 60 ms respectively)
The signals perform the following functions:
• Adjust gains, log amps, and video slopes and offsets.
• Adjust 3 dB bandwidth and center frequencies of LC resolution
bandwidth filters (30 kHz through 1 MHz).
• Adjust 3 dB bandwidth, symmetry, and gain of the crystal resolution
bandwidth filters (300 Hz through 10 kHz).
The cal oscillator uses a phase-locked loop (PLL). The oscillator
(function block X) is locked to the instrument 10 MHz reference. The
reference divider (function block U) divides the reference and delivers a
100 kHz TTL signal to the phase detector (function block V). The
divide-by-N circuitry (function block Y) divides the oscillator output of
9.9 MHz to 11.5 MHz (by 99 to 115) resulting in a 100 kHz output to the
phase detector. When the cal-oscillator PLL is locked, narrow positive
and negative of equal width pulses occur at the phase detector output.
Since the phase detector drives a low-input impedance at the loop
integrator, observe the positive pulses at A4CR808 anode and negative
pulses at A4CR809 cathode.
The loop integrator acts as a low-pass filter that filters the pulses and
inverts the result. If the anode of A4CR808 is more positive (with
respect to ground) than the cathode of A4CR809 is negative, the loop
integrator output should saturate to approximately
−
13 V. Conversely,
if the anode of A4CR808 is less positive than the cathode of A4CR809 is
negative, the integrator should saturate to a positive voltage.