HP 8560A, Руководство по обслуживанию

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A4 Assembly’s Cal Oscillator Circuit
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
bandwidth, bandwidth amplitude, and crystal-filter symmetry.
The A4 Cal Oscillator provides three output frequencies (all -35
n
10.7 MHz
n
9.9 to 11.5 MHz in 100 steps
n
Frequency sweeps from 20 to 2 centered at 10.7 MHz (lasting 5 to ms
respectively)
The signals perform the following functions:
n
Adjust gains, log amps, and video slopes and offsets.
n
Adjust 3 bandwidth and center frequencies of LC resolution bandwidth filters (30
through 1 MHz).
n
Adjust 3 bandwidth, symmetry, and gain of the crystal resolution bandwidth filters
(300 Hz through 10
The cal oscillator uses a phase-locked loop (PLL). T he 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 logic 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
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
anode and negative pulses at cathode.
The loop integrator acts as a low-pass filter that filters the pulses and inverts the result. If the
anode of
is more positive (with respect to ground) than the cathode of is
negative, the loop integrator output should saturate to approximately -13 V. Conversely, if
the anode of
is less positive than the cathode of is negative, the integrator
should saturate to a positive voltage.
Note If error messages
581 AMPL
or
582 AMPL
appears, refer to error message 582
AMPL in Chapter 6 and perform the procedure provided.
1. The oscillator output frequency should exceed 11.5 MHz if the CAL OSC TUNE line,
pin 14, exceeds V. The oscillator frequency should be less than 9.9 MHz if
CAL OSC TUNE is below -9 V. The oscillator only operates when CALOSC-OFF is low
2. If the Cal Oscillator remains locked (no error code 499 displayed) but does not have the
correct output level, troubleshoot the output leveling circuitry (function blocks AA, AB,
and AC) or output attenuator (function block AD).
8-34 IF Section