Agilent Technologies 8564EC, Service Manual

Page: 312 / 512

402 Chapter 8
ADC/Interface Section
A3 Assembly Video Circuits
9. Set the spectrum anal"ADC Control Signals" in this chapter.yzer to
the following settings:
Sweep time ................................................................. 100 ms
Span ......................................................................... 100 MHz
10.Press
CAL
and
IF ADJ ON
and check for the presence of the CAL OSC
TUNE signal by monitoring A3J401 pin 25 with an oscilloscope. If
ERR 499 CAL UNLK
is displayed and a signal within the range of
−
10 V to +10 V is present during part of the retrace period, the fault
is on the A3 assembly.
11.If a constant dc voltage is present during the sweep and all of the
retrace period, refer to the IF Section troubleshooting procedure in
Chapter 9 , “IF Section.”
Variable Gain Amplifier (VGA)
Refer to function block AB of A3 Interface Assembly Schematic
Diagram (sheet 6 of 6) in the 8560 E-Series Spectrum Analyzer
Component Level Information.
The VGA provides adjustable gain in the video path. Its nominal gain of
7 can be adjusted
±
10%. U112 removes dc offset to keep U113 in its
monotonic range. (Both U112 and U113 are set to the same value.) The
DAC settings cannot be changed from the front panel.
Track and Hold
Refer to function block AC of A3 Interface Assembly Schematic
Diagram (sheet 6 of 6) in the 8560 E-Series Spectrum Analyzer
Component Level Information.
1. Press
PRESET
on the spectrum analyzer and set the controls as
follows:
Center frequency .................................................. 300 MHz
Span .............................................................................. 0 Hz
Detector mode .......................................................... Sample
Reference level .......................................................
−
70 dBm
Log dB/division ..................................................... 2 dB/DIV
Sweep time ................................................................. 50 ms
2. Disconnect any signal from the spectrum analyzer input. A full scale
display of sampled noise should be present.
3. Trigger an oscilloscope on the positive going edge of HHOLD
(A3U506 pin 16).
4. The waveform at A3TP10 should be random noise with an average
level of approximately 4 V. The noise should have a flat spot in its
response while HHOLD is high, indicating proper operation of U114.