Agilent Technologies 8564EC, Service Manual

Page: 374 / 512

Chapter 9 479
IF Section
Cal Oscillator (P/O A4 Assembly)
Sweep Generator
Refer to function block Z of A4 log amplifier schematic diagram (sheet 4
of 4) in the 8560 E-Series Spectrum Analyzer Component Level
Information.
A properly operating sweep generator generates a series of
negative-going parabolas. Before the sweep, switches A4U802C and
A4U802D turn on, shorting A4C802 and A4C801 (the output is at
0 volts). These switches open to start the sweep. The output of
A4U804A, pin 1, is 0.35 V to 10 V, depending on the sweep width
selected by A4U802A and A4U803A. This voltage appears across
A4R801. Capacitor A4C801 integrates the current through A4R801.
The output of A4U804B is a straight, negative-going ramp. Capacitor
A4C802 and resistor A4R802 integrate the output of A4U804A which
starts a negative ramp (A4U804C) at the beginning of the sweep. The
ramp from A4U804B is added to the current in A4R802 via A4U803B.
Integrating this ramp results in the parabolic output waveform.
AM/FM Demodulation, Audio Amplifier, and Speaker
Refer to function blocks R, S, and T of A4 Log Amplifier Schematic
(sheet 4 of 4) Diagram in the 8560 E-Series Spectrum Analyzer
Component Level Information.
If the audio circuits are not functioning use the following procedure to
isolate the problem:
1. Set an AM signal generator controls as follows:
Frequency ................................................................ 100 MHz
Amplitude ..................................................................
−
6 dBm
Modulation type ....................................................... 80% AM
Modulation frequency ................................................. 400 Hz
2. Set the 8564EC or 8565EC spectrum analyzer controls as
follows:
Center frequency ........................................................ 100 MHz
Span ................................................................................... 0 Hz
Sweep time ....................................................................... 50 ms
Reference level ................................................................ 0 dBm
Resolution bandwidth ................................................... 10 kHz
Amplitude scale ........................................................... LINEAR
3. Adjust the 8564EC or 8565EC spectrum analyzer reference level
and center frequency to display the 400 Hz modulation frequency
eight divisions peak-to-peak.
4. On the 8564EC or 8565EC spectrum analyzer, press
AUX CTRL
,
AM/FM DEMOD
,
AM DEMOD ON
, and set the sweep time to 5 seconds.