HP 8560E Service Manual

Chapter 7 371
ADC/Interface Section
A3 Assembly Video Circuits
Log Offset/Log Expand
Refer to function block X of A3 Interface Assembly Schematic
Diagram (sheet 5 of 6) in the
HP 8560 E-Series Spectrum Analyzer
Component Level Information.
The log scales are modified using a combination of amplification and
digital trace manipulation. The video input to the A3 assembly is either
10 dB/div or linear. To obtain the 5 dB/div scale, the CPU manipulates
the trace data from the 10 dB/div scale. To obtain the 2 dB/div scale, the
video signal is amplified and offset so that top-screen in 10 dB/div
corresponds to top-screen in 2 dB/div. To obtain the 1 dB/div scale, the
CPU manipulates trace data from the 2 dB/div scale.
In 2 dB/div, Log Offset/Log Expand amplifies the top 20 dB of the
display. This is done by offsetting the video signal by −0.8 V and
providing a gain of 5 to the top 0.2 V of the video signal. The −0.8 V
offset is accomplished by sinking 2 mA through R114 by current source
U105/Q101.
1. On the HP 8560E/EC, press
PRESET, SPAN, ZERO SPAN, CAL, and IF
ADJ OFF.
2. Disconnect W26 (coax 2) from A3J101 and connect the output of a
function generator to A3J101.
3. Set the function generator to the following settings:
Output ................................................................. Sinewave
Amplitude ........................................................ 1V pk-to-pk
DC Offset .............................................................. +500mV
Frequency ................................................................... 50Hz
4. Set the HP 8560E/EC sweep time to 50 ms.
5. Adjust the function generator amplitude and offset until the sine
wave fills the entire graticule area.
6. Measure and note the function generator peak-to-peak voltage using
an oscilloscope.
V
(10 dB/div)
=______________________________V
7. Set the HP 8560E/EC to 2 dB/div.
8. Readjust the function generator amplitude and offset until the sine
wave again fills the entire graticule area.
9. Measure the function generator peak-to-peak voltage and dc offset.
V
(2 dB/div)
= ______________________________V
10.The ratio of voltage recorded in step 6 to the voltage recorded in step
9 should be 5 ±3%. If the ratio is not 5, troubleshoot the A3 assembly.
11.Reconnect W26 to A3J101.

HP 8560E Specifications

General

Brand	HP
Model	8560E
Category	Measuring Instruments
Language	English