Chapter 8 393
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 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 spectrum analyzer, 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 spectrum analyzer 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.
7. Set the spectrum analyzer 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
(10 dB/div)
= ________________________________V
V
(2 dB/div)
= ______________________________V
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