Chapter 10 509
Controller Section
Video Input Scaling Amplifiers and Limiter
Video Input Scaling Amplifiers and Limiter
The video input scaling amplifiers help provide scaling (10 dB/div,
5 dB/div, 2 dB/div, or 1 dB/div) and buffer the flash video output. When
the GAINX2 control line is low, switch U44D is open and switch U44C
is closed. Thus, the scaled video at TP26 virtually follows the video
input (0 - 1 V). When the GAINX2 control line is high, switch U44C is
open and switch U44D is closed. Amplifier U43 then provides a gain of
2(V
in
) −1 V. Voltage clamp CR4 prevents the scaled video input to
amplifier U45 from going more negative than −0.35 V or more positive
than +1.25 V.
NOTE When measuring voltages or waveforms on the Fast ADC section of the
A2 controller assembly, connect the ground (or common) lead to the
ground-plane trace associated with the shield. This digital ground
plane is totally isolated from the chassis.
1. Press
PRESET on the 8560 EC-series spectrum analyzer and set the
controls as follows:
Center frequency ............................................. 300MHz
Span .........................................................................0Hz
Reference level ................................................. −10dBm
Log/division ....................................................10dB/DIV
Sweep time ............................................................ 20ms
2. Connect the CAL OUTPUT to the INPUT 50Ω connector.
3. Adjust the spectrum analyzer reference level to place the signal at
the top graticule line on the LCD display.
4. Measure the dc level at pin 3 of U10. If the voltage measured is not
+1.0 ±0.15 V, troubleshoot the A3 interface assembly.
5. Measure the dc level at pin 3 of U17. The level should be
approximately the same as the level measured at pin 3 of U10. If not,
suspect switch U9.
6. Set the spectrum analyzer scale to 5dB per division.
7. Adjust the spectrum analyzer reference level to place the signal at
the top graticule line on the LCD display.
8. Measure the dc level at pin 3 of U10 and pin 3 of U17. The level
should be +1.0 ±0.25 V. If the level measured at pin 3 of U17 differs
from the level measured at pin 3 of U10 by more than 0.25 volts,
troubleshoot U10 and associated circuitry.
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