Log Offset/Log Expand - HP 8560A Service Manual

22. Change the reference level from -10 dBm to 0 dBm. The voltage change on A3TP7
should be between 670 mV and 730 mV. If the voltage change is outside of these limits,
refer to "Variable Gain Amplifier (VGA)" in this chapter. The gain of the VGA should be
7 &lO%.

Log Offset/Log Expand

See function block X of A3 Interface Assembly Schematic Diagram (sheet 5 of 6).
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/d iv. 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
2. Disconnect W26 (coax 2) from A3JlOl and connect the output of a function generator to
3. Set the function generator to the following settings:
Output . . . . . . . . . ..~..........................................Sine wave
Amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1V pk-to-pk
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . t500 mV
DC Offset
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Hz
Frequency
4. Set the HP 8560A 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's peak-to-peak voltage using an oscilloscope.
7. Set the HP 8560A's dB/div to 2 dB.
8. Readjust the function generator amplitude and offset until the sine wave again fills the
entire graticule area.
9. Measure the function generator's peak-to-peak voltage and dc offset.
10. The ratio of voltage recorded in step 6 to the voltage recorded in step 9 should be 5 f3%.
If the ratio is not 5, troubleshoot the A3 Interface assembly.
11. Reconnect W26 to A3JlOl.
7-14 ADC/lnterface Section
ZERO SPAN.
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