I
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Channel Measurements
Making Channel Measurements
Improving Accuracy
Use the External 75 Ohm Preamp
(85905A)
for the Oifferential Gain and Oifferential Phase tests when
measuring peak visual carrier levels equal to or less than
+lO
dElmV. This improves the repeatability of
the measurement. Repeatability is one of the components of accuracy Accuracy is improved from 6%
and 4 degrees to 4% and 3 degrees.
The external preamp must be located ahead of the coupler so the TV input can take advantage of the
preamplifier’s gain.
3. If the desired test signal is not displayed, press SELECT LINE then enter
a line number, or press
AUTO
SIG SEARCH to execute an automatic
search routine.
4. Press FIELD ODD EVEN if you need to change the field.
5.
Press Select Test Sig if you need to change the test signal selection.
a.
Press FCC COMPOSIT to select the FCC composite test signal, or
NTC 7 COMPOSIT to select the NTC 7 composite test signal. To assist
in test signal identiEcation a small schematic diagram of the test signal
is displayed at the top of the screen.
b.
Press 5th PKT YES NO to determine whether the 5th riser of the
staircase is included in the measurement. The default is NO (NO is
underlined) and the changes are permanent until changed again by
pressing this key. The status of this key is reported on the results
screen just below the results box. In system mode, the 5th packet is
never measured. Most modulators have 90% limiting which causes the
5th packet to be clipped on an NTC-7 composite test signal because
the 5th step exceeds the white bar by more than 1.5%. Under these
conditions, it is recommended that you do not use the 5th packet
in your measurement. This will calculate the differential gain and
differential phase result based on the reference and the Erst four
packets.
2-39
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