User's Manual

3300-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures
335B, Rev. 0 5-11
jumper W7 on jack J4 to the
Clamp-Off, Disable, position.
3. Connect a 5-step staircase video
test signal to the input of the
transmitter.
4. Monitor TP2 with an oscilloscope.
Adjust R12, the video gain pot, for
1 Vpk-pk.
5. Change the video input test signal
to a multiburst test pattern. While
monitoring TP2, adjust C8 and R32
for a flat-frequency response.
Change the input video test signal
back to the 5-step staircase.
6. Monitor TP2 with an oscilloscope.
Adjust pot R41, manual offset, for a
blanking level of -0.8 VDC. The
waveform shown in Figure 5-1
should be observed. Move the
jumper W2 on J4 to the Clamp
Enable position. Adjust pot R152,
depth of modulation, for a blanking
level of -0.8 VDC.
NOTE: This waveform represents the
theoretical level for proper
modulation depth. Step 9 below
describes how to set the modulation
depth through the use of a television
demodulator or a zero-spanned
spectrum analyzer tuned to the
visual IF frequency.
Figure 5-1. Waveform
7. The following test setup is for the
adjustment of the depth of
modulation and ICPM at IF:
A. Remove the cable that is on
J18 and connect the double-
sideband, 45.75-MHz visual IF
signal from J18 to a 10-dB
splitter/coupler. Connect the
coupled port of the
splitter/coupler to the RF input
of a television demodulator.
Connect the direct port to a
spectrum analyzer.
B. Connect the 75Ω video output
of the demodulator to the video
input of a waveform monitor.
For ICPM measurements, also
connect the quadrature output
of the demodulator to the
horizontal input of the
waveform monitor using a 250
-kHz, low-pass filter. (An
oscilloscope can be used in
place of a waveform
monitor).
C. Set the controls of the
demodulator to the following:
Detector mode – Cont
Sound trap – In
Zero carrier – On
Auto – Sync
Audio source – Split
De-emphasis – In
8. Move jumper W7 on J4 to the
Clamp Disable position. Readjust
pot R41, manual offset, for the
correct depth of modulation by
observing the demodulated
waveform on the waveform monitor
or on the spectrum analyzer set to
zero span.
9. Check the demodulated video for a
proper sync-to-video ratio (sync is
28.6% of the total white video
signal). If sync stretch is needed,
adjust R45, sync stretch cut-in,
until sync stretch occurs. Adjust
R48, sync stretch magnitude, for
the proper amount of stretch.
Readjust R41, manual offset, if
needed, for the correct depth of
modulation.