Data Sheet Chapter 4

3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures
835-3, Rev. 0 4-12
Move switch S1 on the amplifier control board to the Manual position. Insert a visual-
only with sync-only test signal to the system and verify 40 IRE units of sync. Adjust R16
for 360 watts on the wattmeter. Monitor a sample of the output with a spectrum
analyzer set to the zero-span position and adjust the spectrum analyzer so that the
level of the output is at the top line of the gaticule. Insert -13 dB of aural to the visual
sync-only test signal and adjust R16 for the same visual-only input reference level on
the spectrum analyzer. The output power on the wattmeter should be approximately
450-watts visual +/-13-dB aural. Calibrate the front panel output power meter to 100%
with R2, forward calibration, on the amplifier control board.
Reduce manual gain pot R16 to a 50% reading on the front panel meter in the %
Output Power position. Turn the tray off. Remove the load from the output of the tray
and switch the front panel meter to the Reflected Output Power position. Switch the tray
on. Adjust reflected power adjust pot R22, on the amplifier control board, for a 50%
reading. Then adjust R29, the VSWR threshold cutback pot on the amplifier control
board, until the VSWR Cutback LED DS1 on the front panel just lights. This sets up the
VSWR cutback circuitry.
After the setup is completed, move switch S1 on the amplifier control board to the
Automatic Gain Control position; this is the normal operating position for the switch.
This completes the alignment procedures for the 600-watt UHF amplifier tray. The tray
is now aligned, calibrated, and ready for normal operation.
4.3 Alignment of (A9) the Bandpass Filter and (A10) the Output Trap Filter
The (A9) bandpass filter (A9) and (A10) the output trap filter are tuned to reject
unwanted distortion products generated when the signals are diplexed and also during
the amplification process. The bandpass filter is factory tuned to the proper bandwidth
and should not need to be re-tuned. Please consult the ADC Field Support Department
before tuning the filters.
The traps on the output trap filter are labeled with their center frequency relative to the
frequency of the carrier (for example, the traps labeled -4.5 MHz are tuned for a center
frequency of 4.5 MHz lower than the frequency of the visual carrier).
The trap sections are reflective notches that can be adjusted across the entire UHF
frequency band. The electrical length of the outer sleeve and the center rod of the notch
can be adjusted to tune the notch frequency. The depth of the notch is set by the gap
between the center conductor of the trap section and the center conductor of the main
line. Tight coupling makes a deep notch, while loose coupling makes a shallow notch.
The trap sections have been factory tuned and should not need major adjustments. The
frequency, relative to the visual carrier, that the trap is tuned to is marked on that trap
section.
Fine tuning of the center frequency of the notches can be accomplished with the tuning
bolts on the side of the filter section. Loosen the nut locking the bolt in place and adjust
the bolt to change the frequency of the notch. Monitor the output of the transmitter
with a spectrum analyzer and null the distortion product with the bolt. Red field is a
good video test signal to use to see the +8.08-MHz product. Tighten the nut when the
tuning is completed. Hold the bolt in place with a screwdriver as the nut is tightened to
prevent it from slipping.