User's Manual

300-Watt Digital UHF Transmitter Chapter 5, Detailed Alignment Procedures
DT830A, Rev. 1 5-16
are adjusted until there is no leak-
through of the 46.69056 MHz on the
spectrum analyzer.
5.3.12 I and Q Baseband Frequency
Response Adjustments
1. With the setup in the same
configuration as it was in step 3 of
the Local Oscillator Leak-Through
Adjustment procedure, move the
spectrum analyzer back to J5.
2. Using the MENU button, go to the
OPERATING MODE menu. Use the up
arrow to select NORMAL.
3. Observe the spectrum analyzer until
a 12 MHz-wide spectrum appears.
Set the spectrum analyzer to read .2
dB/division on the vertical scale and
turn the video averaging on to 200
averages or greater. Set the
reference level so that the top of the
spectrum is visible on the display.
Some time may be required to allow
the averaging feature to produce a
spectrum.
4. Adjust C16 until the spectrum looks
flat within .1 dB.
5. Repeat this process for the Q
channel, connecting the spectrum
analyzer to J9 and adjusting the
frequency response at C43.
This completes the frequency-response
adjustment.
5.3.13 Gain and Quadrature
Adjustments
1. With the setup in the same
configuration as step 5 of the I and
Q Baseband Frequency Response
Adjustments procedure, replace the
jumpers that were removed on J5/J6
and on J9/J10. Connect the spectrum
analyzer to J1, the IF output at the
rear of the tray. A spectrum will be
produced that is 12-MHz wide.
Perform the gain and quadrature
adjustments described in the next
two steps to remove the upper 6
MHz of this spectrum.
2. Rotate quadrature adjust capacitor
C27 until the upper 6 MHz of the
spectrum is reduced as low as
possible.
3. Adjust I channel gain potentiometer
R32 until the upper 6 MHz of the
spectrum is reduced as low as
possible.
4. Repeat steps 2 and 3 until the right
side of the spectrum is reduced as
far as possible. When this is
complete, the out-of-band on the
right side should look similar to the
out-of-band on the left side.
5.3.14 Output Level Adjust
The final output level of the modulator is
set by R3. A spectrum analyzer with a
power measurement capability, or a
vector signal analyzer, can be used to
adjust the output level. This level is set
at the factory to a nominal –18 dBm.
At this point, the alignment of the
modulator tray is complete. When the
modulator tray has been aligned
correctly, the out-of-band rejection
should be at least 40 dB (typically 42
dB) and the signal-to-ratio should be at
least 34 dB (typically 36 dB). If either of
these specifications is not met, the steps
described above for aligning the vector
modulator should be repeated. When the
desired modulator performance has
been achieved, the test jumper on the
(A2) switch board (1561-1202) should
be returned to position 1-2 to disable
the test modes. Power will need to be
cycled off and back on to return the unit
to the OPERATE mode.