User's Manual

300-Watt Digital UHF Transmitter Chapter 5, Detailed Alignment Procedures
DT830A, Rev. 1 5-13
The potentiometers on this board consist
of the I and Q gain and offset
adjustments. Since the overall gain
balance between the I and Q channels
will be adjusted in the vector modulator,
the gain is not critical. As a result, gain
potentiometers R50 and R71 should be
set to the middle of their range. The
offset potentiometers are adjusted in the
following order:
1. Power up the modulator with the test
jumper placed in the enable position
on the rear of the (A2) switch board.
2. Using the up arrow button, change
the operating mode from NORMAL to
CW ZERO POWER.
3. Remove the connector to J26 on the
VSB modulator board. Connect a
voltmeter to J26, with the positive
lead connected to the center of the
3-pin connector and the negative
lead connected to one of the outer
pins.
4. Set the voltmeter to read in millivolts
and adjust R47 until the voltmeter
reads zero.
5. Repeat this procedure for J29 while
adjusting R68.
6. Return all jumpers and connectors to
the standard configuration.
This completes the I and Q offset
adjustment.
5.3.4 (A2) Switch Board (1561-1202;
Appendix D)
The switch board is mounted on the rear
of the front panel. It houses the front
panel pushbuttons for the LCD readout.
There is one jumper, J2, to enable or
disable the TEST mode. The TEST mode
enables several MPEG digital signals that
are used for the test and the alignment
of the tray. In order to enable the TEST
mode, turn off the tray and move jumper
J2 from position 1-2 to position 2-3.
Note: The tray must be turned off,
jumper J2 moved back to the disable
position (1-2), and the tray powered
back up in order for the tray to be
restored to the NORMAL mode.
5.3.5 (A7) Local Oscillator Board
(1561-1203; Appendix D)
The local oscillator board does not have
any adjustments for the VCXO oscillator;
however, there are several jumpers and
DIP switches on the board to set the PLL
circuits.
Verify that jumpers J7 and J3 are in the
1-2 position. Jumper J3 is set based on
the polarity of the VCXO that has been
installed. The VCXO installed in the 8-
VSB modulator requires that J3 be set in
position 1-2. Jumper J7 is the most
significant part of the divide ratio that
determines the final local oscillator (LO)
frequency. If J7 is in the 1-2 position, the
MSB is set to 1. Using J7, along with SW1
through SW3, the LO frequency is
determined from the following equation:
LO Frequency = 2*(Switch Setting+3)
or in terms of the switch setting:
Switch Setting = (LO Frequency/2)-3.
For the LO card to produce 46.690560
MHz, verify that DIP switches J7, SW1,
SW2, and SW3 are set as shown in Table
5-6.
Table 5-8. DIP Switch Settings
J7 SW1 SW2 SW3 LSB
1 01100100 00111000 01111101