User's Manual Part 2

Digital ATSC Exciter-Modulator System Chapter 3, Upconverter/Downconverter
Tray or Module Assemblies
Axcera Axciter, Rev. 0 45
U12 converts +12V to +9V for the L-Band PLL board and the First
Conversion board. U13 and U14 generate +5V for the microcontroller.
The board also selects whether the internal or external 10 MHz reference
source will be used. There is an onboard 10 MHz oscillator, U3, which is
used when no external 10 MHz source is present. The Relay K1 is
automatically switched to the external 10 MHz reference whenever it is
present. The LED DS1 illuminates whenever the internal 10 MHz
reference is used. The diode detector CR1 detects the presence of the 10
MHz external reference, that connects to U2, which compares the detected
level to a reference level and switches the relay whenever the reference is
present. It also disables the internal oscillator whenever the external 10
MHz reference is being used. The output of the relay is split to drive
multiple outputs, some internal and some external. The external outputs
leave the board at J1-22C and J1-31B and are used by the external
receiver and modulator modules.
The board also contains AGC circuitry, which controls a pin diode
attenuator on the Downconverter Board. There are three references used
by the AGC circuit. The first is the AGC reference #1, which comes from
the transmitter's driver module. The second is the AGC reference #2,
which is a diode-ORed sample of the output stages of the transmitter.
The IC U5 normalizes the level of the AGC reference # 1 and sets it at a
level that is 0.2V less than the level of AGC reference #2. The AGC
reference #1 and #2 are diode-ORed with only the highest reference used
by the AGC circuit.
The highest reference is compared to the ALC reference, which originated
on the IF processor module, and the error voltage generated by U4D and
applied to the external pin attenuator. The AGC will try to maintain a
constant ratio between the ALC voltages and the higher of the two AGC
voltages. If something in the output amplifier of the transmitter fails, the
AGC reference #1 voltage will take over and the power will be regulated
at the output of the driver.
Located on the board is the DIP switch SW1. See the picture above. The
function of each position is stated in the following text. We have added
the use of position 5 to allow or disable the changing RF output power
while the Upconverter is in Auto AGC Mode.
Upconverter DIP
Switch SW1
Function
Position 1
Master/Slave, ON turns control board into RS-485 master. This is
used when there is ONLY a tray based upconverter and Axciter in
the system.
Position 2
Mute Orientation - Reverses the polarity of the MUTE input from the
remote connector on the rear panel of the Upconverter. In the OFF
position, the Upconverter requires a pull down to come out of mute.
Position 3-4 Not Used
Position 5
Disable Auto AGC Power Changes. OFF allows power adjustments
to be made while the Upconverter is in Auto AGC mode. ON does
not allow power adjustment when the Upconverter is in Auto AGC
mode.