User Manual

DM8-R Digital Modulator Tray Board Descriptions
DM8-R Digital Modulator, Rev. 0 4
Analog Output Section
The baseband I and Q signals from the D/A converter are applied to differential analog
filters that remove some of digital artifacts from the D/A conversion process. The output
of the I channel filter is then mixed with the pilot frequency, 46.69 MHz, using mixer
U30. The output of the Q filter is mixed with the pilot frequency that is phase shifted 90
degrees using mixer U34. The mixers are current driven devices so that when the
outputs of U30 and U34 are connected together, they provide a combined output. This
combined output is subsequently input to a final differential output filter which provides
the final IF output at the SMA connector, J38. This output is connected through a RG-
174 cable to the BNC connector J4, the IF output jack, located on the rear panel of the
tray.
Pilot Frequency Generation
The 46.69 MHz pilot signal, which is used in the mixing process, is generated from a
46.69 MHz VCXO that is phase locked to a 10 MHz reference. The VCXO and the 10 MHz
are divided down to a common frequency, which is then compared internal to the FPGA.
The FPGA subsequently provides error signals to a analog phase locked implemented
with op amp stages U45-A, B and C. The output of these compensation stages is used
as the control voltage to the VCXO, U37. The phase locked output of U37 is applied to
a analog filter to remove harmonics of the pilot and then input to quadrature splitter Z1.
The outputs of Z1 are used as the inputs to the mixers in the analog output section.
DM8-R Front Panel Board (1307113)
The front panel board contains four LEDs, Power, MPEG, PLL A and PLL B, which are
viewable on the front panel. J2 connects to the Modulator Board, which controls the
operation of the LEDs. When the system is operation normally, all LEDs will be Green.
If a problem occurs, that LED will not be lit.