User's Manual Part 2

Digital ATSC Exciter-Modulator System Chapter 3, Upconverter/Downconverter
Tray or Module Assemblies
Axcera Axciter, Rev. 0 46
Upconverter DIP
Switch SW1
Function
Position 6
AGC 1 and 2 Gain Modify Enable. ON enables the user to modify
AGC1 gain and AGC 2 gain through the Axciter
Position 7 Upconverter Lockout- ON locks out all commands from the Axciter
Position 8
Frequency Modify Enable. ON enables the user to modify the
Frequency through the Axciter
The upconverter provides the frequency translation necessary to convert
the IF output signal of the Axciter to the assigned channel frequency. The
upconverter is 100% synthesized using PLL techniques so no crystal
changes are required to operate on any standard U.S. TV channel.
A 10 MHz signal is required as a reference for the two PLL systems on the
IF and RF board. The IF board contains a 10 MHz oven controlled crystal
oscillator (OCXO) for this purpose. If synchronous or precise frequency
control is required, an external 10 MHz reference can be applied through an
external connector that is connected to the IF board. The 10 MHz reference
(internal or external) is also provided as a buffered output for use by the
Axciter Upconverter/Downconverter.
Downconverter Board, Axciter (1306807)
A sample of the transmitter’s RF output is applied to the downconverter
board, mounted in the tray or the downconverter module, at a nominal
input level of –6 dBm. The signal is attenuated by a 10 dB pad, and then
converted to an IF of 1044 MHz by mixer U1. A sample of the
upconversion LO from the L-Band PLL Board mounted in the upconverter
module assembly is sent through the exciter’s backplane board, or directly
to the board in the stand alone tray. On the downconverter board, the LO
is amplified and then filtered to remove any spurious energy before being
applied to U1.
A filter selects the appropriate conversion product, with the resulting signal
being applied to the mixer U9, which converts the signal to a second IF of
44 MHz. A 1 GHz LO frequency that is generated externally, and either sent
through the exciter’s backplane board to the downconverter module or
connected from the 1
st
conversion board in the stand alone tray. The 1 GHz
LO is applied to a high pass and low pass filter designed to eliminate any
other interfering signals that might be coupled into the 1 GHz LO. This 44
MHz second IF signal is then applied to a low pass filter to remove any out
of band energy, amplified and connected to a frequency response correction
circuit intended to compensate for any linear distortions in the
downconversion path. Adjustments R50-R52 and C78-C80 are used to
control the frequency response of the downconverter. The resulting signal
is sent to a pin diode attenuator, which allows the operator to adjust the
gain of the downconversion path. The signal is then amplified again to a
level of +4 dBm average and applied to a cascaded high pass low pass
filter, which removes any out of band energy that would be aliased in the
demodulation process.