TX RX Systems Inc. Manual 7-9487-3 05/05/11 Page 21
Attenuation Settings
Spectrum Analysis
Operational Tests
Sensitivity with Load Connected
Sensitivity with Antenna Connected
Operational testing must be performed in a
methodical manner to provide the correct perfor-
mance evaluation and ensure that the information
obtained is correct. For each procedure it is impor-
tant that the data be recorded accurately and is
available anytime assistance is required or when
performance is in question. Before a receive sys-
tem problem is suspected, the appropriate opera-
tional tests must be performed. Before operational
tests for sensitivity can be verified, the programma-
ble attenuation settings and spectrum analysis
must be performed. If these are not correct, the
sensitivity and degradation may appear out of tol-
erance.
ATTENUATION SETTINGS
The system contains programmable attenuators for
optimizing the reserve gain. And for MCU style sys-
tems there are also programmable attenuators for
adjusting the receiver multicoupler distribution.
Both of these attenuators must be adjusted as part
of the system installation. The attenuation adjust-
ments allow the system to maintain maximum pro-
tection of the receivers, while obtaining the best
sensitivity possible.
Reserve Gain
MCU reser ve gain is defined as the net gain
between the input of the tower top LNA and the
input of the base LNA. CMU reser ve gain is
defined as the net gain between the input of the
tower top LNA and the base control unit output to
the GTR-8000. The amount of programmable
attenuation that your system requires in order to
reach an ideal amount of reserve gain will vary
depending on the length of your main transmission
line.
The value of the reserve gain programmable atten-
uator can be adjusted either manually by the cus-
tomer or automatically by the microprocessor in the
control unit. The choice of manual or automatic is
determined by a menu choice in the Webpage
User Interface which is accessible via the control
units front panel LAN connector using a separate
PC. For a complete description of the Webpage
User Interface software refer to the TXRX Systems
Inc. technical manual
7-9437
.
When automatic has been selected the message
“AUTO MODE” will be shown on the control unit
front panel display when you scroll down the menu
choices to the reserve gain value, refer to the
menu selections chart shown in figure 13. Manual
changes to the attenuator value using the front
panel ARROW buttons are not allowed when in the
automatic mode.
Receiver Multicoupler Distribution (MCU Only)
Receiver Multicoupler Distribution is defined as the
gain between the input of the base LNA and the
input of the station receiver. The receiver multicou-
pler distribution adjustment allows for proper com-
pensation of the MCU to receiver cable/distribution
losses and should ideally be set to unity. The
amount of attenuation that your system will require
in order to reach an ideal amount of receiver multi-
coupler distribution will vary depending on the
length of the cables from the multicoupler outputs
at the rear of the MCU deck to your receivers. Dis-
tribution attenuation is always adjusted manually
using the front panel ARROW buttons. There is no
automatic mode for distribution attenuation adjust-
ments.
Setting the MCU Reserve Gain Attenuation
The purpose of the MCU reserve gain attenuation
adjustment is to insure that the net gain from the
input of the tower top LNA to the input of the distri-
bution LNA equates to
14 dB
.
The gain of the tower top LNA is specified at 26 dB
(refer to table 2). There are internal losses associ-
ated with the LNA assembly such as a transfer
switch circuit and trace/interconnect losses. The
typical gain of the entire tower-top box is therefore
specified at 23 dB as noted in table 2. However,
there is 0.8 dB of tower box loss associated with
the tower-top preselector so the actual gain of the
entire tower top box is 23.8 dB which we will round
up to 24.0 dB for this discussion.
Between the input of the tower-top LNA and the
input of the distribution LNA there are fixed losses
including the ceramic filter assembly (with 2.3 dB of
loss) as well as the programmable reserve gain
attenuator (with 1.7 db of fixed loss in its minimum
attenuation state). These two additional losses
equal 4.0 dB. Subtracting the 4.0 dB of loss from
the 24 dB of tower-top LNA gain gives us a net 20
dB of reserve gain between the input of the tower-
top LNA and the input of the distribution LNA.
Fig-
ure 23
is a graphical representation.
