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TX RX Systems Inc. Manual 7-9487-3 05/05/11 Page 22
From this net reserve gain of 20 dB we need to
subtract the main line cable loss. Once the main
line loss is subtracted we can then use the pro-
grammable attenuator to trim the reserve gain
down to 14 dB. Keep in mind that the reserve gain
programmable attenuation circuitry has 2 types of
loss. A fixed portion (1.7dB) and a variable portion
with a range of 0 to 15.5 dB. The fixed portion is
always present regardless of where you set the
variable portion.
Because the gain and loss values of the subas-
semblies in the tower box and on the base unit are
fixed, we can simplify our discussion of setting the
reserve gain attenuation. The total amount of
reserve gain attenuation for your system is com-
posed of two parts, the main line cable loss which
is fixed (based on cable type and length) as well as
the programmable reserve gain attenuation setting
which is variable (depends on software selection).
This is illustrated in the formula below.
Main Line Loss + Software Setting = 14 dB
By transposing our formula we can determine the
correct software setting as follows;
Software Setting = 14 db - Main Line Loss
Because we know the reserve gain we want is 14
dB and we know the main line loss (determined in
earlier section titled “Feedline Loss”) it is a simple
matter to determine the correct Software Setting. If
the main line cable loss is less than 6 dB then pro-
grammable attenuation will need to be added. The
total loss of the main line cable and the program-
mable attenuator must be maintained at 6 dB.
A d d i t i o n a l a t t e n u a t i o n m ay b e
required in cases where carriers are
encountered above -35 dBm as mea-
sured at the receiver. Refer to the dis-
cussion “Spectrum Analysis” later in
the manual. In such cases 1 dB of additional atten-
uation is recommended for each 2 dB of signal
over -35 dBm. Good sensitivity will never be
obtained if the signals entering the system are
above -25 dBm.
Setting the CMU Reserve Gain Attenuation
The proper setting of both the GTR8000 and TTA
attenuators is required to establish the correct
reserve gain of the entire system. This must be
done before any verification of sensitivity is per-
formed.
The TTA contains software programmable attenua-
tion for optimizing the TTA attenuator adjustment.
When interfacing to the GTR8000 the TTA’s atten-
uation should be adjusted using the formula;
11 dB - main line cable loss
For example, if there is 3 dB of main line cable loss
between the control unit and the tower top box then
the TTA’s attenuation should be adjusted for 8 dB
which is equivalent to 11db - 3 dB.
NOTE
Filter
0.8 dB
Fixed
Loss
LNA
24 dB *
Gain
Ceramic
Filter
2.3 dB
Fixed
Loss
Programmable
Attenuator **
1.7 dB
Fixed Loss
(In Minimum
Attenuation State)
Distribution
LNA
Tower Top Box (23 dB of Gain)
* LNA Gain Rounded
to 24.0 dB
Base Unit
Ant
IN
To
Dividers
IN IN
Programmable Attenuator ships from factory
preset to 3.0 dB of programmable attenuation **
Total of 14 dB Reserve Gain
between LNA Inputs desired
Main
Line
Figure 23: Determining the MCU reserve gain programmable attenuation level.