Bird Technologies 429-83H-01 Series-Manual - Manual (Page 25)

TX RX Systems Inc. Manual 7-9439-6 09/03/09 Page 17
4) After setting the TTA Net Gain press the
ENTER button to return back to the ATTENUA-
TORS menu. This will save your setting choice.
Then pressing the CANCEL button while at the
ATTENUATORS menu will return you to the
default display.
Setting Distribution Attenuation
The MCU to receiver cable loss can be determined
from the cable manufacturers specifications. For
your system these values will be fixed once the
cable type is chosen and cut to length. Typically 2
to 3 dB of Distribution attenuation is usually
required to achieve unity gain. Use the attenuation
values shown in Table 6 to set the Distribution
attenuation to the correct value.
The DISTRIBUTION attenuation is adjustable from
0.0 to 15.5 dB in 0.5 dB increments. To adjust the
multicoupler attenuation perform the following
steps.
1) Press the DOWN ARROW button on the front
panel to scroll through the menu choices until
you reach the ATTENUATORS menu.
2) With the ATTENUATORS menu displayed
press the ENTER button to step down to the
TTA Net Gain sub-menu.
3) Press the UP ARROW button to move to the
Distribution sub-menu.
4) Press the ENTER button again to advance to
the Distribution adjustment screen. The current
distribution value will now be displayed. Use the
UP and DOWN ARROW buttons to set the dis-
tribution to the desired value. The attenuation
setting works in a forward loop fashion, starting
at 0.0 and increasing to 15.5, a button press
after 15.5 returns the setting back to 0.
5) After setting the distribution value press the
ENTER button to return back to the ATTENUA-
TORS menu. This will save your setting choice.
Then pressing the CANCEL button while at the
ATTENUATORS menu will return you to the
default display.
SPECTRUM ANALYSIS
Obtaining good sensitivity requires an understand-
ing of the levels applied to the receiver. A receiver,
like any electronic device, has a dynamic range of
operation. As long as this dynamic range is main-
tained, the specifications of the receiver are main-
tained. When the levels applied to the receiver
exceed this range, the sensitivity, intermodulation
rejection, as well as the adjacent channel selectiv-
ity will deteriorate. To properly perform a Spectrum
Analysis, a spectrum analyzer must be connected
to the output of the multicoupler as if it were a
receiver, essentially monitoring what the receiver
sees.
Figure 15 shows the equipment interconnection for
this measurement while Figure 16 is a graph which
indicates the maximum desired measured-signal
levels both inside and outside of the transmit and
receive bands. TTA filter selectivity and antenna
space isolation are the dominant factors that deter-
mine the signal levels observed. Excessively
strong receive signals indicate the need for addi-
tional attenuation in the MCU. There are three
areas of the spectrum that must be evaluated:
1) Receive Band - The spectrum where the
receive frequencies reside must not have carri-
ers above -35 dBm. These are the carriers that
are intended to enter the receiver. If subscribers
or control stations are near the infrastructure
the levels can be very high. If the levels are
above -35 dBm, the gain of the system must be
reduced or the source of the high level carrier
must be reduced.
2) Transmit Band - The highest carrier that the
receive system will consistently see is its own
transmitter. The preselector of the receive sys-
tem must adequately remove these carriers to
prevent over-drive. The goal of the preselector
is to reduce all transmit carriers below -55 dBm.
If the level of a transmitter is above -55 dBm the
Set DISTRIBUTION
attenuation to
for cable / distribu-
tion losses of
3 dB 1 dB
2 dB 2 dB
1 dB * 3 dB
0 dB more than 3 dB
* factory default setting
Table 6: Distribution attenuation settings.