User's Manual

61-38-05 UserMan page 12 of 38
TXRX Systems Inc. Manual 7-9408-1.2 07/25/05 Page 12
4) Referring to figure 4, connect the generator test
lead to one side of the signal distribution system
(external antenna) and the spectrum analyzer
lead to the other (internal distribution system)
and observe the signal level. The difference
between this observed level and 0 dBm is the
isolation between the sections. If the signal is
too weak to observe, the spectrum analyzer's
bandwidth may have to be narrowed and its
input attenuation reduced. Record the isolation
value.
The isolation value measured should
exceed the amplifier gain figure by at least
15 dB.
It is wise to repeat the procedure listed above for
measuring antenna isolation, with the signal gener-
ator set to frequencies at the passbands edges in
order to see if the isolation is remaining relatively
constant over the complete width of the passband.
Also, the procedure should be repeated for each of
the remaining channels in the system.
Increasing Isolation
If the measured isolation does not exceed the
amplifier gain figure by at least 15 dB then modifi-
cation of the signal distribution system is required.
Alternately, the gain of the signal booster can also
be reduced to insure the 15 dB specification is met.
If the isolation cannot be increased then the
amount of gain reduction required is determined as
shown in the following example.
Input Signal Levels
Excessive input signal levels can damage the sig-
nal booster. Although this problem is less severe in
OLC protected branches, strong signals may cause
sudden reductions in gain and an associated
decrease in the desired output signal strength.
Even in the most carefully designed signal distribu-
tion systems, unpredictable situations can arise
that can cause this trouble. A few of the more com-
mon causes are:
a) Unintended signals entering the system. Prima-
rily caused by radios operating on channels that
are within the operational bandwidth of the sig-
nal booster. Sometimes this will be a transient
problem caused by mobile units when they
transmit while in close proximity to your system.
b) Hand-held and mobile units that approach
much closer than expected to one of the anten-
nas in the signal distribution system.
c) Unexpected signal propagation anomalies.
Building geometry can cause signal ducting and
other phenomena that cause signal levels that
are much stronger (or lower) than expected.
d) Lower than estimated signal attenuation causes
signals to be unusually strong. Higher losses
can also occur giving weaker signals than
desired.
e) Signal booster model with excessive gain. In
systems that have an existing signal booster, it
is sometimes assumed that an identical unit
should be installed when expanding the system
to provide extended coverage. In most cases, a
signal booster with far less gain than the first is
required.
f) Improper installation or application of signal
splitters or directional couplers in the signal dis-
tribution system. This is usually the cause of too
low a signal level but deserves mentioning here.
Signal splitting needs to be done with constant
impedance signal splitters so that the proper
power splitting ratios and VSWR are main-
tained. Using tee connectors by themselves is
inviting trouble. Directional couplers must be
connected with regard to their directionality and
coupling levels or improper system signal levels
may result.
Procedure for Measuring Input Signal Levels
1) Set a spectrum analyzer for the center fre-
quency of one of the branches (look at the
specification drawing for this information).
2) Set the analyzer sweep width so that the entire
passband frequency range can be observed.
EXAMPLE
Gain Reduction (dB) = Minimum Isolation (dB) -
Measured Isolation (dB)
If the measured isolation is -75dB and the mini-
mum isolation is -80dB then the amount of gain
reduction required is: -80dB - (-75) = -5 dB