TX RX Systems Inc. Manual 7-9469-1.1 06/10/09 Page 2
Down / Up Conversion
A channelized signal booster has much in common
with a superheterodyne (superhet) receiver. The
incoming signal is converted to a lower frequency
so that single channel selectivity can be obtained.
It is then filtered. Unlike the superhet receiver how-
ever, the signal is not demodulated. Instead, it is
up-converted back to its original frequency where it
is further amplified to reach a useful power level.
Figure 1
shows a simplified block diagram that
illustrates the down/up conversion principle. An
incoming signal at (Freq IN) is applied to the first
mixer along with a signal from a local oscillator
(Freq LO). A third signal at an intermediate fre-
quency (Freq IF) is produced as a result of the mix-
ing. The intermediate frequency is given by the
following relationship:
(1) Freq IF = Freq IN - Freq LO
The IF signal from the mixer then passes through
filtering with single channel bandwidth before being
amplified and passed on to the second mixer. The
second mixer also receives the same local oscilla-
tor signal (Freq LO). The result is a mixing product
frequency at the output of mixer 2. The output fre-
quency (Freq OUT) is given by the following rela-
tionship:
(2) Freq OUT = Freq IF + Freq LO
Substituting equation (1) for the “Freq IF” term in
equation (2) allows the “Freq LO” terms to be can-
celed yielding:
(3) Freq Out = Freq IN
The implication of equation (3) is that the frequency
stability of the signal that is processed by this type
of signal booster is not affected by the frequency
stability of the signal booster itself. Frequency sta-
bility depends only on the stability of the signal
source producing the signal to be boosted. A shift
in the LO frequency of the signal booster will only
cause a shift in the IF frequency with a possible
loss of signal amplitude but no change in the out-
put signal frequency.
UNPACKING
It is important to report any visible damage to the
carrier immediately. It is the customers responsibil-
ity to file damage claims with the carrier within a
short period of time (1 to 5 days). Care should be
taken when removing the unit from the packing box
to avoid damage to the unit.
INSTALLATION
The following sub-sections of the manual discuss
general considerations for installing the booster. All
work should be performed by qualified personnel
and in accordance with local codes.
Location
The layout of the signal distribution system will be
the prime factor in determining the mounting loca-
tion of this unit. However, safety and serviceability
are also key considerations. The unit should be
located where it can not be tampered with by the
general public, yet is easily accessible to service
personnel. Also, consider the weight of the unit and
the possibility for injury if it should become
detached from its mounting for any reason.
The booster needs to be installed such that there
can be unobstructed air flow over and around the
subassemblies. The various subassemblies will
stay warm to the touch during normal operation so
in the interest of equipment longevity, avoid loca-
tions that carry hot exhaust air or are continually
hot.
Antenna Isolation
Antenna isolation between uplink and downlink
should be measured before connecting the signal
booster to the antenna system. This step is neces-
sary to insure that no conditions exist that could
possibly damage the signal booster and should not
be skipped for even the most thoroughly designed
system.
Just like the feedback squeal that can occur when
the microphone and speaker get too close together
in a public address system, a signal booster can
start to self oscillate. This can occur when the iso-
lation between the Uplink and Downlink antennas
does not exceed the signal boosters gain by at
least 15 dB. Oscillation will reduce the effective-
ness of the system and may possibly damage
amplifier stages. Isolation values are relatively
easy to measure with a spectrum analyzer and sig-
nal generator.
REQUIRED EQUIPMENT
The following equipment is required in order to per-
form the antenna isolation measurements.
