Specifications

[
StarServe Installers Guide
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page 31
Attenuators
As there are many signals received by an antenna, there
may be a wide variation in signs levels. In order to ensure the
same picture quality on all channels, the signal levels may
require equalisation to prevent the stronger signals from
overriding the weaker ones. Equalisation is achieved by
using attenuators, which reduce the incoming stronger signals
by a specified amount.
Attenuators can be either fixed or variable. They are either
designed for one specific attenuation level, or they are
switchable so that the signals can be reduced in increments
to the required level.
Attenuators reduce all signals that pass through by the same
amount. Therefore, frequencies that need reducing need to
be separated from the rest of the signals so that only the
stronger signals are reduced.
Amplifiers
Amplifiers increase the strength of signals received to a level
greater than the losses in the distribution system.The
amplifier gain determines the level of increase, which should
be high enough to provide an acceptable signal level to all
televisions in the system.
Although an amplifiers gain is important, the output capability is
just as important. The amplifier’s specifications should be
checked to ensure that the output level is sufficient to feed the
system and that the strength of the input signal plus the gain of
the amplifier doesn’t exceed the amplifiers rated output capability.
Exceeding the output capability will result in overloading, cross
modulation distortion, and overall signal deterioration.
System Losses
Cable Loss
A certain amount of signal will be lost as it travels through
coaxial cable. This loss depends on the type of cable used
and the frequency of the signal being carried.
Losses are greater at higher frequencies, the greatest loss
occurring at channel 67 in UHF/VHF systems. The cable loss
should always be calculated at the highest frequency received
or the highest frequency to be received in the future.
Splitter Loss
When a two-way splitter is inserted in-line, the signal in each
leg will be approximately 3.5 dB less than that of the main
line. If a 4-way splitter is inserted in-line, the signal in each
leg is 6.5 dB less than that in the main line. The signal sent
to each branch of the system will be equal to the signal sent
into the splitter minus the splitter loss. That is, an input of 30
dB into a 2-way splitter will deliver a signal of 30 dB minus
3.5 dB splitter losses, or 26.5 dB to each branch of the System.
Insertion Loss
All tapoff devices inserted into the distribution system create
signal loss. This type of loss is called insertion loss, (some-
times called feed-through loss). On the line, the insertion
loss of each tapoff is subtracted from the signal carried by
that line. When estimating total system losses, the insertion
loss of each unit is added together to find the total insertion
loss for that system. For example, if there are 10 tapoffs on
the line, and each tapoff has an insertion loss of 0.5 dB, the
total insertion loss is 5 dB.
For initial calculation, the tapoff values and the insertion losses
are estimated as the output of the amplifier will influence the
final selection tapoff values.
Isolation Loss
Each tapoff attenuates the signal by a specified number of
dB to prevent one set from interfering with another. For
example, if there is a 25dB signal in the line, and a 23dB
isolation wall tapoff is inserted in the line, the signal available
at the tapoff would be 2dB. The 23dB loss is called Isolation
Loss. In computing the total distribution system losses,
calculate the isolation loss of the last tapoff only. Since the
system design requires a minimum of 0dB (1000 µV) to each
set, the lowest isolation value should be used. For most MATV
tapoffs this value is 12dB.