| The third part of the CATV / RF distribution system consists of
the active devices like modulators, signal processors and broadband amplifiers. Here
are brief descriptions of each type of device: |
| Modulators take video and audio signals
from satellite receivers, video tape machines or cameras and produce a standard modulated
television channel for viewing on a television set. The signal output level
from a modulator is constant and can be as much as +60 dBmV. This value is 60
decibels above ONE millivolt as measured across a 75 ohm source impedance. This
value is ONE volt of RF carrier at the peak amplitude of the signal. |
| Signal processors (or strip amplifiers) are single channel
amplifiers designed to receive a single television signal from an antenna and control its
output to a steady level. This automatic gain control (AGC) function is essential for the
success of the distribution network of broadband amplifiers. Strip amplifiers do
not alter the input frequency of the received television signal - - what goes in, comes
out on the same frequency. Signal processors convert received television signals to
an intermediate frequency ( I.F. ) for filtering out unwanted adjacent television signals
that may be present at the antenna input. Precise signal level control (AGC) is
accomplished at this point inside the signal processor. The sound carrier RF level
is reduced from the 6 to 10 dB level difference transmitted by the TV station to a level
between 13 and 17 dB lower than the associated visual RF carrier. This allows
adjacent TV channels to be received by a television set with no visual impairment on the
viewing screen. The final stage of a signal processor converts the I.F. signal to a
standard or CATV channel for distribution to the cable system. A typical maximum RF
output level for a processor may also be as much as +60 dBmV. |
| Some types of frequency AGILE processors and modulators can also
produce broadband noise across the full range of CATV frequencies. When a large
number of these low-cost devices are coupled together into a single cable, the result is a
noisy or "snowy picture" on all channels before leaving the headend. The
best types of AGILE modulators and processors will have some type of "tracking
bandpass filter" to minimize the broadband noise output. Fixed channel
processors and modulators may contain an internal single-channel bandpass filter to
prevent this type of broadband noise problem. |
| Now that we have television signal sources, all that remains is
to combine them using directional couplers and splitters. When combined, the VISUAL
RF carriers should all be at the same level. Each associated sound carrier should
be set -15 dB in relation to the visual carrier level for that channel. The losses
of the couplers and splitters may require a broadband amplifier after all the signals are
combined. This will insure that we have enough RF signal level to begin signal
distribution to the many television outlets in the RF distribution system. |
| Depending on the number of channels combined together, the
coupler and splitter losses could result in a combined signal level on all channels from
+10 dBmV to +30 dBmV on each channel. At +30 dBmV, the signal levels may be strong
enough to feed directly into cables leaving the headend to go to other buildings. At
lower signal levels, an amplifier is used to provide a signal level from +35 dBmV to +45
dBmV or more to depart the headend. The maximum signal level will vary depending on
the number of channels and amplifiers in cascade in the distribution network. This
is due to the inter-mixing of signals when they are passed through the active components
of the amplifiers. The most common intermodulation distortion
is known as composite triple beat (CTB). You know you have it when you see various
types of "lines" in the pictures. This may be called a "busy
background" by some individuals. |
| When the headend amplifier output levels are established, the
design of the cable distribution network can begin. For our
750 MHz system, let us assume an output level of +40 dBmV on cable channel 118 and a level
of +35 dBmV on channel 2. There are two reasons why the amplifier output is
not FLAT (equal level at all channels). The first reason is the coaxial cable that
follows the amplifier has lower insertion loss at channel 2 than it has at cable channel
118. The second reason is to provide a lower ratio of CTB distortion from the
amplifier with the reduced signal levels at the lower frequencies. |
| The choice of the type of coaxial cable to be used in the
distribution system is based on the distances to be covered and whether or not the cable
is aerial, direct buried, or in a conduit or other underground passage ways. Common
sizes of outdoor cables are specified by the outside diameter of the metalic outer sheath
of the cable. The outdoor cable sizes most used are 0.500" (1/2 inch) and 0.750
(3/4 inch). The larger cable has lower insertion loss per 100 feet of cable. |
| Systems that are completely inside a single building may use the
extruded aluminum cable commonly used outdoors or may require special plenum types of
cable in sizes known by their RG numbers: RG-11, RG-6 or RG-59. The RG-11
cable is the largest and has the lowest loss per 100 feet. The RG-59 cable is
hardly used anymore in systems with bandwidths above 300 MHz, since it has the highest
loss of the three types of flexible coaxial cable. |