COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION)
Satellite reception at the headend doubled or tripled the number of channels that could
be available to the CATV subscriber. Each satellite has the potential of adding 24 channels
to the system. Note how the usable cable bandwidth is "broadened" as channels are added.
We assume contiguous channels across the band, starting at 55 MHz. For 30 channels, we
have 55270 MHz; for 35 channels, 55300 MHz; for 40 channels, 55330 MHz; for 62
channels, 55450 MHz; and for 78 channels, 55550 MHz. These numbers of channels
were beyond the capability of many TV sets of the day. Set-top converters were provided
that converted all channels to a common channel, an unoccupied channel, usually channel
2, 3, or 4, to which the home TV set is tuned. This approach is still very prevalent today.
In the next section we discuss CATV transmission impairments and measures of system
performance. In Section 17.4, hybrid-coaxial cable/fiber-optic systems are addressed. The
fiber replaced coaxial cable trunks, which made a major stride toward better performance,
greater system extension, and improved reliability/availability.
SYSTEM IMPAIRMENTS AND PERFORMANCE MEASURES
A CATV headend places multiple TV and FM (from 30 to 125) carriers on a broadband
coaxial cable trunk and distribution system. The objective is to deliver a signal-to-noise
ratio (S/N) of 4245 dB at a subscriber's TV set. From previous chapters we would
expect such impairments as the accumulation of thermal and intermodulation noise. We
find that CATV technicians use the term beat to mean intermodulation (IM) products.
For example, there is triple beat distortion, defined by Grant (Ref. 1) as "spurious signals
generated when three or more carriers are passed through a nonlinear circuit (such as
a wideband amplifier)." The spurious signals are sum and difference products of any
three carriers, sometimes referred to as "beats." Triple-beat distortion is calculated as a
The wider the system bandwidth is and the more RF carriers transported on that system,
the more intermodulation distortion, "triple beats," and cross-modulation we can expect.
We can also assume combinations of all of the above, such as composite triple beat (CTB),
which represents the pile up of beats at or near a single frequency.
Grant (Ref. 1) draws a dividing line at 21 TV channels. On a system with 21 chan-
nels or fewer, one must expect Xm to predominate. Above 21 channels, CTB will
dBmV and Its Applications
We define 0 dBmV as 1 mV across 75
impedance. Note that 75
is the standard
impedance of CATV, coaxial cable, and TV sets. From Appendix A, the electrical power
law, we have
is the power in watts,
E the voltage in volts, and R the impedance, 75 .
Substituting the values from above, we obtain
= 0.0133 × 10
W or 0.0133
By definition, then, 0.0133 W
= +60 dBmV
If 0 dBmV
= 0.0133 × 10
W and 0 dBm
= 0.001 W,andgainindB = 10 log(P
or, in this case, 10 log[0
.001/(0.0133 × 10
)], then 0 dBm = +48.76 dBmV.