The cable television industry throughout the years has seen various technologies and developments to the broadband amplifier. Amplifier
manufacturers have designed and distributed everything from a simple Push-Pull circuit to more complex circuits such as Feed-Forward,
Power Doubling, and GAS-FET amplifier circuit's. With proper alignment and maintenance, each can be used effectively
in todays high-tech infrastructures.
Why do I want to balance my amplifier cascade and properly sweep the network?
The correct balancing of RF amplifiers and the maintenance of broadband network's through the utilization of a "System
Sweep" is for many reasons.
The reduction and control of amplifier and non-amplifier created distortions in the broadband network are the main reason
for correct system alignment and maintenance.
Distortions are directly proportional to picture quality at the customers home. The sweep is a good field tool that helps
the technician maintain a solid and acceptable two-way broadband network. If properly used it can locate poor connectors,
substandard amplifiers and damaged or bad cable.
What are "distortions" and how do they make the picture quality bad at my customers television?
The cable operator strives to produce the best possible picture quality for his customers throughout the entire
network. This task can be either easy or a difficult depending on the variables the technician must overcome.
A distortion is a change in a waveform (or picture) from its orginal signal that results in degradation
from its inital form. All active components in a cable
network will produce distortions and is normal. What is not normal is excessive distortions created by active
devices because of incorrect alignment, defective components or substandard workmanship.
Why do some pictures in the cable system look good if distortions are always present? I am confused.
There are many types of distortions assosiated with the broadband network. The most common are Carrier-to-Noise
(CNR), Composite Second Order (CSO), Composite Triple Beat (CTB) and Hum. These distortions are related to the picture
quality problems that are seen on the customers television that may be a snowy picture, diagonal lines, flutter and
rolling lines to mention a few. Every RF amplifier in the cable system has a manufacturers specification sheet and distortion
analysis that provides the designer with "worse case" distortion numbers under specific conditions. Using this information,
the designer can calculate his system distortion numbers based on differential formulas and dictate amplifier levels
and tilt necessary for his requirements. These levels must be achieved at all amplifier locations with correct attenuation
and equalization including the interstage location if required. If this is inconsistant throughout the network, distortions
will be random and some areas worse than others. Distortions are minimal at the 1st amplifier location and get worse as the
cascade deepens under normal conditions given a good design with proper balancing and a respectable cascade count within
reason.
How do I tell if I have distortions?
You will always have distortions present in amplifiers and pictures in the broadband network, this is a fact-of-life. The
real question is how much distortion do you have and is it of an acceptable level. This is done through "proof-of-performance"
testing and the use of test equipment along with a specification sheet of what should be acceptable in
the system design.
How do I eliminate distortions in my cable system?
You will never eliminate the distortions in your broadband network, but you can control them to work with you
to achieve the best possible picture quality and optimum amplifier performance. This is best done through proper
alignment of the amplifiers and a good sweep of the entire network.
