Understanding TV Signal Distribution Networks: Components and Types
Understanding TV Signal Distribution Networks
Distribution networks in receiving systems for terrestrial TV utilize passive elements. These elements include terminals for interconnecting components and output connectors for users. Bandwidth, or frequency range, is a crucial parameter. Common bandwidths include 47 to 862 MHz for TV and radio, and 950 to 2150 MHz for terrestrial TV and satellite radio. The distribution of TV signals occurs via coaxial cable from the headend to the user.
Key Components of a TV Signal Distribution Network
The main elements of the grid are:
Splitters
Splitters, also known as line dividers, are used when a single connection needs to be distributed in multiple directions. Unused outputs should be terminated with a 75Ω resistor to maintain proper impedance and prevent signal reflections. Key characteristics include:
- Attenuation (Insertion Loss): The signal loss as it passes through the splitter, measured in dB.
- Rejection Between Outputs: Indicates how much one output signal affects others, with higher values indicating better isolation (dB).
- Input-Output Adaptation (ROE): Resistor values should be 75 Ω for proper impedance matching.
Types of Splitters:
- Resistive: These consist of a network of resistors designed to produce a balanced output signal.
- Inductive: Made from an insulating material with copper toroidal coils, resulting in lower insertion loss and reduced stray signals. Recommended for installations with more than 8 or 10 user outlets.
Derivatives (Taps)
Derivatives, or taps, are placed on a main distribution line to create branches. They minimize stray signals and typically have 2 or 4 outputs. Unused outputs should be terminated with a 75Ω resistor. Key characteristics include:
- Attenuation (dB): Indicates the signal loss as it passes through the tap.
- Derivation Shunt Attenuation (dB): Indicates the signal loss in the shunt when removing part of the TV signal. Taps closer to the headend have higher shunt attenuation.
- Rejection Between Outputs (dB): Indicates how much one output signal affects others. Higher values indicate better isolation.
Types of Taps:
- Resistive Shunts: Use a network of resistors to create the necessary voltage drop for attenuation. Rarely used in modern installations, typically only in systems with fewer than 10 taps.
- Inductive Shunts: Built with high-frequency toroidal transformers, offering lower attenuation than resistive shunts. These are the most commonly used and provide the best results.
- Hybrid Shunts: A combination of resistive and inductive elements, but no longer commonly used.
User Outlets (Wall Plates)
User outlets allow users to connect their devices to the network. They often filter and separate mixed signals from TV bands, FI (Intermediate Frequency), and FM, providing separate connectors for TV and radio receivers.
Dispersion Network
The dispersion network connects the distribution network to the user’s devices. It includes two types of user outlets:
- Step User Outlets: Key characteristic is shunt attenuation (dB), indicating the signal loss in the outlet (0 to 30dB). Step attenuation should be as low as possible, between 0.4 and 0.8 dB. Rejection between user outlets should be no less than 30 dB.
User Outlets Step
Step user outlets are installed on the internal network and are considered derivatives, also called shots in series. They have two terminals and one, two, or three output connectors.
- Terminals: Allow the signal to pass to other elements in the chain.
- Connectors: Provide the user interface, delivering a portion of the filtered and/or divided signal to one, two, or three output connectors.
Connector configurations include:
- Single connector TV/R
- Solo-R or TV connector
- TV/R-only connector
- TV-FI
- R-FI