Guide to TV Signal Distribution & Amplification Systems
Devices and Amplifiers Selection for Headend Teams
Choosing the Right Device
Selecting the right device involves considering the maximum output voltage required to deliver a strong signal without distortion. This is linked to intermodulation and cross-modulation, which limit the maximum undistorted signal from the amplifier output.
Intermodulation is typical of single-channel systems, while cross-modulation arises from broadband mixtures or interactions between signals from different channels.
In broadband systems, to avoid cross-modulation issues, it’s crucial that the amplified output signal levels across channels remain very similar.
The Z Technique
When using the Z technique, consider the following:
- In VHF, leave two unused channels between two used channels.
- Place amplifiers from the highest to the lowest channel, drawing power from the grid via the highest channel.
- When using a two-output amplifier, utilize the power supply output. If only one output is used, connect a 75W load to the other.
In a headend equipped with single-channel Z amplifiers, the overall maximum output level is lower than the maximum output level of each individual amplifier.
When amplifying adjacent channels using connected single-channel Z amplifiers, arrange the amplifiers in two or more groups with an equal number of amplifiers in each group. Place and connect the amplifiers within each group, varying the channel and drawing power from the highest channel or the power supply of each group. However, this setup has the drawback of each group having its own TV signal output.
Converters
Converters transform the TV signal to a different channel than the original transmission channel. Reasons for using a converter include:
- Higher attenuation in a specific band across all system components.
- Excessively high TV signal levels, leading to double images or ghosting.
- Presence of channels from neighboring countries or regions broadcasting on very high frequencies, preventing reception.
Converter Characteristics
- Maximum output power
- Adjacent channel rejection in dB (higher is better)
- Noise factor in dB (lower is better)
- Channel handling capabilities
Filters
Filters eliminate specific frequencies when adjacent channel interference or other disturbances occur. They attenuate unwanted signals by up to 30dB. Types of filters include:
- Low-pass filters: Remove signals above a certain frequency.
- Suppressors: Tuned to remove specific TV signal frequencies.
- Band-pass filters: Allow reception within a specific frequency range.
Install filters before the amplifier to mitigate or eliminate unwanted signal frequencies. Sometimes, multiple amplifiers are needed to ensure strong signal attenuation.
Filter Characteristics
- Channels passed or attenuated
- Insertion loss
- Adjacent channel rejection
- Number of inputs
Mixers
Mixers combine multiple input signals into the distribution network. They combine and distribute TV signals captured by one or more antennas, which may belong to different channels within the same or different bands. They can be mounted on the antenna mast, amplifier output, or integrated into antennas.
Mixer Characteristics
- Attenuation per channel (input-output voltage loss in dB)
- Bandwidth
- Decoupling between inputs (should be less than 20dB)
- Number of input channels
Equalizers
Equalizers balance the output levels of TV signals. They are installed directly at the input of amplification equipment and receive the signal directly from the antenna(s).
Attenuators
Attenuators reduce the level of received TV signals. They are connected before the amplifier and can be mounted directly on the amplifier input. They are characterized by having the same impedance at both input and output.
Terrestrial TV Reception Network Distribution Systems
This crucial part of a TV installation ensures good signal levels at the user end. TV signal distribution is achieved using a coaxial conductor from the headend output to the user outlets.
Distributors (Splitters)
Distributors are used when multiple output signals are needed from a single source. Unused outputs should be terminated with a 75W resistance.
Distributor Characteristics
- Insertion loss (attenuation)
- Rejection between outputs (higher dB values indicate better quality)
- Input and output impedance matching
Distributor Types
- Resistive splitters: Use resistors for balanced signal distribution, suitable for installations with few outlets.
- Inductive splitters: Utilize coils on a copper ring for lower insertion loss and spurious signal elimination.
Taps
Taps are used in distribution networks to provide signal access to multiple users or homes. They are essential for distributing the signal within the network.
Tap Characteristics
- Through loss (attenuation in dB)
- Tap loss (attenuation in dB when extracting signal for a user)
- Isolation (ability to prevent signal leakage between outputs, higher dB values are better)
Tap Types
- Resistive taps: Use resistors to achieve desired attenuation levels.
- Inductive taps: Employ toroidal transformers for lower attenuation, especially at higher frequencies.
User Outlets
User outlets allow connection to the network for accessing services. They mark the end of the dispersion network, which links the distribution network to the user’s internal network.
User Outlet Types
- Pass-through outlets: Allow the signal to continue to other outlets.
- Terminating outlets: The final outlet in the network.
User Outlet Characteristics
- Connection attenuation (signal loss at the outlet)
- Through loss (attenuation for pass-through outlets)
- Isolation between outlets (especially for TV and radio signals)
User Outlet Types
- Resistive outlets: Use resistors for attenuation, suitable for simple installations.
- Inductive outlets: Employ coils for better performance, often with separate TV and radio connectors.
- Hybrid outlets: Combine resistive and inductive elements for improved isolation, suitable for larger installations with multiple channels.