Coaxial Cable Termination: Understanding the Types and Applications of Connectors
Coaxial cable termination is a crucial process in many electronic and telecommunications applications. This article explores the use of connectors such as BNC and the various types of coaxial termination to ensure optimal signal transmission and minimize reflections.
Introduction to Coaxial Cable Termination
Coaxial cable termination involves the proper connection of a resistor across the end of the cable to match the inherent characteristic impedance of the cable. This characteristic impedance is typically 75 ohms for cable TV cables and 50 ohms for other applications, allowing for efficient signal transmission with minimal reflections.
Types of Coaxial Connectors
The most common type of connector used for coaxial cable termination is the BNC (Bayonet Neil-Concelman) connector. BNC connectors are widely used in RF equipment, test instruments, radio, television, and video signal applications. They are designed for quick connection and disconnection, making them ideal for these applications.
BNC Connectors in Detail
BNC connectors are miniature-to-subminiature RF connectors originally developed for military use. They feature a unique bayonet-style locking mechanism, which ensures a secure and reliable connection. Due to their compact size and reliability, BNC connectors are frequently used in applications requiring quick setup and simple disassembly.
Coaxial Termination Mechanism
During coaxial termination, a resistor is connected across the end of the cable. This resistor, along with a capacitor in parallel to the resistor, creates a termination load that absorbs the pulses sent down the cable and prevents reflections. In cable TV applications in the USA, the main coaxial cable ends in a housing, which typically contains a passive tap or splitter. Outputs from this housing need to be terminated to prevent reflections on the line.
Termination in Cable TV Applications
In cable TV systems, the main coaxial "hardline" cable typically ends in a housing equipped with passive taps or splitters. The outputs from this housing require termination to prevent reflections. This is achieved using a chassis "pin-type" terminator connector, which screws into a 5/8-inch chassis port. The connector has a center pin secured with a seizure screw inside the device being terminated.
Inside the terminator connector, a resistor is placed in series with a capacitor across the pin and the shield of the connector body. The capacitor serves to block the AC 60 Hz line power needed for repeater amps from shorting across the resistor, ensuring the integrity of the signal.
Never Terminate the Main Coaxial Cable
It is important to note that the main line coaxial cable typically ends at the point where the tap is installed. Therefore, there is no need to terminate the cable itself, as the termination occurs at the last device in the chain.
Efficient Coaxial Termination
Efficient coaxial termination is achieved by connecting the appropriate resistor to the cable to match its characteristic impedance. This ensures that pulses sent down the cable are absorbed and not reflected. This minimizes signal distortions and ensures a clean transmission, particularly in applications like old "thinwire" Ethernet standards, where 50 ohm coaxial cables are used.
Conclusion
Coaxial cable termination is a critical process in many electronic and telecommunications applications. Understanding the types of connectors and the proper termination methods is essential to ensure optimal signal transmission and minimize reflections. BNC connectors and the termination process described here can significantly enhance the performance of RF equipment, test instruments, radio, television, and video signals.
By following proper coaxial termination techniques, users can achieve reliable and efficient signal transmission in a wide range of applications, from cable TV systems to Ethernet networks.