A Guide to Coaxial Technology

Coaxial Cable

Coaxial or Coax cable is a type of electrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield all within a plastic outer sheath. This is called a coaxial cable as the inner conductor and the outer shield are concentric.

Coaxial cable is a type of transmission line. A transmission line is a specialised structure designed to conduct alternating current of radio frequency. Current at this frequency has a wave characteristic. Transmission lines are used for connecting radio transmitters and receivers with their antenna, distributing cable television signals, trunk lines routing calls between switching centres, computer network connections and high-speed data buses. In general, coaxial cables and connectors are used in broadcast of television signals and machinery that has a video feed, for example machine vision for production lines or medical imagery.

RF connectors

A coaxial RF connector (Radio Frequency Connector) is an electrical connector designed to work at radio frequencies in the multi megahertz through gigahertz range. RF connectors are typically used with coaxial cables and should be designed to match the electrical characteristics of the cable whilst providing a means of connection. RF connectors are also available in PCB mounting version which provide the connection from the source equipment or receiver equipment.

All coaxial connectors have three main parts to them: Outer Conductor– to provide an electrical path for the cable shield through the connector couple. Centre Pin – to provide and electrical path for the cable centre conductor through the connector couple. Insulator – to provide and electrical isolation between the outer and centre conductor.

A coaxial cable connector will also have: The means to electrically and mechanically connect the coaxial cable. This function of the connector is specific to each cables mechanical and electrical specifications.

RF Connector TypesBNC (Bayonet Neill-Concelman)

The BNC is a quick disconnect radio frequency connector used with coaxial cable. It features two bayonet lugs on the female connector and a quarter turn coupling nut on the male connector. The majority of board mounted connectors are female while cable mounted connectors tend to be male.

Micro BNC

The Micro BNC has the same characteristics of the BNC connector in a miniaturised form, which allows significantly higher densities when designing PCB board layouts. The Micro BNC is also known as a they HD-BNC TM or Ultra Tiny BNC.

DIN 1.0/2.3

The DIN 1.0/2.3 was introduced in the 1990’s initially for telecommunication application. They feature a push/pull lock and release feature and are suited to applications where space it limited.

How Does Broadcasting Work ?

Digital data containing all the information necessary to define a video transmission is sent from a transmitter to a receiver through a transmission media such as air, copper or fibre. The transmitters typically form part of the broadcast infrastructure and the receiver is typically located in the home as a TV or set top box.

Television creates the moving pictures by repeatedly sending still pictures called frames, so quickly that your eyes view them as a moving image, as opposed to separate pictures. This is a more technically advanced and faster moving version of a flick book animation.

In a television, the frame rate is the number of times in a second that the pictures are refreshed. Most standard televisions refresh the screen at 60 frames per second (60Hz), but 4K televisions may have a 120 or 240 frames per second (120Hz/240Hz) refresh rate. This means that the picture that is shown appears smoother due to the speed of the frames changing.

All frames (pictures) are composed of pixels (picture element). The higher the number of pixels in each frame the clearer the final picture will be and images are more defined, hence why this is called the screen definition or resolution. The most common resolutions used now are:

720p - @900 pixels -HD Ready 1080p - @2005 pixels – Full HD 4K - @4000 pixels – 4K UHD TV 8K - @8000 pixels – 8K UHD TV

Serial Digital Interface

Serial Digital Interface (SDI) is a professional digital video interface standard that has been used since the early 90s. The SDI is a family of digital video interfaces that was first standardized in 1989 by SMPTE (The Society of Motion Picture and Television Engineers). Additional versions of the SDI standard have been developed to support continuously increased resolutions (HD, UHD, 4K, 8K), higher bitrates and more colors.

CEI offers connectivity products to fully support the following standards:

The 3G-SDI standard is a 3 Gbit/s interface which was standardised in 2006 and supported the 1080p full HD over a single channel at a frame rate of 60Hz (fps), following the SMPTE standard 424M.

The 6G-SDI standard is a 6 Gbit/s interface, which was announced in 2015, supports the 4K resolution over a single channel at a frame rate of 30Hz (fps) or 60Hz (fps) over a dual channel, following the SMPTE standard ST-2081.

The 12G-SDI standard is a 12 Gbit/s interface which was introduced in 2015 and updated in 2018, supports the 4K resolution over a single channel at 60Hz or 8K resolution over a single channel at 30Hz, following the SMPTE standard ST- 2082.

Impedance

There are mainly two impedances used in RF transmission systems. 50 ohm, for higher power signals, such as transmission base stations and 75 ohm for low power signals. 75 ohm offers a lower signal loss than 50 ohm.

It is important to transfer radio frequency energy from a source to a receiver through the transmission lines with minimum power loss. An impedance mismatch in a transmission line will cause some of the signal power reaching the receiver to be reflected back on itself causing a return loss.

You should never mix 50ohm and 75ohm cables and connectors in the same system. To achieve optimum results it is best practice to use cables and connectors of the same impedance.

What is the difference between a 50 ohm and 75 ohm connector ?The internal materials and structure.

Materials

Connector Body Zinc : Zinc bodies are normally die cast and plated with nickel or gold to give a wear and corrosion resistance and make solderable to a PCB.

Brass : Brass bodies are normally machined and can be plated with nickel, tin or gold to give resistance to wear and corrosion and also aid with soldering to a PCB.

Centre Pin Most centre pins are made from a metal alloy for strength with a gold plating to improve conductivity. The main metal alloys are:

Beryllium Copper

Phosphor Bronze

BrassPEEK

Insulators Insulators are chosen for their mechanical, electrical and thermal properties. PEEK is used in nuclear applications as it has radiation resistance properties.

PTFE

UL94 HB TPX

Mounting Types

PCB mountings Right Angle – A connector with through hole right angled centre pin. The connector body will have legs that go through the board.

Edge Mount – The centre pin of an edge mounted connectors is straight and can be surface mount. Posts go through the board at right angles to hold the connector in place.

Surface Mounted - The centre pin is the surface mounted part of this connector and the legs are right angled through hole for mechanical stability.

Top Entry – A top entry connector has a straight pin that goes through the board and the connectors is perpendicular to the PCB.

All PCB mounted connectors mentioned above are available with a number of leg or post lengths suitable to different board thicknesses.

End Launch – The pin of an end launch connector is straight and the PCB goes between the legs of the connector. The distance between the legs on an end launch connector can differ to suit different board thicknesses.

Cable Mountings Straight – With a straight pin that will be crimped or soldered onto the cable.

Right Angled – The cable is crimped or soldered at the right angle join with the straight pin.

Bulkhead Mounting A connector that is fixed to a bulkhead either by a single jam nut or by four screws and is hardwired on one side and has a mating free connector on the other side.

For more information on the full range of connectivity products offered by Cambridge Electronic Industries, visit our website at:

www.CambridgeElectronics.com

Cambridge Electronic Industries Ltd Pembroke Avenue Denny Industrial Centre Waterbeach Cambridge Cambridgeshire CB25 9QR

Tel No: +44 (0)1223 860041 Tel No: +44 (0)1223 863377 Fax No: +44 (0)1223 863625 E-mail: sales@cambridgeelectronics.com