Some fact and some folklore
Designed originally as a tightly specified co-axial connector for low power RF applications, the BNC has become the connector of preference for many communications, video and, until recently, computer network installations. Its use over the years for increasingly varied applications has seen numerous variants, in terms of adapting to various cable types and sizes, of assembly (mainly compression gland or crimp styles) and, maybe most controversially, of characteristic impedance.
Despite attempts over the years to standardize cable sizes and characteristics by the British (in the URM series) and the Americans (in the RG series) these efforts have failed spectacularly. Despite also initial intentions to constrain the use of BNCs to cables with diameters of less than 6.5mm, this has failed with equal success!
These variations led to ever more variants of BNC connectors being introduced to match the growing choice of cables, a trend which continues today.
Two main variants of the BNC connector exist in terms of assembly style. The first of these is the compression gland type. In this style the centre pin of the connector is usually a solder pin whilst the braid and sheath of the cable are held by an expanding compression gland fixed by a nut at the rear of the connector. This type of connector by its nature can cope with a (limited) range of cable sizes and requires no specialised tooling to assemble.
The second is the crimp connector. In this type the centre pin is usually (but not exclusively) crimped to the centre conductor. This crimped pin is then pushed into position through an inner ferrule which separates the inner insulation sheath and the braid of the cable. An outer ferrule is then crimped over the braid and outer insulation which fixes the cable to the connector. Due to the accuracy required, virtually every cable type requires a corresponding crimp connector variant. Further, assembly requires very accurate crimping tools to optimise the integrity of the connection, and must be correct first time.
In both types of connector it is essential that the exact amount of insulation is stripped from each section to ensure accurate assembly. For volume production, the crimp style connection is always preferred.
In its original incarnation, the BNC was designed for RF applications and had a characteristic impedance of 50 ohms, together with a predictable reflection factor. Increasing use in applications such as video equipment, with source and load impedance of 75 ohm and with 75 ohm cables, led inevitably to the creation of the 75 ohm BNC.
At frequencies of up to 10MHz, the characteristic impedance of the connector is completely swamped by the source and load impedance of the circuit of which it is part, thus in analogue video applications it is simply irrelevant whether a 50ohm or a 75ohm connector is used. However the increasing likelihood that the connector is being fitted in a digital video application means that it is absolutely essential that 75ohm connectors and cable are used. This requirement is becoming ever more important as the introduction of HDTV requires significantly higher bit rates so the demands on the cable and connector infrastructure increases. Canford carries a wide range of BNC connectors for HDTV applications where both the frequency and phase response of the product are closely controlled whilst ensuring compatibility with existing infrastructure.
It is implicit in IEC 169-8 that 75 ohm BNCs made to comply with that standard will mate in a non-destructive manner with the 50 ohm BNC connectors described in IEC 169-8. All BNC connectors supplied by Canford as stand-alone connectors are compliant in that respect with the IEC standard. In over 15 years and a many million BNC connectors we have no first hand experience of incompatibility between 50 ohm and 75 ohm types, other than extremely rare (and very obvious) manufacturing faults.
As will be evident from the foregoing, it is unlikely that any supplier will ever stock the complete range of BNC connectors. In the case of Canford we attempt to stock at least one BNC which is appropriate to the types of cable held in stock.
Usually, for coaxial cables they are allocated a ‘group’ code. (WARNING: There is no standard for group coding, so Canford’s group code will not necessarily be the same group code for the same cable type from an alternative supplier, although Canford’s codes are the same as the BBC’s where the cables are still in use). A BNC connector will similarly be allocated a group code which indicates that that connector is dimensionally suitable for the corresponding group cable.
The user must make the choice between the options offered when more than one manufacturer’s version or impedance type is available. Having chosen the group, the impedance and the manufacturer, the user should then consult the chart showing the required crimp jaws sets, if appropriate (obviously not required for solder types). Canford’s range is a selection, made over many years, of types chosen by Canford or the BBC for combinations of reasons, but always with quality and consistency in mind. Thus, although versions with more expensive contact or plating materials often exist as non-listed options, all of the versions listed have had many years of reliable operation in broadcast environments, and, except for client preference, we have no evidence to suggest a need for more expensive types. However, we can usually obtain these other variants to special order if required, subject to manufacturers’ minimum manufacture quantities.
Given the lack of evidence that 75 ohm connectors are less robust in use than 50 ohm versions, it is suggested that users avoid the potential confusion mentioned above by using 75 ohm connectors for 75 ohm systems and 50 ohm versions for 50 ohm systems.