BNC connector
- details and information about the BNC connector, - BNC socket, plug and
adapter with links to suppliers.
The BNC coax connector is a form of rf connector that is one
of the most widely used coaxial connectors. The BNC connector is intended as an
RF connector that can be used in a wide number of applications from any form of
RF equipment including radio communications equipment to test equipment
including everything from oscilloscopes to audio generators, and power meters to
function generators. In fact BNC connectors are used in applications where
coaxial or screened cable is required.
The BNC connector has many attributes. One is that it has a
bayonet fixing. This is particularly useful because it prevents accidental
disconnection if the cable is pulled slightly or repeatedly moved. Another
advantage is that it is what is termed a constant impedance connector. This is
particularly important for RF applications and means that the connector presents
the same impedance throughout its length.
BNC development
The BNC connector was developed in the late 1940s and it
gains its name from a combination of the fact that it has a bayonet fixing and
from the names of the designers, the letters BNC standing for Bayonet
Neill Concelman. In fact the BNC connector is essentially a miniature
version of the C connector which was a bayonet version of the N-type connector.
It was developed as a result of the need to provide a high
quality, robust connector that would be capable of being used in a wide variety
of applications. Additionally it needed to be smaller than either the N-type or
C-type connectors which were much larger
BNC connector specifications
The specifications of the BNC connector naturally vary from
one manufacturer to another and it is always best to ensure that the particular
component being purchased is suitable for the intended application. However
there are a number of guidelines that can be used. The connector comes in two
basic types:
Of the two versions of the BNC connector, the 50 ohm version
is more widely used. Often the BNC connector is specified for operation at
frequencies up to 4 GHz and it can be used up to 10 GHz provided the special top
quality versions specified to that frequency are used. However it is wise to
fully check the specification.
BNC connector formats
BNC connectors come in a variety of formats. Not only are
there plugs and sockets but there are also adapters and also other items such as
attenuators.
BNC plugs are designed not only for the required impedance,
but also to accept a particular coax cable format. In this way all the internal
piece parts are compatible with the coaxial cable used. It is therefore
necessary to specify the BNC plug for use the cable to be used. Although there
is some latitude, it is naturally best to select the correct cable format.
In addition to this there are straight and right angled
variants. Of these the straight connectors are the most widely used, although
right angled connectors where the cable leaves the plug at right angles to the
centre of the connector centre line are also available. These are ideal in many
applications where the cables need to leave the connector in this manner to
ensure cables are in a tidy fashion, or where space is at a premium.
Unfortunately right-angled connectors have a marginally higher level of loss
than their straight through counterparts. This may not be significant for most
applications, but at frequencies near the operational limit of the connector
there may be a small difference.
The sockets or female BNC connectors also come in a number of
flavours. The very basic BNC connector consists of a panel mounting assembly
with a single connection for the coax centre. The earthing is then accomplished
via the panel to which the connector is bolted using a single nut. Large washers
can be used to provide an earth connection directly to the connector. Some of
these connectors may also use four nuts and bolts to fix them to the panel.
These arrangements are only suitable for low frequency applications, and not for
RF. Where impedance matching and full screening is required. Bulkhead mounting
connectors where coaxial cable entry is provided are available for this. Again
these are available for a variety of cable dimensions and the correct type
should be used.
There are two main variants of the BNC connector assembly
method:
- Compression gland type
- Crimp type
The compression gland type has the centre pin of the
connector which is usually a solder pin and 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. This makes it
ideal for small quantity production, either for one off cables for laboratory
use of for limited production runs.
The crimp connector has the centre pin which is normally
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. Greater accuracy is required
for the crimp style connectors and therefore the correct connector variant must
be chosen for the cable being used. This may result in a crimp style connector
not being practicable for some cable types. In addition to this the assembly
requires the use of the correct crimping tools to ensure that the connector is
correctly crimped. While these connectors are always preferred for large
production runs because they are much faster to assemble, it is not possible for
them to be reworked for obvious reasons.
For both styles of BNC connector it is essential that the
exact amount of insulation is stripped from each section to ensure accurate
assembly and the required RF performance.
Finally a variety of BNC adapters and other ancillary items
are available. One popular BNC adapter is the straight through adapter, allowing
two cables with male connectors fitted to be connected end to end. Other "T"
adapters are also available. These have a male plug at the bottom of the "T" and
two female connections at either end of the horizontal of the "T". These are
ideal for use with oscilloscopes where a through connection needs to be
measured, and the "T" BNC adapter enables the required connections to be made.
In addition to this a variety of inter-series adapters are
available to enable transitions to be made between different connector types.
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