Directional Couplers
Directional Couplers get their name from the fact that
they have a
much greater loss between tap to output than from tap to
input.
The WISI DM21, Fig.1, is a good example with tap to in,
(side loss),
at 8dB and tap to out, (isolation), being 32dB. The
through, or
insertion loss is 2dB.
Directional Couplers may be used as 1 way taps or signal
combiners.
However not all 1 way taps make good directional
couplers; it
depends on the tap to out loss which should be 20dB or
greater.
When used as a tap, Fig. 2, the high tap to out isolation
ensures that
back channel information is directed to the headend
rather that to
other users outlets. Taps also attentuate unwanted
signals injected
into a network from VCR's and similar devices between
customer
outlet points.
Attenuation occurs equally upstream or downstream of the
injection
point and illustrates why taps are preferred over
splitters as the last
passive device before the user port.
In combiner mode a Directional Coupler has it's inputs
and outputs
reversed to ensure high isolation between the two signals
inputs.
The tap or side loss then becomes the coupling loss and
the
through loss remains the same. Fig.3 gives a practical
application.
Glossary of Terms
Insertion Loss between input 'E' and input 'A'.
For Splitters between input 'E' and the outputs 'A'
Side or tap loss between input 'E' and tap 'Ab'
Isolation between output 'A' and tap 'Ab'
Also known as tap to out loss
Isolation between two taps 'Ab'
For splitters between the output 'A'
Attenuation of a reflected signal in proportion to the forward signal.
The performance parameters are only valid when all outputs are
terminated. Outputs not used must always be terminated.
