ABCD Parameters
Introduction
Since a transmission line is symmetrical, the measurement of the open circuit and short circuit impedances is enough to
determine the ABCD constants.
ABCD Parameters
In figure as shown below V
s
and V
r
represents the sending end voltage and receiving end voltage respectively Also the
currents I
S
and I
R
are entering and leaving the network respectively. The two-port network has a pair of input terminals and a
pair of output terminals. When a voltage V
s
is supplied to input terminals, the input & output currents, I
s
and I
r
, flow & an
output voltage V
r
is produced. There are therefore 4 variable quantities V
s
,I
s
,V
r
and I
r
for a two-port network. If the elements
are supposed to be linear, then there are a number of ways in which we can write the relationships. One such way is
termed ABCD parameters the sending end voltage and current are then defined in terms of the ABCD parameters as
From (2.1) we see that
This implies that Ais the ratio of sending end voltage to the open circuit receiving end voltage. This quantity is dimension less.
Similarly,
i.e., B, given in Ohm, is the ratio of sending end voltage and short circuit receiving end current. In a similar way we can also
define
The parameter Dis dimension less.
Two port representation of a transmission
network.
Eq. (2.2)
Eq. (2.1)
Eq. (2.3)
Eq. (2.4)
Eq. (2.5)
Eq. (2.6)
