Model XC0450A-03
Rev B
PIn POut(RL)
POut(ISO)
Input Port
Pin 1
Isolated Port
Coupled Port Pin 4
Direct Port
POut(CPL)
POut(DC)
Figure 1
The coupler is excited at the input port with Pin (watts) of power. Assuming the coupler is not ideal, and that there are
no radiation losses, power will exit the coupler at all four ports. Symbolically written, Pout(RL) is the power that is
returned to the source because of impedance mismatch, Pout(ISO) is the power at the isolated port, Pout(CPL) is the
power at the coupled port, and Pout(DC) is the power at the direct port.
At Anaren, insertion loss is defined as the log of the input power divided by the sum of the power at the coupled and
direct ports:
Note: in this document, insertion loss is taken to be a positive number. In many places, insertion loss is written as a
negative number. Obviously, a mere sign change equates the two quantities.
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
P
in
IL =10⋅log10
(dB)
(2)
(3)
Pout(CPL) + Pout(DC)
In terms of S-parameters, IL can be computed as follows:
2
2
⎛
⎝
⎞
⎟
⎠
IL = −10 ⋅log10 S31 + S41
(dB)
⎜
We notice that this insertion loss value includes the power lost because of return loss as well as power lost to the
isolated port.
For thermal calculations, we are only interested in the power lost “inside” the coupler. Since Pout(RL) is lost in the
source termination and Pout(ISO) is lost in an external termination, they are not be included in the insertion loss for
thermal calculations. Therefore, we define a new insertion loss value solely to be used for thermal calculations:
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
P
in
ILtherm =10 ⋅log10
(dB)
(4)
P
+ P
+ P
+ P
out(CPL)
out(DC)
out(ISO)
out(RL)
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