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Measurement Concepts
36
It is convenient to express the transducer functions
logarithmically in decibels. The system power gain from a Z
source to a Z
load can be defined using the above
0
relationships:
2
a
=
1
b
2
20log
The responsivities R
value in order to have meaning as individual quantities
expressed logarithmically, just as 0 dB represents an S
unity or gain of 1.
Consequently source responsivity will be expressed in Watts
per Amp, which in decibels will be related to a conversion
efficiency of 1 W/A. Similarly, receiver conversion efficiency
will be relative to 1 A/W.
20log
R
10
The individual responsivities can now be expressed
individually in decibels:
This now allows us to express the original equations for
responsivity in logarithmic terms:
Responsivity measurements are now based on the LCA's
ability to accurately measure optical modulation power (∆P
and modulation current (∆I
The measurement of modulation current is derived from the
system characteristic impedance and a measurement of
electrical power.
The measurement of optical modulation power is based on a
"standard" lightwave receiver whose characteristics are
predetermined and known by the LCA.
Agilent N4373A Lightwave Component Analyzer, First Edition
Power incident on the E/O converter
2
=
Power delivered to a Z0 load
2
b
2
2
------------
S
=
21
2
a
1
•
2
=
R
R
s
r
=
System power gain
S
=
System gain in dB
10
21
•
=
20log
R
R
10
s
r
and R
need to be related to some
s
r
(
⁄
R
W A
s
•
----------------------------
R
=
20log
⁄
s
r
10
W A
(
⁄
R
W A
s
(
)
----------------------------
R
dB
=
20log
⁄
s
10
W A
(
⁄
R
A W
r
(
)
--------------------------- -
R
dB
=
20log
⁄
r
10
A W
∆
P
(
)
-------- -
R
dB
=
20log
∆
s
10
I
1
∆
I
2
(
)
-------- -
R
dB
=
20log
∆
r
10
P
).
1,2
of
21
)
(
⁄
)
R
A W
r
•
--------------------------- -
⁄
A W
)
)
0
)
0
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