Keysight Technologies N4373D, User Manual

Page: 59 / 176

Measurement Concepts
Keysight
N4373D Lightwave Component Analyzer, User’s Guide 57
The responsivities R
s
and R
r
need to be related to some value in
order to have meaning as individual quantities expressed
logarithmically, just as 0 dB represents an S
21
of 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.
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 (
D P
0
) and
modulation current ( D I
1,2
).

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.
20log
10
R
s
R
r

20log
10
R
s
W A
  
W A

----------------------------
R
r
A W
  
A W

----------------------------

=
R
s
dB
 
20log
10
R
s
W A
  
W A

----------------------------
=
R
r
dB
 
20log
10
R
r
A W
  
A W

----------------------------
=
R
s
dB
 
20log
10
P

I
1

---------
=
R
r
dB
 
20log
10
I
2

P

---------
=