Tektronix CSA7404B, User Manual

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Acquiring Waveforms
CSA7000B Series & TDS7000B Series Instruments User Manual
3- 61
In CSA7000B Series instruments, the vertical units displayed for an optical
module are not in voltage, but watts; this is a unit of power. The optical-to-elec-
trical converter inside the instrument outputs a voltage whose amplitude is
linearly dependent on the incoming optical power; in this condition the voltage
applied at the electrical input already represents optical power in its linear form
(as opposed to having to square the voltage and divide by R). For the optical
input then, the bandwidth where the displayed optical power is one half that
approaching DC is:
dB
= 10 log
Ꮛ
0.5
response at DC
Ꮠ
= − 3
dB
The V(f) is the frequency at which the vertical swing is one half (0.5) the V(DC)
not 0.707. The optical bandwidth therefore corresponds to the traditional
electrical bandwidth of --6 dB. During testing of optical systems by impulse
testing, the resulting impulse waveform is converted to frequency by Fourier
transform and the bandwidth is defined as --3 dB = 10 log (vertical swing at
frequency / vertical swing at DC). During reference receiver curve calculation,
however, the definition is changed to match the industry standard definition
which assumes electrical bandwidths are --3 dB = 20
×
log (vertical swing at
frequency / vertical swing at DC).
The curve calculation of frequency response for the unfiltered settings uses the
definition for dB and bandwidth where --3 dB = 10 log (vertical swing at
frequency / vertical swing at DC); that is, the optical bandwidth.
The curve calculation of frequency response for reference receiver settings uses
the definition of dB and bandwidth that matches the industry standard which
assumes electrical bandwidths where --3 dB = 20 log (vertical swing at frequen-
cy / vertical swing at DC).
Bandwidth for Unfiltered
Settings
Bandwidth for Reference
Receiver settings