Tektronix CSA8000 Series, Service Manual

Page: 192 / 354

Optical Sampling Modules
4–88
CSA8000 & TDS8000 Instruments and Sampling Modules
Minimum Optical Bandwidth & Reference Receiver Frequency Response
Before performing the checks for minimum optical bandwidth and reference
receiver frequency response, you need to have an understanding of what optical
bandwidth is and how it is measured.
Traditionally, the bandwidth of a device or system is defined as the frequency at
which the power out of the same device or system is one half as compared with a
frequency near DC. In the voltage domain, the power dissipated into a resistive
load (for example, a 50
W
termination of a sampler) is the V
RMS
2
/R where V
RMS
is the RMS of the voltage swing seen at the resistive load, and R is the resistance
value. A logarithmic scale using decibels is typically used to describe a
frequency dependent response of a system.
A value expressed in terms of a decibel relative to a reference is defined as:
dB
+
10 log
ǒ
value
reference
Ǔ
For electrical bandwidths, the power ratio is used so:
dB
+
10 log
ǒ
Power
f
Power
DC
Ǔ
when
Power
f
Power
DC
+
1
2
10 log
ǒ
1
2
Ǔ
+
–3 dB
In terms of voltage, and resistance, the bandwidth is expressed as:
*
3 dB
+
10 log
ȧȡȢ
V
f
2
R
V
DC
2
R
ȧȣȤ
where V
f
is the RMS of the voltage swing response at the bandwidth frequency
and V
DC
is the RMS voltage swing response at a frequency approaching DC.
Further math yields that V
f
= 0.707 V
DC
. The expression is simplified by the
cancellation of the R and the movement of the squared term inside the log
expression to a multiple outside the log expression:
log
ȧȡȢ
V
f
2
R
V
DC
2
R
ȧȣȤ
+
2 10 log
ǒ
V
f
V
DC
Ǔ
+
20 log
ǒ
V
f
V
DC
Ǔ