Tektronix DSA8200 Series, Service Manual

Page: 40 / 196

Theory of Operation
2- 6
DSA8200 Digital Serial Analyzer and Modules Service Manual
1
No filter is used, but the sampler’s bandwidth is adjusted.
1
The O/E bandwidth is adjusted and dominates the response.
Information about the available bandwidth and reference receiver selections, and
the method used to set the bandwidth for the optical modules starts on
page 2--7.
The power monitor is a second measure of the photodiode current that is
independent of the sampler signal path. Analog circuitry continuously senses the
current flowing into the bias side of the photodiode. The signal is amplified by a
programmable gain amplifier and input to an 8 bit AD converter. The AD
converter and amplifier are controlled through the I
2
C interface. Compensation
performs two functions in the power meter: First, two offset inputs are adjusted
in the amplifier so that the signal stays in range for all of the gain settings. Next,
offset is measured for all gain settings and stored so it can be subtracted from the
raw measured current. Because the measurements are made through independent
paths the power monitor is useful in debugging module/mainframe problems.
The main board of the module only provides power and control bits to the clock
recovery board. A small fraction of the input signal is split off and applied to the
clock recovery components. The type of splitter for each module is shown in
Table 2--3.
Table 2- 3: Module optical/electrical split
Module Optical Split Electrical Split
C01
T
C02
T
C03
T
C04
T
C07
T
C07B
T
C08
T
C08B
T
C08C
T
C09
T
C10B (with CRTP option)
T
C11
T
C12
T
C25GBE (with CRTP option)
T