HP 83447A, Руководство пользователя и сервисное руководство

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Description of the HP 83447A
The HP 83447A lightwave trigger receiver is used to generate an electrical
trigger signal from an optical waveform. A typical usage is for eye-diagram
analysis of telecommunications optical signals where an external electrical
clock signal is not available as a timing reference .
F or more information on using the lightwave trigger receiver refer to
Chapter 3.
Learning the inside
story . . .
The block diagram for the HP 83447A is shown in Figure 1-1. The optical
input uses 9 m single-mode ber . The optical signal to be measured is
routed to the HP 83447A. Inside the HP 83447A, the signal is passed through
an asymmetric coupler , where approximately 10% of the power is tapped.
The remaining 90% of the optical signal is sent directly out of the HP 83447A
and can be connected to the measuring instrument. The 10% signal is
detected by a high-gain avalanche photodiode (APD). The electrical signal
from the APD is amplied by an A C-coupled, two-stage limiting amplier .
The output can be used as a timing reference , such as a trigger signal for an
instrument like the HP 83480A digital communications analyzer .
This technique is similar to \splitting" an electrical signal from an
optical-electrical (O/E) converter to trigger an oscilloscope . However , a key
advantage to using the HP 83447A technique is that the optical signal to be
measured experiences only a minor level of attenuation, typically 1 dB
o
.
Splitting an electrical signal usually results in a 6 dB
e
loss . Because the
O/E element of the HP 83447A is an APD and two stages of amplication,
a preamp and a limiting amplier , the output electrical signal exceeds
200 mV
p-p
with input optical modulation powers lower than 20 W
p-p
,
(such as a 020 dBm average power with a 10 dB or higher extinction ratio).
Placing an electrical splitter after an O/E converter can degrade the frequency
response of the measurement channel, resulting in inaccurate measurements .
The HP 83447A triggering technique preserves the integrity of the electrical
signal path.
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