New Focus 1621, Руководство пользователя

Страница: 12 / 20

12 • General Features and Principles NEW FOCUS, Inc.
capacitance of the photodiode and the load resistance
value. Figures 4 and 5 give typical frequency responses
and pulse responses for the Model 162X
photodetector.
Figure 4:
Typical
frequency
response for
the Model 1623
on the 50-
Ω
setting
When performing high-speed measurements, imped-
ance-matching effects between your measurement
apparatus and the photo-detector must be taken into
consideration in order to achieve optimal high-speed
response.
Typical laboratory measurement equipment may pre-
vent you from achieving nanosecond response times.
BNC connectors do not match 50-
Ω
transmission lines
well for frequencies above 100 MHz. The frequency
response is therefore not flat between 100 MHz and
500 MHz.
Above 500 MHz, the situation can be worse, with typi-
cal BNC connectors and cables proving to be quite
lossy. Therefore, while the time response of the nano-
second detector is quite fast, it is not expected to pro-
vide flat frequency response in the 100-MHz to 500-
MHz regime.
Selecting the internal 50-
Ω
resistor will usually allow
you to achieve a response speed which is limited by
your detection equipment. Typical laboratory
0 100 200 300 400 500 600
Frequency, MHz
Gain (10 dB/div)
162x nanosec revB.fm Page 12 Friday, August 17, 2001 5:51 PM