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adjustable over the range 10 to 20 dBm (100 µW to 21 mW). In the
XFA version, it is automatically set to a fixed value of 10 dBm. The XF
version allows the user to access wavelengths over the ranges 1250 to
1350 nm and 1500 to 1600 nm in 10-nm steps. The XFA version is fixed
at 1300 nm to 1550 nm. To control all functions, the XF version has six
keys; the XFA has only two. Although both instruments perform
identical tasks equally well, the XF may be seen as the more flexible
version, and the XFA as the simpler-to-use version.

3.17.1 Test Sets/Talk Sets
The handheld Photodyne 21XTL fiber-optic test set/talk set (Fig. 3.56)
is for use in installation and maintenance of fiber cables. This instru-
ment functions as a power meter and test set, as well as a talk set. For
maintenance purposes, the user may establish voice communication
over the same fiber pair being measured.
The 21XTL, as a power meter, covers an exceptionally wide
dynamic range (–80 to +3 dBm). As an option, the receiver may
include a silicon or an enhanced InGaAs photodiode. With the InGaAs
version, the user can perform measurements and voice communica-
tion at short and long wavelengths. The silicon version achieves a
superior dynamic range at short wavelengths only.
The highly stabilized LED ensures repeatable and accurate mea-
surements. Precision optics couples the surface-emitter LED to the
fiber core. With this technique, the fiber end will not wear out or
scratch. The transmitter is interchangeable, providing complete flex-
ibility of wavelengths and connector types.

FIGURE 3.56 Photodyne 21XTL fi ber-optic test set/talk set. (Courtesy 3M
Corporation)