5 MBd Logic Link Design
If resistor R1 in Figure 2 is 70.4
W,
a forward current I
F
of 48 mA
is applied to the HFBR-14x4
LED transmitter. With I
F
= 48
mA the HFBR-14x4/24x2 logic
link is guaranteed to work with
62.5/125 µm fiber optic cable
over the entire range of 0 to
1750 meters at a data rate of dc
to 5 MBd, with arbitrary data
format and pulse width
distortion typically less than
25%. By setting R
1
= 115
W,
the
transmitter can be driven with
I
F
= 30 mA, if it is desired to
economize on power or achieve
lower pulse distortion.
The following example will
illustrate the technique for
selecting the appropriate value
of I
F
and R
1
.
Maximum distance required =
400 meters. From Figure 3 the
drive current should be 15 mA.
From the transmitter data V
F
=
1.5 V (max.) at I
F
= 15 mA as
shown in Figure 9.
R
1
=
V
CC
−
V
F
=
5V
−
1.5V
I
F
15 mA
R
1
=
233
Ω
The curves in Figures 3, 4, and 5
are constructed assuming no
inline splice or any additional
system loss. Should the link
consists of any in-line splices,
these curves can still be used to
calculate link limits provided
they are shifted by the
additional system loss expressed
in dB. For example, Figure 3
indicates that with 48 mA of
transmitter drive current, a 1.75
km link distance is achievable
with 62.5/125 µm fiber which
has a maximum attenuation of 4
dB/km. With 2 dB of additional
system loss, a 1.25 km link
distance is still achievable.
TTL DATA OUT
+5 V
SELECT R
1
TO SET I
F
I
F
R
1
HFBR-14xx
TRANSMITTER
2
6
7
3
HFBR-24x2
RECEIVER
2
V
CC
0.1 µF
1 KΩ
T
R
6
7&3
R
L
DATA IN
�½ 75451
TRANSMISSION
DISTANCE =
NOTE:
IT IS ESSENTIAL THAT A BYPASS CAPACITOR (0.01 µF TO 0.1 µF
CERAMIC) BE CONNECTED FROM PIN 2 TO PIN 7 OF THE RECEIVER.
TOTAL LEAD LENGTH BETWEEN BOTH ENDS OF THE CAPACITOR
AND THE PINS SHOULD NOT EXCEED 20 MM.
Figure 2. Typical Circuit Configuration.
11
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