10
5 MBd Logic Link Design
If resistor R
1
in Figure 2 is
70.4 Ω, 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 Ω, the transmit-
ter can be driven with I
F
=30mA,
if it is desired to economize on
power or achieve lower pulse
distortion.
The following example will illus-
trate 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.
V
CC
- V
F
5 V - 1.5 V
R
1
= ––––––– = –––––––––
I
F
15 mA
R
1
= 233 Ω
The curves in Figures 3, 4, and 5
are constructed assuming no in-
line 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.
Figure 2. Typical circuit configuration.
