11

5 MBd Logic Link Design

The resistor R1 is the only significant element in the drive

circuit (see Figure 2) that limits the current through the

LED, apart from the gate´s output port. Depending

on the actual gate used, the voltage drop on the output

and on the temperature (see Figure 9). Make sure you

take this behavior into account for the calculations.

The curves in Figure 3, Figure 4, and Figure 5 are con-

structed assuming no inline splice or any additional

system loss. Besides fiber attenuation, for correct power

budget calculation, make sure you take into account the

effect of bending, humidity, ambient temperature, aging

and other relevant influences. All these additional losses

reduce the achievable link distance accordingly.

For calculating the LED´s aging effect, an additional loss

of about 1.5 dB is recognized.

The following example will illustrate the technique for

Figure 2. Typical Circuit Configuration

The following diagrams (Figure 3 to Figure 5) serve as an

aid in Link Design and are based on theoretical calcula-

tions. For broad use, no additional effects such as aging

were taken into account. The additional losses and the

individual safety buffer values should be added sepa-

rately. These diagrams reflect the pure viewing of power

budget and do not allows conclusions about the actual

link quality.

Overdrive: Maximum optical output power of Tx com-

bined with receiver sensitivity of -10 dBm over the entire

temperature range.

Typical 25

°C: Typical optical output power of Tx com-

Worst Case: Minimum optical output power of Tx com-

bined with receiver sensitivity of -24 dBm over the entire

temperature range.

Maximum distance required = 2000 meters by using

HFBR-14x4Z/24x2Z logic link with 62.5/125

µm fiber.

Figure 4 shows the“worst-case”drive current of about 43

mA for reaching a distance of about 2000 meters.

Figure 9 shows the transmitter forward voltage of about

choosen to 78.6

Ω (3.38 V/43 mA) for reaching driver

current of about 43 mA.

Page 16 shows the guaranteed HFBR-14x4Z´s optical

output power limit of -16.0 dBm (for driver current of 60

mA) over the entire temperature range.

Figure 10 shows the normalized typical output power.

When the transmitter will be driven with 43 mA the opti-

cal output power is about 0.70 or -1.55 dB lower than at

60 mA.

With an assumed fiber attenuation of 3.2 dB/km and the

reduced driver current of 43 mA, the minimum optical

output power at fiber end is about -24 dBm, which is

equal to the receiver sensitivity over the entire tempera-

ture range.

For balancing the individual additional system losses, the

driver current must be increased accordingly.

-

=

I

V

V

R

F

F

CC

1

+5 V

SELECT R

R

1

I

F

1 K

DATA IN

½ 75451

2

6

7

3

T

HFBR-14xxZ

TRANSMITTER

TRANSMISSION

DISTANCE =

HFBR-24x2Z

RECEIVER

R

TTL DATA OUT

2

6

7 & 3

R

L

V

CC

0.1 µF

Note:

A bypass capacitor (0.01

µF to 0.1 µF ceramic) must 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.