7
The transmitters shown in Figures 5 and 6 use the following
techniques to improve LED performance. When the output
of U1 is a logic “1”, resistor R11 applies a small residual prebias
current to the LED. This small prebias current minimizes
the propagation delay distortion of the LED. Prebias also
improves LED linearity sufficiently to permit the use of a
frequency compensation circuit, which reduces the optical
rise/fall time of the fiber optic transmitter.
This frequency compensation technique is often called
drive current peaking, because it adds brief current spikes
to the LED drive current pulses.When prebiased, the HFBR-
15X7Z LED has an amplitude versus frequency response
which is roughly equivalent to a first order low-pass filter.
Without prebias and peaking, the HFBR-15X7Z LED has a
typical 10% to 90% optical rise time of 12 ns. When prebias
is provided by R11, and frequency compensation (peaking)
is provided by R10, and C8, the 10% to 90% optical rise time
of the HFBR-15X7Z LED decreases to a typical value of 3 ns,
when using 1 mm plastic fiber. Optical rise times of 3.5 ns
are typical when the peaked LED driver is used with 200
µm HCS fiber. The LED’s on-state current is primarily deter-
mined by the values of resistors R8 and R9, but Equation 1
shows that some on-state current is also provided by R11.
Transistor Q3 is connected to form a low cost high speed
diode. This diode allows LED prebias current to be set inde-
pendent of the resistance chosen for R8 and R9. The LED’s
prebias current can be calculated as shown in Equation 2.
Capacitance between the emitter and collector of Q3
changes as a function of the diode connected transistor’s
forward current. Current dependent changes in the capaci-
tance of Q3 ensure that the current peak which turns the
LED off will have a larger amplitude than the current peak
applied when the LED is switched on. LEDs are character-
istically harder to turn off than on. The difference between
the amplitude of the peak current applied at turn on, and
turn off, helps to reduce the optical pulse width distortion
of the fiber optic transmitter. One of the best features of this
recommended LED driver circuit is that all of the active and
passive components needed to build 10,000 of the trans-
mitters shown in Figures 5 or 6 can be purchased for about
$8.00 per circuit.
Recommended Receiver
The recommended receiver is shown in Figure 7. The HFBR-
25X6Z component used in this receiver linearly converts
changes in received optical power to a corresponding
change in voltage. The output of the HFBR-25X6Z is an
analog signal which can easily be converted to logic by a
post amplifier and comparator. This post amplifier com-
parator function is often called a quantizer. A very inexpen-
sive quantizer can be implemented using an MC10H116
ECL line receiver. The MC10H116 provides three low cost
differential amplifiers in a single package. The MC10H116
can accommodate a large range of input voltages. The
large dynamic range of the MC10H116 is very important! The
quantizer must have a large dynamic range because the
output of the HFBR-25X6Z can change from a few millivolts
to hundreds of millivolts when fiber length and attenuation
are varied.
Several subtle techniques are used to maximize the re-
ceiver’s sensitivity to optical pulses, while minimizing the
receiver susceptibility to electromagnetic interference (EMI).
In most systems, the same +5 V DC supply which powers
the fiber optic receiver is also used to power microproces-
sors and digital logic. The receiver must be isolated from noisy
dc power supplies! This isolation is provided by low-pass
filters that prevent noise injection into the HFBR-25X6Z, and
quantizer, through the +5 V power connections. The HFBR-
25X6Z is a miniature hybrid circuit that, due to its small
physical size, is relatively immune to environmental noise.
In most applications, the HFBR-25X6Z has sufficient noise
immunity to operate without any additional electrostatic
shielding, but the connection between the HFBR-25X6Z
and the non-inverting input of the MC10H116 forms a loop
antenna with sufficient area to receive significant amounts
of EMI. The receiver’s susceptibility to EMI is minimized by
connecting a second loop antenna with equal area to the
inverting input of the MC10H116 quantizer. When connec-
tions to the quantizer’s input are symmetric, and have equal
loop areas, the common mode rejection of the MC10H116’s
difference amplifiers will assure that the fiber optic receiver
provides good EMI immunity.
Design techniques which improve the EMI immunity of the
receiver help to minimize crosstalk between the transmitter
and the receiver. Crosstalk will also be reduced when the
printed circuit for the fiber optic transceiver is designed so
that pin 4 of the HFBR-15X7Z LED transmitter is next to pin
1 of the HFBR-25X6Z receiver. This arrangement maximizes
the distance between pin 2 of the HFBR-15X7Z LED and the
power supply lead (pin 4) of the HFBR-25X6Z. When the
distance between pin 4 of the HFBR-25X6Z and pin 2 of
the LED is maximized, the crosstalk between the LED trans-
mitter and the HFBR-25X6Z receiver’s power pin is reduced.
The typical transmitter to receiver crosstalk which occurs
when using the printed circuit shown in this application
note is equivalent to a 0.5 dB reduction in receiver sensi-
tivity. The effect of transceiver crosstalk has already been
factored into the recommended distances and data rates
shown in Figures 1 and 2.
I
F
ON
=
(V
cc
- V
F
ON
)
R11
+
[V
cc
- (V
F
ON
+ V
CE
Q3
+ V
OL
U1
)]
[(R8)(R9)/(R8 + R9)]
I
F
OFF
=
(V
cc
- V
F
OFF
)
R11
Equations
Equation 1:
Equation 2:
