Data Sheet
January 2000
1340-Type Lightwave Receiver
output signal. The outputs of the LT1016 are TTL-com-
patible and provide both true and inverted logic levels.
The Q output of this circuit will be a TTL high (>2.5 V)
when the 1340 is receiving an optical signal greater
than the FLAG switching threshold and will be a TTL
low (<0.4 V) whenever the optical signal is absent or is
below the FLAG switching threshold. The FLAG and
FLAG outputs of the OC-12/STM-4 and 1.25 Gbits/s
receivers are 5 V TTL logic level compatible. The FLAG
output is provided directly by the comparator IC. How-
ever, the FLAG output is derived from the FLAG output
through an inverter. Excessive loading of the FLAG out-
put can cause the FLAG output to malfunction.
Data and Flag Outputs (continued)
The FLAG and FLAG outputs of the OC-3/STM-1
155 Mbits/s version of the 1340 receiver are PECL
logic levels driven by open emitter transistors with the
same characteristics as the data outputs. These out-
puts must be properly terminated in order to obtain the
correct logic levels. Since the FLAG function is basi-
cally a dc switch that indicates the loss of optical input
signal, it can be interfaced to much slower TTL or
CMOS logic circuits.
The circuit shown in Figure 2 provides one example of
how to create a TTL logic output from the PECL FLAG
+5 V
+5 V
11
1340
R X
12
+
Q
Q
TTL (TRUE)
FLAG
LT1016*
14
–
FLAG
TTL (INVERTED)
10 k•
10 k•
1-800(C).a
* Part available from Linear Technology Corporation of Milpitas, CA 95035.
Figure 2. Converting PECL FLAG Outputs to TTL
4
Agere Systems Inc.
AGERE [ AGERE SYSTEMS ]
