ML4622, ML4624
critical, the pole it creates can effect the stability of the
feedback loop. To avoid stability problems, the value of
this capacitor should be at least 10 times larger than the
input coupling capacitors.
COMPARATOR
Two types of comparators are employed in the output
section of these Quantizers. The high speed ECL
comparator is used to provide the ECL level outputs and in
turn drives the TTL comparator. The enable pin,
CMP
ENABLE,
is provided to control the ECL comparator. When
CMP ENABLE
is low the comparators function normally.
When it’s high, it forces ECL+ high, ECL– low, and TTL
OUT high. The
CMP ENABLE
pin can be controlled with
TTL level signals when the Quantizer is powered by 5V
and ground.
LINK DETECT CIRCUIT
The Link Detect circuit monitors the input signal and
provides a status signal indicating when the input falls
below a preset voltage level. When the input falls below
the preset voltage level, the
TTL Link Mon
output changes
from active (low) to inactive (high). This signal can be fed
to the ML4662 10BASE-FL transceiver or a similar type of
function to indicate a Low Light Condition. This output
can also be used to disable the output data by tying it to
the
CMP Enable
input.
In many fiber optic systems, including Ethernet and Token
Ring, a bit error rate is given at a minimum power level.
For example, in a 10Base-FL receiver there must be less
than 1 x 10
–9
bit errors at a receive power level of
–32.5dBm average. Designers of these systems must
insure that the bit error rate is lower than the specification
at the given minimum power level. One procedure to
determine the sensitivity of a receiver is to start at the
lowest optical power level and gradually increase the
optical power until the BER is met. In this case the Link
Detect circuit must not disable the receiver (i.e.
CMP
ENABLE
should be tied to Ground). Once the sensitivity of
the receiver is determined, the Link Detector circuit can
be set just above the power level that meets the BER
specification. This way the receiver will shut off before the
BER is exceeded.
The ML4622 and ML4624 quantizers have greater Link
Detect sensitivity, noise immunity, and accuracy than their
predecessor the ML4621.
The threshold generator shifts the reference voltage at
V
TH
ADJ through a circuit which has a temperature
coefficient matching that of the limiting amplifier. The
relationship between the V
TH
ADJ and the V
TH
(the peak to
peak input threshold) is:
V
TH
ADJ = 417 V
TH
(ML4624)
V
TH
ADJ = 500 V
TH
(ML4622)
In most cases, including 10Base-FL, 10Base-FB and
Token-Ring, V
TH
ADJ can be tied directly to V
REF
. However
if greater sensitivity is required the circuit in figure 3 can
be used to adjust the V
TH
ADJ voltage. Even if V
REF
is tied
to V
TH
ADJ, it is a good idea to layout a board with these
two resistors available. This will allow potential future
adjustments without board revisions.
The response time of the Link Detect circuit is set
by the C
TIMER
pin. Starting from the link off state (i.e.,
TTL LINK MON
is high), the link can be switched on
if the input exceeds the set threshold for a time given by:
T=
C
TIMER
×
0.7V
700µA
(4)
(3)
To switch the link from on to off, the above time will be
doubled.
V
REF
R
1
V
TH
ADJ
R
2
(–V
RF
)
THRESH
GEN
REF
Figure 3.
BURST MODE
In some fiber optic links, the idle signal is DC, or of a
frequency that is substantially different from the data. For
these links, a faster response time of the DC loop and the
Link Monitor is required.
The ML4622 and ML4624 has been designed to
accommodate these two requirements. The input coupling
capacitors can be relatively small and still maintain
stability. With smaller input coupling capacitors and V
DC
capacitor a faster DC loop response time can be achieved.
The Link Monitor is also enhanced to have a faster
response time.
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
