MT9042 데이터 시트보기 (PDF) - Mitel Networks
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MT9042 Datasheet PDF : 16 Pages
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Preliminary Information
MT9042
Automatic Mode
In normal AUTOMATIC mode operation, the RSEL
input is set to 0. This will allow the state machine to
control PLL operation and select the reference input
based on the state of the LOSS1 and LOSS2 inputs
(see state transitions in Table 4). If the PRI reference
signal is lost (LOSS1 = HIGH, LOSS2 = LOW), then
the PLL will enter HOLDOVER mode immediately
and stay there for a time determined by the RC time
constant connected to the Guard Time input (GTi,
GTo).
R (Ω)
GTo
GTi
C (f)
(a)
VGTi
1.77v
tgt
(b)
time
Figure 4 - a) RC circuit for guard time,
b) exponential waveform on GTi
When the primary reference signal has not been
regained and the guard time has been exceeded, the
reference will be switched to SEC. The time
constant determined by the RC circuit connected to
the GTi input provides the hysteresis on automatic
switching between PRI and SEC during very short
interruptions of the primary reference signal. The
Guard Time, tgt, can be predicted using the step
response of an RC network. The capacitor voltage
on the RC circuit is described by an exponential
curve. When the capacitor voltage reaches the
positive going threshold of GTi (typically 1.77 volts
for Schmitt trigger TTL inputs, see Figure 4) a logic
HIGH level results. This causes the state machine to
move from the holdover state of PRI to the state of
using SEC as the input reference. The following
equation can be used to determine the Guard Time
tgt:
tgt
=
–R
C
ln
-V---d---d----–----1---.--7---7-
Vdd
The state machine will continue to monitor the LOSS1
input and will switch back to the PRI reference once
the primary reference becomes functional as indicated
by the LOSS1 input. A logic level HIGH on both the
LOSS1 or LOSS2 inputs indicates that none of the
external references are available. Under these
circumstances, the PLL will be switched into the
HOLDOVER state (within a specified rate of frame
slip) until a fuIly functional reference input is available.
FSEL FSEL Input Reference Frequency
2
1
0
0
Reserved
0
1
8 kHz
1
0
1.544 MHz
1
1
2.048 MHz
Table 5 - Input Frequency Selection of the MT9042
Time Interval Error Correction Circuit
(TIE)
The TIE correction circuit generates a virtual input
synchronized to the selected primary input
reference. After a reference rearrangement the TIE
corrects the phase of this new reference in such a
way that the virtual input preserves its phase. In
other words, reference switching will not create
significant phase changes on the virtual input, and
therefore, the outputs of the PLL.
The TIE reset (TRST) aligns the falling edge of the
current input with the falling edge of the primary
input reference. When TRST is held LOW for at least
100 ns, the next falling edge of the reference input
becomes aligned and passes through the TIE circuit
without additional delay.
PLL Measures of Performance
To meet the requirements of AT & T TR62411 and
ETSI 300 011, the following PLL performance
parameters were measured:
• locking range and lock time
• slip rate in holdover mode
• free-run accuracy
• maximum time interval error and slope
• intrinsic jitter
• jitter transfer function
• output jitter spectrum
• wander
3-101
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