MT9044 Mitel Networks Corporation, MT9044 Datasheet - Page 16

• Manufacturer Part Number: MT9044
• Description: T1/E1/OC3 System Synchronizer
• Manufacturer: Mitel Networks Corporation

MT9044
Rise & Fall Time:
Duty Cycle:
The output clock should be connected directly (not
AC coupled) to the OSCi input of the MT9044, and
the OSCo output should be left open as shown in
Figure 9.
Crystal
Oscillator may be used. A complete oscillator circuit
made up of a crystal, resistor and capacitors is
shown in Figure 10.
The accuracy of a crystal oscillator depends on the
crystal tolerance as well as the load capacitance
tolerance. Typically, for a 20MHz crystal specified
with a 32pF load capacitance, each 1pF change in
load capacitance contributes approximately 9ppm to
the frequency deviation.
tolerances, and stray capacitances have a major
effect on the accuracy of the oscillator frequency.
The trimmer capacitor shown in Figure 10 may be
used to compensate for capacitive effects.
accuracy is not a concern, then the trimmer may be
removed, the 39pF capacitor may be increased to
56pF, and a wider tolerance crystal may be
substituted.
The crystal should be a fundamental mode type - not
an overtone. The fundamental mode crystal permits
a simpler oscillator circuit with no additional filter
components and is less likely to generate spurious
responses. The crystal specification is as follows.
Frequency:
Tolerance:
Oscillation Mode:
Resonance Mode:
Load Capacitance:
Maximum Series Resistance:
Approximate Drive Level:
16
1uH inductor: may improve stability and is optional
MT9044
Figure 10 - Crystal Oscillator Circuit
Oscillator
OSCo
OSCi
1M
-
56pF
8ns (0.5V 4.5V 50pF)
45% to 55%
Alternatively,
100
Consequently, capacitor
20MHz
20MHz
As required
Fundamental
Parallel
32pF
35
1mW
39pF
1uH
a
3-50pF
Crystal
If
e.g., CTS R1027-2BB-20.0MHZ
( 20ppm absolute, 6ppm 0C to 50C, 32pF, 25 )
Guard Time Adjustment
Excessive switching of the timing reference (from
PRI to SEC) in the MT9044 can be minimized by first
entering
maximum time (i.e., guard time). If the degraded
signal returns to normal before the expiry of the
guard time (e.g. 2.5 seconds), then the MT9044 is
returned to its Normal Mode (with no reference
switch taking place). Otherwise, the reference input
may be changed from Primary to Secondary.
A simple way to control the guard time (using
Automatic Control) is with an RC circuit as shown in
Figure 11. Resistor R
limits the current flowing into the GTi pin during
power down conditions.
calculated as follows.
•
In cases where fast toggling might be expected of
the LOS1 input, then an unsymmetrical Guard Time
Circuit is recommended. This ensures that reference
switching doesn’t occur until the full guard time value
has expired. An unsymmetrical Guard Time Circuit
is shown in Figure 12.
V
GTi Schmitt Trigger input, see DC Electrical
Characteristics
Figure 11 - Symmetrical Guard Time Circuit
SIH
MT9044
is the logic high going threshold level for the
guard time
guard time
example
guard time 150k
Holdover
GTo
GTi
=
Mode
150k
RC
RC
R
P
Advance Information
is for protection only and
0.6
The guard time can be
ln
1k
10u 0.6
R
for
P
--------------------------------- -
V DD V SIH
+
a
V DD
10uF
–
=
C
predetermined
0.9s