MT9044
Advance Information
Rise & Fall Time:
Duty Cycle:
8ns (0.5V 4.5V 50pF)
45% to 55%
e.g., CTS R1027-2BB-20.0MHZ
(±20ppm absolute, ±6ppm 0C to 50C, 32pF, 25Ω)
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.
Guard Time Adjustment
Excessive switching of the timing reference (from
PRI to SEC) in the MT9044 can be minimized by first
entering Holdover Mode for
a
predetermined
Crystal Oscillator
-
Alternatively,
a
Crystal
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.
Oscillator may be used. A complete oscillator circuit
made up of a crystal, resistor and capacitors is
shown in Figure 10.
MT9044
OSCi
MT9044
GTo
20MHz
1MΩ
R
+
150kΩ
56pF
39pF
3-50pF
C
10uF
OSCo
100Ω
1uH
GTi
1uH inductor: may improve stability and is optional
R
P
1kΩ
Figure 10 - Crystal Oscillator Circuit
Figure 11 - Symmetrical Guard Time Circuit
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. Consequently, capacitor
tolerances, and stray capacitances have a major
effect on the accuracy of the oscillator frequency.
A simple way to control the guard time (using
Automatic Control) is with an RC circuit as shown in
Figure 11. Resistor R is for protection only and
P
limits the current flowing into the GTi pin during
power down conditions. The guard time can be
calculated as follows.
V
DD
– V
---------------------------------
guard
guard
= RC × ln
≈ RC × 0.6
The trimmer capacitor shown in Figure 10 may be
time
V
DD
SIH
used to compensate for capacitive effects.
If
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.
time
example
guard
≈ 150k × 10u × 0.6= 0.9s
time
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.
•
VSIH is the logic high going threshold level for the
GTi Schmitt Trigger input, see DC Electrical
Characteristics
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.
Frequency:
20MHz
As required
Fundamental
Parallel
32pF
Tolerance:
Oscillation Mode:
Resonance Mode:
Load Capacitance:
Maximum Series Resistance:
Approximate Drive Level:
35Ω
1mW
16
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