MT9044AP1 Zarlink Semiconductor, Inc., MT9044AP1 Datasheet - Page 19

• Manufacturer Part Number: MT9044AP1
• Description: Framer: Framer Circuit: T1/E1/OC3 System Synchronizer
• Manufacturer: Zarlink Semiconductor, Inc.

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.
The trimmer capacitor shown in Figure 10 may be 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.
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:
e.g., CTS R1027-2BB-20.0MHZ
(
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 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.
±
20ppm absolute,
±
6ppm 0C to 50C, 32pF, 25
20MHz
As required
Fundamental
Parallel
32pF
35
1mW
1uH inductor: may improve stability and is optional
Ω
MT9044
Figure 10 - Crystal Oscillator Circuit
OSCo
Ω
OSCi
1MΩ
)
Zarlink Semiconductor Inc.
MT9044
56pF
19
100Ω
20MHz
39pF
1uH
3-50pF
Data Sheet