MT9041 Zarlink Semiconductor, MT9041 Datasheet - Page 10

• Manufacturer Part Number: MT9041
• Description: Single Reference Frequency Selectable Digital PLL With Multiple Clock Outputs For T1/E1 Trunk And Backplane Synchronization
• Manufacturer: Zarlink Semiconductor

Crystal Oscillator - Alternatively, a Crystal Oscillator may be used. A complete oscillator circuit made up of a
crystal, resistor and capacitors is shown in Figure 6.
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 6 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
(
±
20ppm absolute,
±
6ppm 0C to 50C, 32pF, 25
20MHz
As required
Fundamental
Parallel
32pF
35
1mW
1uH inductor: may improve stability and is optional
MT9041B
Ω
Figure 6 - Crystal Oscillator Circuit
OSCo
OSCi
1MΩ
Ω
)
Zarlink Semiconductor Inc.
MT9041B
56pF
10
100Ω
20MHz
39pF
1uH
3-50pF
Data Sheet