MT9040AN Zarlink Semiconductor, MT9040AN Datasheet - Page 11

• Manufacturer Part Number: MT9040AN
• Description: Description = Single Reference Frequency Selectable, 3.3V Digital PLL With Multiple Clock Outputs For Stratum 4 Applications ;; Package Type = Ssop ;; No. Of Pins = 48
• Manufacturer: Zarlink Semiconductor

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., R1B23B32-20.0MHz
(20ppm absolute,
±
6ppm 0C to 50C, 32pF, 25
20MHz
As required
Fundamental
Parallel
32pF
35
1mW
1uH inductor: may improve stability and is optional
Ω
Figure 6 - Crystal Oscillator Circuit
Ω
OSCo
OSCi
1MΩ
)
MT9040
Zarlink Semiconductor Inc.
MT9040
56pF
11
100Ω
20MHz
39pF
1uH
3-50pF
Data Sheet