MT9046 Zarlink Semiconductor, MT9046 Datasheet - Page 19

• Manufacturer Part Number: MT9046
• Description: T1/E1 System Synchronizer with Holdover
• 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 9.
The accuracy of a crystal oscillator depends on the crystal tolerance as well as the load capacitance tolerance.
Typically, for a 20 MHz crystal specified with a 32 pF load capacitance, each 1 pF change in load capacitance
contributes approximately 9 ppm 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 9 may be used to compensate for capacitive effects. If accuracy is not a
concern, then the trimmer may be removed, the 39 pF capacitor may be increased to 56 pF, 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., R1B23B32-20.0 MHz
(20 ppm absolute,
TIE Correction (using PCCi)
When Primary Holdover Mode is entered for short time periods, TIE correction should not be enabled. This will
prevent unwanted accumulated phase change between the input and output.
For instance, 10 Normal to Holdover to Normal mode change sequences occur, and in each case Holdover was
entered for 2s. Each mode change sequence could account for a phase change as large as 350 ns. Thus, the
accumulated phase change could be as large as 3.5 us, and, the overall MTIE could be as large as 3.5 us.
±
6 ppm 0C to 50C, 32 pF, 25
20 MHz
As required
Fundamental
Parallel
32 pF
35
1 mW
1 uH inductor: may improve stability and is optional
Ω
Figure 9 - Crystal Oscillator Circuit
OSCo
OSCi
1 MΩ
Ω
MT9046
Zarlink Semiconductor Inc.
)
MT9046
56 pF
19
100 Ω
20 MHz
39 pF
1 uH
3-50 pF
Data Sheet