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參數(shù)資料
| 型號： | DSP1627 |
| 英文描述： | TVS 400W 6.5V BIDIRECT SMA |
| 中文描述： | DSP1627數(shù)字信號處理器 |
| 文件頁數(shù)： | 149/154頁 |
| 文件大?。?/td> | 2365K |
| 代理商： | DSP1627 |
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Data Sheet
March 2000
DSP1627 Digital Signal Processor
Lucent Technologies Inc.
149
13 Crystal Electrical Characteristics and Requirements
(continued)
13.4 Frequency Accuracy Considerations
For frequency accuracy implications of using the PLL, see Section 4.12, Clock Synthesis.
For most applications, clock frequency errors in the hundreds of parts per million can be tolerated with no adverse
effects. However, for applications where precise average frequency tolerance on the order of 100 ppm is required,
care must be taken in the choice of external components (crystal and capacitors) as well as in the layout of the
printed-circuit board. Several factors determine the frequency accuracy of a crystal-based oscillator circuit. Some of
these factors are determined by the properties of the crystal itself. Generally, a low-cost, standard crystal will not be
sufficient for a high-accuracy application, and a custom crystal must be specified. Most crystal manufacturers provide
extensive information concerning the accuracy of their crystals, and an applications engineer from the crystal vendor
should be consulted prior to specifying a crystal for a given application.
In addition to absolute, temperature, and aging tolerances of a crystal, the operating frequency of a crystal is also de-
termined by the total load capacitance seen by the crystal. When ordering a crystal from a vendor, it is necessary to
specify a load capacitance at which the operating frequency of the crystal will be measured. Variations in this load
capacitance due to temperature and manufacturing variations will cause variations in the operating frequency of the
oscillator. Figure 89 illustrates some of the sources of this variation.
Notes:
C
ext
= External load capacitor (one each required for CKI and CKI2).
C
D
= Parasitic capacitance of the DSP1627 itself.
C
B
= Parasitic capacitance of the printed-wiring board.
C
O
= Parasitic capacitance of crystal (not part of C
L
, but still a source of frequency variation).
Figure 89. Components of Load Capacitance for Crystal Oscillator
The load capacitance, CL, must be specified to the crystal vendor. The crystal manufacturer will cut the crystal so
that the frequency of oscillation will be correct when the crystal sees this load capacitance. Note that C
L
refers to a
capacitance seen across the crystal leads, meaning that for the circuit shown in Figure 89, C
L
is the series combi-
nation of the two external capacitors (C
ext
/2) plus the equivalent board and device strays (C
B
/2 + C
D
/2). For exam-
ple, if 10 pF external capacitors were used and parasitic capacitance is neglected, then the crystal should be
specified for a load capacitance of 5 pF. If the load capacitance deviates from this value due to the tolerance on the
external capacitors or the presence of strays, then the frequency will also deviate. This change in frequency as func-
tion of load capacitance is known as pullability and is expressed in units of ppm/pF. For small deviations of a few
pF, pullability can be determined by the equation below:
where C
0
= parasitic capacitance of crystal in pF.
C
1
= motional capacitance of crystal in pF (usually between 1 fF to 25 fF, value available from crystal vendor).
C
L
= total load capacitance in pF seen by crystal.
5-4045 (F).a
CKI
CKI2
XTAL
C
D
C
B
C
EXT
C
D
C
B
C
EXT
C
L
C
O
pullability (ppm/pF)
1
)
10
6
(
C
L
+
)
)
2
2 C
0
-----C
=
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