CSP1027 Datasheet, PDF (43/64 Pages) Agere Systems – CSP1027 Voice Band Codec for Cellular Handset and Modem Applications

Data Sheet
December 1999
CSP1027 Voice Band Codec for
Cellular Handset and Modem Applications
7 Application Information (continued)
7.4.5 Frequency Accuracy Considerations
For most applications, clock frequency errors in the hundreds of parts per million (ppm) can be tolerated with no
adverse effect. However, for applications where precise frequency tolerance on the order 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 suffi-
cient 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 ven-
dor 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
determined by the total load capacitance seen by the crystal. When ordering a crystal from a vendor, it is neces-
sary 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 fre-
quency of the oscillator. Figure 36 illustrates some of the sources of this variation.
XTAL
CEXT
CB
XLO
CD
CO
CEXT
CB
XHI
CD
CL
Notes:
CEXT =
CD =
CB =
C0 =
External load capacitor (one each required for XLO and XHI).
Parasitic capacitance of the CSP1027 itself.
Parasitic capacitance of the printed-wiring board.
Parasitic capacitance of crystal (not part of CL, but still a source of frequency variation).
Figure 36. Components of Load Capacitance for Crystal Oscillator
5-7615 (F)
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 CL refers to a
capacitance seen across the crystal leads, meaning that for the circuit shown in Figure 36, CL is the series combi-
nation of the two external capacitors (CEXT/2) plus the equivalent board and device strays (CB/2 + CD/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.
Lucent Technologies Inc.
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