CSP1027 datasheet(43/64 Pages) AGERE

43 / 64 page

Lucent Technologies Inc.

Data Sheet

CSP1027 Voice Band Codec for

December 1999

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.

Notes:

CEXT =

External load capacitor (one each required for XLO and XHI).

CD =

Parasitic capacitance of the CSP1027 itself.

CB =

Parasitic capacitance of the printed-wiring board.

C0 =

Parasitic capacitance of crystal (not part of CL, but still a source of frequency variation).

Figure 36. 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 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.

XLO

XHI

XTAL

CEXT

5-7615 (F)

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