CSP1027 Datasheet, PDF (41/64 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)
7.4.3 Printed-Circuit Board Layout Considerations
The following guidelines should be followed when designing the printed-circuit board layout for a crystal-based
application:
1. Keep crystal and external capacitors as close to XLO and XHI pins as possible to minimize board stray capaci-
tance.
2. Keep high-frequency digital signals such as CKO1 and CKO2 away from XLO and XHI traces to avoid coupling
into the oscillator.
7.4.4 LC Network Design for Third Overtone Crystal Circuits
For operating frequencies of greater than 30 MHz, it is usually cost advantageous to use a third overtone crystal as
opposed to a fundamental mode crystal. When using third overtone crystals, it is necessary, however, to filter out
the fundamental frequency so that the circuit will oscillate only at the third overtone. There are several techniques
that will accomplish this; one of these is described below.
Figure 35 shows the basic setup for third overtone operation.
XLO
XHI
XTAL
L1
C1
C2
C3
Figure 35. Third Overtone Crystal Configuration
5-7614 (F)
The parallel combination of L1 and C1 forms a resonant circuit with a resonant frequency between the first and third
harmonic of the crystal such that the LC network appears inductive at the fundamental frequency and capacitive at
the third harmonic. This ensures that a 360° phase shift around the oscillator loop will occur at the third overtone
frequency but not at the fundamental. The blocking capacitor, C3, provides dc isolation for the trap circuit and
should be chosen to be large compared to C1.
For example, suppose it is desired to operate with a 40 MHz, third overtone, crystal.
Let:
f3 = operating frequency of third overtone crystal (40 MHz in this example)
f1 = fundamental frequency of third overtone crystal, or f3/3 (13.3 MHz in this example)
fT = resonant frequency of trap = ------------¹--------------
2π L1C1
C2 = external load capacitor (10 pF in this example)
C3 = dc blocking capacitor (0.1 µF in this example)
Lucent Technologies Inc.
41