
Data Sheet
March 2000
DSP1627 Digital Signal Processor
Lucent Technologies Inc.
147
13 Crystal Electrical Characteristics and Requirements
(continued)
13.3 LC Network Design for Third Overtone Crystal Circuits
For certain crystal applications, it is cheaper to use a third overtone crystal instead of a fundamental mode crystal.
When using third overtone crystals, it is necessary, however, to filter out the fundamental frequency so that the cir-
cuit will oscillate only at the third overtone. There are several techniques that will accomplish this; one of these is
described below. Figure 88 shows the basic setup for third overtone operation.
Figure 88. Third Overtone Crystal Configuration
The parallel combination of L
1
and C
1
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, C
3
, provides dc isolation for the trap circuit and should
be chosen to be large compared to C
1
.
For example, suppose it is desired to operate with a 40 MHz, third overtone, crystal:
Let:
f
3
=
operating frequency of third overtone crystal (40 MHz in this example)
f
1
=
fundamental frequency of third overtone crystal, or f
3
/3 (13.3 MHz in this example)
f
T
=
resonant frequency of trap =
C
2
=
external load capacitor (10 pF in this example)
dc blocking capacitor (0.1
μ
F in this example)
C
3
=
Arbitrarily, set trap resonance to geometric mean of f
1
and f
3
. Since f
1
= f
3
/3, the geometric mean would be:
5-4043 (F).a
CKI
CKI2
XTAL
C
1
C
2
C
3
L
1
2
π
L
1
C
1
----------1
f
T
f
3
------
3
40 MHz
3
23 MHz
=
=
=