FS6132
AMERICAN MICROSYSTEMS, INC.
VCXO Clock Generator IC
Preliminary
January 2000
3.0
3.1
Functional Block Description
Phase-Locked Loop (PLL)
250
200
150
Deviation - ppm
100
50
0
0
-50
-100
-150
-200
V(XTUNE) - volts
0.5
1
1.5
2
2.5
3
VCXO Deviation vs. XTUNE Input (typical)
The on-chip PLL is a standard frequency- and phase-
locked loop architecture. The PLL multiplies the refer-
ence oscillator to the desired frequency by a ratio of
integers. The frequency multiplication is exact with a
zero synthesis error.
3.2
Voltage-Controlled Crystal
Oscillator (VCXO)
The VCXO provides a tunable, low-jitter frequency ref-
erence for the rest of the FS6132 system components.
Loading capacitance for the crystal is internal to the
FS6132. No external components (other than the
crystal resonator itself) are required for operation of the
VCXO.
Continuous fine-tuning of the VCXO frequency is ac-
complished by varying the voltage on the XTUNE pin.
The oscillator operates the crystal resonator in the par-
allel-resonant mode. Crystal warping, or the “pulling” of
the crystal oscillation frequency, is accomplished by
altering the effective load capacitance presented to the
crystal by the oscillator circuit. The actual amount that
changing the load capacitance alters the oscillator fre-
quency will be dependent on the characteristics of the
crystal as well as the oscillator circuit itself.
Specifically, the motional capacitance of the crystal
(usually referred to by crystal manufacturers as C
1
),
the static capacitance of the crystal (C
0
), and the load
capacitance (C
L
) of the oscillator determine the “warp-
ing” or “pulling” capability of the crystal in the oscillator
circuit.
A simple formula to obtain the warping capability of a
crystal oscillator is:
C
1
×
(
C
L
2
−
C
L
1
)
×
10
∆
f
(
ppm
)
=
2
×
(
C
0
+
C
L
2
)
×
(
C
0
+
C
L
1
)
6
where C
L1
and C
L2
are the two extremes of the applied
load capacitance.
EXAMPLE: A crystal with the following parameters is
used. With C
1
= 0.02pF, C
0
= 6pF, C
L1
= 10pF, and C
L2
= 20pF, the tuning range (peak-to-peak) is
0
.
025
×
(
20
−
10
)
×
106
∆
f
=
=
300
ppm
.
2
×
(
6
+
20
)
×
(
6
+
10
)
3
ISO9001
1.24.00
AMI [ AMI SEMICONDUCTOR ]
