33.5.02
FS6345
FS6345FS6345
FS6345
Triple PLL VCXO Cl ock Generator I C
Triple PLL VCXO Cl ock Generator I CTriple PLL VCXO Cl ock Generator I C
Triple PLL VCXO Cl ock Generator I C
ISO9001
ISO9001ISO9001
ISO9001
3.0 Functional Bl ock Descripti on
3.1 Phase-Locked Loop (PLL)
The on-chip PLL is a standard frequency- and phase-
lock ed loop archi tec tur e. The PLL multiplies the refer enc e
oscillator to the desired frequency by a ratio of integers.
The frequency multiplication is exact with a zero synthe-
sis error (unless otherwise noted).
3.2 Voltage-Controlled Crystal
Oscillator (VCXO)
The VCXO provides a tunable, low-jitter frequency refer-
ence for the rest of the FS6345 system components.
Loading capacitance for the crystal is internal to the
FS6345. No external components (other than the crystal
resonator itself) are required for operation of the VCXO.
Continuous fine-tuning of the VCXO frequency is accom-
plished by varying the voltage on the XTUNE pin.
The oscillator operates the crystal resonator in the paral-
lel-reso nant mode. Cr ystal warpin g, or the “pul ling” of the
crystal oscillation frequency, is accomplished by altering
the eff ective load ca pacitance presented to the crystal b y
the oscillator circuit. T he actual amount that changin g the
load capacitance alters the oscillator frequency will be
dependent o n the charac teristics of the cr ystal as well as
the oscillator circuit itself.
Specifically, the m otional capacitance of the cr ystal (usu-
ally referr ed to by cr ystal m anufactur ers as C1), the st atic
capacitance of the crystal (C0), and the load capacitance
(CL) of the oscillator determine the “warping” or “pulling”
capability of the crystal in the oscillator circuit.
A simple formula to obtain the warping capability of a
crystal oscillator is:
()
()()
CCCC CCC
ppmf LL
LL
1020
6
121
210
)( +×+× ×−×
=∆
where CL1 and CL2 are the two extremes of the applied
load capacitance.
EXAMPLE: A crystal with the following parameters is
used. W ith C1 = 0.025pF, C0 = 6pF, CL1 = 10pF, and CL2
= 20pF, the tuning range is
()
()()
ppm
.
f300
1062062 106
10200250 =
+×+× ×−×
=∆ .
FS6245 Typical VCXO Deviation vs. XTUNE Input
-200
-150
-100
-50
0
50
100
150
200
250
00.511.522.53
V(XTUN E) - volts
Deviation from 13.500MHz - ppm