PLL Clock Generator IC with VCXO
AMERICAN MICROSYSTEMS, INC.
Table 2: Pin Descriptions
Key: AI = Analog Input; AO = Analog Output; DI = Digital Input; DIU = Input with Internal Pull-Up; DID = Input with Internal Pull-Down; DIO = Digital Input/Output; DI-3 = Three-Level Digital Input,
DO = Digital Output; P = Power/Ground; # = Active Low pin
1 AI XIN VCXO Feedback
Power Supply (+3.3V)
Power Supply (+3.3V)
3.0 Functional Block Description
3.1 Voltage-Controlled Crystal
The VCXO provides a tunable, low-jitter frequency refer-
ence for the rest of the FS6128 system components.
Loading capacitance for the crystal is internal to the
FS6128. No external components (other than the reso-
nator 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
value of this voltage controls the effective capacitance
presented to the crystal. The actual amount that this load
capacitance change will alter the oscillator frequency de-
pends on the characteristics of the crystal as well as the
oscillator circuit itself.
It is important that the crystal load capacitance is speci-
fied correctly to “center” the tuning range. See Table 5.
A simple formula to obtain the “pulling” capability of a
crystal oscillator is:
C1 × (C L2 − C )L1 ×106
2 × (C0 + C L2)× (C0 + C )L1
C0 = the shunt (or holder) capacitance of the crystal
C1 = the motional capacitance of the crystal
CL1 and CL2 = the two extremes (minimum and maximum)
of the applied load capacitance presented by the
EXAMPLE: A crystal with the following parameters is
used: C1 = 0.025pF and C0 = 6pF. Using the minimum
and maximum CL1 = 10pF, and CL2 = 20pF, the tuning
range (peak-to-peak) is:
0.025 × (20 − 10)×106
2 × (6 + 20)× (6 + 10)
= 300 ppm .
3.2 Phase-Locked Loop (PLL)
The on-chip PLL is a standard frequency- and phase-
locked loop architecture. The PLL multiplies the reference
oscillator frequency to the desired output frequency by a
ratio of integers. The frequency multiplication is exact
with a zero synthesis error (unless otherwise specified).