FS6127 Datasheet, PDF (2/6 Pages) List of Unclassifed Manufacturers – VCXO CLOCK GENERATOR IC

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February 1999
Table 2: Pin Descriptions
Key: AI = Analog Input; AO = Analog Output; DI = Digital Input; DI^U = 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
PIN
TYPE
NAME
DESCRIPTION
1
AI
XIN
VCXO Crystal Feedback
2
P
VDD
Power Supply (+5V)
3
AI
XTUNE
VCXO Tune
4
P
VSS
Ground
5
DO
CLK
Clock Output
6
P
VDD
Power Supply (+5V)
7
P
VSS
Ground
8
AO
XOUT
VCXO Crystal Drive
3.0 Functional Block Description
3.1 Phase-Locked Loop (PLL)
The on-chip PLL is a standard frequency- and phase-
locked loop architecture. The PLL multiplies the reference
oscillator to the desired frequency by a ratio of integers.
The frequency multiplication is exact with a zero synthe-
sis error.
3.2 Voltage-Controlled Crystal
Oscillator (VCXO)
The VCXO provides a tunable, low-jitter frequency refer-
ence for the rest of the FS6127 system components.
Loading capacitance for the crystal is internal to the
FS6127. 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
total change (from one extreme to the other) in effective
loading capacitance is t.b.d. nominal.
The oscillator operates the crystal resonator in the paral-
lel-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 frequency will be
dependent on the characteristics of the crystal as well as
the oscillator circuit itself.
Specifically, the motional capacitance of the crystal (usu-
ally referred to by crystal manufacturers as C₁), the static
capacitance of the crystal (C₀), and the load capacitance
(C_L) 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:
∆f
( ppm)
=
C1 ^{× (}C L2 ^− C ⁾L1 ^×10⁶
2 × (_C0 + _{C L2})× (_C0 + _C )_L1
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 C₁ = 0.02pF, C₀ = 5pF, C_L1 = 10pF, and C_L2 =
22.66pF, the tuning range is
∆f
=
0.02 × (22.66 −10)×₁₀₆
2 × (5 + 22.66)× (5 + 10)
=
305
ppm
.
2
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