FS6012-02 Просмотр технического описания (PDF) - AMI Semiconductor
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FS6012-02 Datasheet PDF : 20 Pages
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August 1998
3.3 Voltage-Controlled Crystal Oscillator
(VCXO)
The VCXO provides a tunable, low-jitter frequency refer-
ence for the rest of the FS6012 system components.
Loading capacitance for the crystal is internal to the
FS6012. No external components (other than the reso-
nator itself) are required for operation of the VCXO.
The resonator loading capacitance is adjustable under
register control. This permits factory coarse tuning of in-
expensive resonators to the necessary precision for digi-
tal video applications.
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 1.5pF 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
(CL) of the oscillator determine the warping 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 + CL2)× (C0 + C )L1
where CL1 and CL2 are the two extremes of the applied
load capacitance. A crystal with the following parameters
is used. With C1 = 0.02pF, C0 = 5pF, CL1 = 10pF, CL2 =
22.66pF, the coarse tuning range is:
∆f
=
0.02 × (22.66 −10)×
2 × (5 + 22.66)× (5 +
106
10)
=
305
ppm
4.0 Programming Information
Table 2: Register Summary
BIT D[x]
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
REGISTER BIT DESCRIPTION
ACLK Select (LSB)
ACLK Select
ACLK Select
ACLK Select (MSB)
UCLK Select (LSB)
UCLK Select
UCLK Select
UCLK Select (MSB)
PCLK Select (LSB)
PCLK Select
PCLK Select (MSB)
Crystal Oscillator Coarse Tune (LSB)
Crystal Oscillator Coarse Tune
Crystal Oscillator Coarse Tune
Crystal Oscillator Coarse Tune (MSB)
VCXO Fine Tune Enable/Disable
Bit = 0
Disable Fine Tune
Bit = 1
Enable Fine tune
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8.19.98
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