FS6012-02 查看數據表（PDF） - AMI Semiconductor
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FS6012-02 Datasheet PDF : 20 Pages
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August 1998
4.4 VCXO Coarse Tuning and Enable
The VCXO may be coarse tuned by a programmable ad-
justment of the crystal load capacitance via D[14:11]. The
actual amount of frequency warping caused by the tuning
capacitance will depend on the crystal used. The VCXO
tuning capacitance includes an external load capacitance
of 6pF (12pF from the XIN pin to ground and 12pF from
the XOUT pin to ground).
The fine tuning capability of the VCXO can be enabled by
setting D[15] to a logic-one or disabled by clearing the bit
to a logic-zero.
Table 7: VCXO Coarse Tuning via I2C-bus
D[14] D[13] D[12] D[11]
0
0
0
0
0
0
0
1
0
0
1
0
0
0
1
1
0
1
0
0
0
1
0
1
0
1
1
0
0
1
1
1
1
0
0
0
1
0
0
1
1
0
1
0
1
0
1
1
1
1
0
0
1
1
0
1
1
1
1
0
1
1
1
1
VCXO TUNING
CAPACITANCE (pF)
10.00
10.84
11.69
12.53
13.38
14.22
15.06
15.91
16.75
17.59
18.43
19.28
20.13
20.97
21.81
22.66
4.5 VCXO Range
Figure 3 shows the typical effect of the coarse and fine
tuning mechanisms. The difference in VCXO frequency in
parts-per-million (ppm) is shown as the fine tuning volt-
age on the XTUNE pin varies from 0V to 5V. The coarse
tune range as shown is about 350ppm. As the crystal
load capacitance is increased (with increasing Coarse
Tune setting) the frequency is pulled somewhat less with
each coarse step and the fine tuning range decreases.
The fine tuning range always overlaps a few coarse tun-
ing ranges, eliminating the possibility of holes in the
VCXO response. Note that different crystal warping char-
acteristics will change the scaling on the Y-axis, but not
the overall characteristic of the curves.
Figure 3: VCXO Range
VCXO Range (ppm) vs. XTUNE Voltage (V)
200
150
100
50
0
-50
XTUNE Voltage = 0.0V
XTUNE Voltage = 5.0V
-100
-150
-200
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Coarse Tune Setting D[11:14]
,62
5
8.19.98
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