11274-901 查看數據表（PDF） - AMI Semiconductor
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11274-901 Datasheet PDF : 39 Pages
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FS6131-01
Programmable Line Lock Clock Generator IC
The loop phase angle is:
[ ] Θi = arg K LOOP ( j2πfi) .
Figure 9: Loop Phase vs. Frequency
-100°
-150°
0.1kHz
1kHz
10kHz
Frequency (fi)
100kHz
A Nyquist plot of gain vs. amplitude is shown below.
Figure 10: Loop Nyquist Plot
90°
1.2
1.0
135°
0.8
Amplitude
0.6
0.4
0.2
Gain Margin
180°
45°
0°
Phase
Margin
225°
315°
270°
Phase
4.4 Voltage-Controlled Crystal Oscillator
The VCXO provides a tunable, low-jitter frequency refer-
ence for the rest of the FS6131 system components.
Loading capacitance for the crystal is internal to the de-
vice. No external components (other than the resonator
itself) are required for operation of the VCXO.
The resonator loading capacitance is adjustable under
register control. This feature permits factory coarse tun-
ing of inexpensive resonators to the necessary precision
for digital video applications. Continuous fine-tuning of
the VCXO frequency is accomplished by varying the volt-
age on the XTUNE pin. The total change (from one ex-
treme to the other) in effective loading capacitance is
1.5pF nominal, and the effect is shown in Figure 11. The
oscillator operates the crystal resonator in the parallel-
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.
The motional capacitance of the crystal (usually 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 determine the
total warping capability of a crystal 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 obtained from Table 11.
Example: A crystal with the following parameters is used
with the FS6131. The total coarse tuning range is:
C1=0.02pF, C0=5.0pF, CL1=10.0pF, CL2=22.66pF
∆f
=
0.02 × (22.66 −10)×106
2 × (5 + 22.66)× (5 +10)
= 305 ppm
6
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