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參數(shù)資料
| 型號： | ICS8431I-21 |
| 英文描述： | 350MHZ, LOW JITTER, CRYSTAL OSCILLATOR-TO-3.3V LVPECL FREQUENCY SYNTHESIZER |
| 中文描述： | 350MHz頻段，低抖動，晶體振蕩器到3.3的LVPECL頻率合成器 |
| 文件頁數(shù)： | 9/16頁 |
| 文件大小： | 207K |
| 代理商： | ICS8431I-21 |
8431AMI-21
www.icst.com/products/hiperclocks.html
REV. A AUGUST 2, 2005
9
Integrated
Circuit
Systems, Inc.
ICS8431I-21
350MH
Z
, L
OW
J
ITTER
, C
RYSTAL
O
SCILLATOR
-
TO
-3.3V LVPECL F
REQUENCY
S
YNTHESIZER
S
PREAD
S
PECTRUM
Spread-spectrum clocking is a frequency modulation tech-
nique for EMI reduction. When spread-spectrum is enabled,
a 30kHz triangle waveform is used with 0.5% down-spread
(+0.0% / -0.5%) from the nominal 200MHz clock frequency.
An example of a triangle frequency modulation profile is shown
in Figure 4A below. The ramp profile can be expressed as:
Fnom = Nominal Clock Frequency in Spread OFF mode
(200MHz with 16MHz IN)
Fm = Nominal Modulation Frequency (30kHz)
δ
= Modulation Factor (0.5% down spread)
(1 -
δ
) fnom + 2 fm x
δ
x fnom x t when 0 < t <
,
1
fm
(1 -
δ
) fnom - 2 fm x
δ
x fnom x t when
< t <
1
2 fm
1
2 fm
The ICS8431I-21 triangle modulation frequency deviation will
not exceed 0.6% down-spread from the nominal clock fre-
quency (+0.0% / -0.5%). An example of the amount of down
spread relative to the nominal clock frequency can be seen in
the frequency domain, as shown in Figure 4B. The ratio of
this width to the fundamental frequency is typically 0.4%, and
will not exceed 0.6%. The resulting spectral reduction will be
greater than 7dB, as shown in Figure 4B. It is important to
note the ICS8431I-21 7dB minimum spectral reduction is the
component-specific EMI reduction, and will not necessarily
be the same as the system EMI reduction.
F
IGURE
4B. 200MH
Z
C
LOCK
O
UTPUT
IN
F
REQUENCY
D
OMAIN
(A) S
PREAD
-S
PECTRUM
OFF (B) S
PREAD
-S
PECTRUM
ON
F
IGURE
4A. T
RIANGLE
F
REQUENCY
M
ODULATION
Δ
10 dBm
δ
= .4%
B
A
Fnom
(1 -
δ
) Fnom
0.5/fm
1/fm
C
RYSTAL
I
NPUT
I
NTERFACE
The ICS8431I-21 has been characterized with 18pF parallel resonant
crystals. The capacitor values, C1 and C2, shown in Figure 3 below
were determined using a 25MHz, 18pF parallel resonant crystal and
Figure 3. C
RYSTAL
I
NPUT
I
NTERFACE
were chosen to minimize the ppm error. The optimum C1 and C2
values can be slightly adjusted for different board layouts.
C1
22p
X1
18pF Parallel Crystal
C2
22p
XTAL_OUT
XTAL_IN
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