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参数资料
| 型号： | W137H |
| 英文描述： | CPU System Clock Generator |
| 中文描述： | CPU系统时钟发生器 |
| 文件页数： | 6/13页 |
| 文件大小： | 141K |
| 代理商： | W137H |
W147G
PRELIMINARY
Document #: 38-07165 Rev. *A
Page 6 of 13
Spread Spectrum Clocking
The device generates a clock that is frequency modulated in
order to increase the bandwidth that it occupies. By increasing
the bandwidth of the fundamental and its harmonics, the am-
plitudes of the radiated electromagnetic emissions are re-
duced. This effect is depicted in
Figure 5
.
As shown in
Figure 5
, a harmonic of a modulated clock has a
much lower amplitude than that of an unmodulated signal. The
reduction in amplitude is dependent on the harmonic number
and the frequency deviation or spread. The equation for the
reduction is
dB = 6.5 + 9*log
10
(P) + 9*log
10
(F)
Where
P
is the percentage of deviation and
F
is the frequency
in MHz where the reduction is measured.
The output clock is modulated with a waveform depicted in
Figure 6
. This waveform, as discussed in
“
Spread Spectrum
Clock Generation for the Reduction of Radiated Emissions
”
by
Bush, Fessler, and Hardin produces the maximum reduction
in the amplitude of radiated electromagnetic emissions. The
deviation selected for this chip is
–
0.5% of the selected fre-
quency.
Figure 6
details the Cypress spreading pattern.
Cypress does offer options with more spread and greater EMI
reduction. Contact your local Sales representative for details
on these devices.
Spread Spectrum clocking is activated or deactivated by se-
lecting the appropriate value for bit 3 in data byte 0 of the I
2
C
data stream. Refer to page 8 for more details.
Enabled
EMI Reduction
Spread
Non-
Figure 5. Typical Clock and SSFTG Comparison
MAX.
MIN.
1
2
3
4
5
6
7
8
9
1
1
2
3
4
5
6
7
8
9
1
F
Figure 6. Typical Modulation Profile
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相关代理商/技术参数 |
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| W137HT | 制造商:Cypress Semiconductor 功能描述: |
| W137MPCX-1 | 制造商:Struthers-Dunn 功能描述: |
| W137MPCX-2 | 制造商:Struthers-Dunn 功能描述: |
| W1384AD/GD | 制造商:Kingbright Corporation 功能描述:LED Uni-Color Green 565nm 2-Pin |
| W1384AD/ID | 制造商:Kingbright Corporation 功能描述:LED Uni-Color Red 627nm 2-Pin |
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