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參數(shù)資料
| 型號： | FS781BZB |
| 英文描述： | MISCELLANEOUS CLOCK GENERATOR|SOP|8PIN|PLASTIC |
| 中文描述： | 雜項時鐘發(fā)生器|?？苵 8引腳|塑料 |
| 文件頁數(shù)： | 9/13頁 |
| 文件大?。?/td> | 103K |
| 代理商： | FS781BZB |
LOW EMI SPECTRUM SPREAD CLOCK
Cypress Semiconductor Corporation
525 Los Coches St.
Milpitas, CA 95035. Tel: 408-263-6300, Fax: 408-263-6571
http://www.cypress.com
Document#: 38-07029 Rev. **
05/07/2001
Page 9 of 13
APPROVED
PRODUCT
FS781/82/84
Fc = 20 MHz
Fmin =
19.8 MHz
Fmax =
20.2 MHz
clocks have a very high Q factor, which means that all of the energy at that frequency is concentrated in a very
narrow bandwidth, consequently, higher energy peaks. Regulatory agencies test electronic equipment by the
Amount of peak energy radiated from the equipment. By reducing the peak energy at the fundamental and
harmonic frequencies, the equipment under test is able to satisfy agency requirements for Electro-Magnetic
Interference (EMI). Conventional methods of reducing EMI have been to use shielding, filtering, multi-layer PCB
’
s
etc. These FS781/2 and 4 use the approach of reducing the peak energy in the clock by increasing the clock
bandwidth, and lowering the Q of the clock.
SSCG
The FS781/2/4 products use a unique method of modulating the clock
over a very narrow bandwidth and controlled rate of change, both peak
to peak and cycle to cycle. The FS78x products take a narrow band
digital reference clock in the range 6 - 82 MHz and produce a clock
that sweeps between a controlled start and stop frequency and precise
rate of change. To understand what happens to an SSCG clock,
consider that we have a 20 MHz clock with a 50 % duty cycle. From a
20 MHz clock we know the following;
Clock Frequency = Fc = 20 MHz.
Clock Period = Tc = 1/20 MHz=50 ns
Consider that this 20 MHz clock is applied to the Xin input of the
FS78x, either as an externally driven clock or as the result of a parallel
resonant crystal connected to pins 1 and 2 of the FS78x. Also
consider that the products are operating from a 5-volt DC power
supply and the loop filter is set for a total bandwidth spread of 2%.
Refer to table 6 on page 6.
From the above parameters, the output clock at FSOUT will be
sweeping symmetrically around a center frequency of 20 MHz.
The minimum and maximum extremes of this clock will be +200 kHz
and -200 kHz. So, we have a clock that is sweeping from 19.8 MHz to
20.2 MHz and back again. If we were to look at this clock on a
spectrum analyzer we would see the picture in figure 7. Keep in mind
that this is a drawing of a perfect clock with no noise.
Figure 7.
50%
50%
Tc = 50 ns.
20 MHz Unmodulated Clock
Figure 6.
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相關(guān)代理商/技術(shù)參數(shù) |
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| FS782 | 制造商:CYPRESS 制造商全稱:Cypress Semiconductor 功能描述:Low EMI Spectrum Spread Clock |
| FS782BT | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:MISCELLANEOUS CLOCK GENERATOR|TSSOP|8PIN|PLASTIC |
| FS782BZB | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:MISCELLANEOUS CLOCK GENERATOR|SOP|8PIN|PLASTIC |
| FS784 | 制造商:CYPRESS 制造商全稱:Cypress Semiconductor 功能描述:Low EMI Spectrum Spread Clock |
| FS784BT | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:MISCELLANEOUS CLOCK GENERATOR|TSSOP|8PIN|PLASTIC |
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