IMIFS741 Ver la hoja de datos (PDF) - Unspecified
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IMIFS741 Datasheet PDF : 16 Pages
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Approved Product
FS741
Low EMI Spectrum Spread Clock
Most applications will not have a problem meeting agency specifications at the fundamental frequency. It is the
higher harmonics that usually cause the most problems. With an SSCG clock, the bandwidth and peak energy
reduction increases with the harmonic number. Consider that the 11th harmonic of a 20 MHz clock is 220 MHz.
With a total spread of 200 KHz at 20 MHz, the spread at the 11th harmonic would be 2.20 MHz which greatly
reduces the peak energy content.
The difference in the peak energy of the modulated clock and the non-modulated clock in typical applications will
see a 2 - 3 db. reduction at the fundamental and as much as 8 - 10 db. reduction at the intermediate harmonics,
3rd, 5th, 7th etc. At the higher harmonics, it is quite possible to reduce the peak harmonic energy, compared to the
unmodulated clock, by as much as 12 to 18 db.
The following images are actual scans of the FS741. These scans are from a spectrum analyzer and time domain
analyzer of the FS741 running at 3.3 volts DC.
Figure 9 at the right shows a modulated 10 MHz clock at Modout of the FS741. The following parameters apply to
this scan;
Fin = 10 Mhz.
BW = 2% (total)
Vertical scale = 6 dB/div.
From this scan it can be seen the bandwidth of the
clock is wider than a conventional clock. Notice the
EMI filters displayed at the bottom of the image.
This is the same filter settings that are used by
regulatory agencies.
Figure 9.
It is clear from Figure 9, that the peak amplitude of the modulated clock is lower in amplitude than the non-
modulated clock. In fact, this image indicates that the difference between the two peaks is approximately 2 dB.
International Microcircuits, Inc.
525 Los Coches St., Milpitas, 95035 408-263-6300, FAX 408-263-6571
http:/www.imicorp.com
4/5/1999
Rev. 2.1
Page 12 of 16
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