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PDF FS782 Datasheet ( Hoja de datos )

Número de pieza FS782
Descripción (FS781 - FS784) Low EMI Spectrum Spread Clock
Fabricantes Cypress Semiconductor 
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FS782 datasheet

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FS782 pdf
FS781/82/84
SSCG Modulation Profile
The digital control inputs S0 and S1 determine the modulation
frequency of FS781/2/4 products. The input frequency is
divided by a fixed number, depending on the operating range
that is selected. The modulation frequency of the FS78x can
be determined from Table 4. To compute the modulation
frequency, determine the values of S0 and S1, and find the
modulation divider number in Table 4.
Table 4. Modulation Rate Divider Ratios
S1 S0
00
01
10
11
Input Frequency Range (MHz)
6 to 16
16 to 32
32 to 66
66 to 82
Modulation Divider Number
120
240
480
720
+ .5%
1.0%
Total
Xin
- .5%
TIME (microseconds)
Figure 1. Frequency Profile in Time Domain[5]
Theory of Operation
The FS781/82/84 devices are phase-locked loop-(PLL)-type
clock generators using Direct Digital Synthesis (DDS). ‘By
precisely controlling the bandwidth of the output clock, the
FS781/2/4 products become a low-EMI clock generator. The
theory and detailed operation of these products will be
discussed in the following sections.
EMI
All clocks generate unwanted energy in their harmonics.
Conventional digital clocks are square waves with a duty cycle
that is very close to 50%. Because of the 50/50 duty cycle,
digital clocks generate most of their harmonic energy in the
odd harmonics (e.g., third, fifth, seventh). It is possible to
reduce the amount of energy contained in the fundamental
and harmonics by increasing the bandwidth of the funda-
mental clock frequency. Conventional digital clocks have a
very high Q factor, which means that all of the energy at that
frequency is concentrated in a very narrow bandwidth, conse-
quently, 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 funda-
mental and harmonic frequencies, the equipment under test is
able to satisfy agency requirements for EMI. Conventional
methods of reducing EMI have been to use shielding, filtering,
multi-layer PCBs, etc. These FS781/2 and 4 reduce the peak
energy in the clock by increasing the clock bandwidth and
lowering the Q of the clock.
SSCG
The FS781/82/84 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 of 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 as either an externally driven clock or 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
5V DC power supply and the loop filter is set for a total
bandwidth spread of 2%. Refer to Figure 2.
Note:
5. With the correct loop filter connected to Pin 4, the following profile will provide the best EMI reduction. This profile can be seen on a Time Domain Analyzer.
Document #: 38-07029 Rev. *E
Page 5 of 12

5 Page

FS782 arduino
Package Drawing and Dimensions (continued)
8-Lead Thin Shrunk Small Outline Package (4.40 MM Body) Z8
FS781/82/84
PIN 1 ID
1
6.25[0.246]
6.50[0.256]
4.30[0.169]
4.50[0.177]
DIMENSIONS IN MM[INCHES] MIN.
MAX.
8
0.19[0.007]
0.30[0.012]
0.65[0.025]
BSC.
1.10[0.043] MAX.
0.85[0.033]
0.95[0.037]
2.90[0.114]
3.10[0.122]
0.05[0.002]
0.15[0.006]
0.076[0.003]
SEATING
PLANE
0.25[0.010]
BSC
GAUGE
PLANE
0° -8°
0.50[0.020]
0.70[0.027]
0.09[[0.003]
0.20[0.008]
51-85093-*A
All product and company names mentioned in this document may be the trademarks of their respective holders.
Document #: 38-07029 Rev. *E
Page 11 of 12
© Cypress Semiconductor Corporation, 2005. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility for the use
of any circuitry other than circuitry embodied in a Cypress product. Nor does it convey or imply any license under patent or other rights. Cypress products are not warranted nor intended to be
used for medical, life support, life saving, critical control or safety applications, unless pursuant to an express written agreement with Cypress. Furthermore, Cypress does not authorize its
products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress
products in life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges.

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