SM532 Datasheet, PDF (9/14 Pages) List of Unclassifed Manufacturers – Low EMI Spectrum Spread Clock

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YJGP VKOKPI KU ETKVKECN
Approved Product
SM532
Low EMI Sprectum Spread Clock
SSCG
SSCG uses a patented technology of modulating the clock over a very narrow bandwidth and controlled rate of
change, both peak and cycle to cycle. The SM532 takes a narrow band digital reference clock in the range of 14 -
120 MHz and produces a clock that sweeps between a controlled start and stop frequency and precise rate of
change. The bandwidth of the output clock is programmable. Using two control lines on the SM532, the
bandwidth of the modulated clock can be controlled over four descrete settings, 1.25, 2.50, 5.0 and 10%. To
understand what happens to an SSCG clock, consider that we have a 50 MHz clock with a 50 % duty cycle. From
a 50 MHz clock we know the following;
Clock Frequency = Fc = 50 MHz.
Clock Period = Tc = 1/50 MHz = 20 ns.
50 % 50 %
Tc = 20 ns.
Figure 5. Unmodulated Clock
Consider that this 50 MHz clock is applied to the OSCin input of the SM532, either as an externally driven clock or
as the result of a parallel resonant crystal connected to pins 1 and 2 of the SM532. Also consider that the SM532
is programmed for the following operation;
Range (R0, R1) =
0, 1 Mid Range
Multiplier (S0, S1) =
0, 1 X1
D_C =
1
Center Spread
SSON =
1
Modulation is OFF
% Modulation (S2, S3) =
1, 0 2.50 % Spread
From the above parameters, the output clock at Fout will be 50.625 MHz. With modulation turned off, the
frequency of Fout will always rest at the high end of the programmed
50.00MHz
spectrum. In this case, +1.25 % of 50 MHz is .625 MHz, equals 50.625 ^{49.375 MHz Center}
MHz.
min.
50.625 MH
max.
When modulation is turned ON, the clock at Fout begins sweeping
Modulation
downward to the minimum extreme of -1.25 % of 50 MHz which is 50 MHz -
Off.
.625 MHz = 49.375 MHz. When the clock reaches 49.375, the SM532
begins sweeping back up to the maximum extreme of 50.625 MHz. If we
were to look at this clock on a spectrum analyzer we would see the picture
in figure 6. Keep in mind that this is a drawing of a perfect clock with no
noise.
We see that the original 50 MHz reference clock is at the Center frequency,
Cf, and the minimum and maximum extremes are positioned symmetrically
about the center frequency. This type of modulation is called Center-
Spread. Note that when modulation is turned off, the Fout clock is at the
maximum extreme of the bandwidth.
Figure 6.
International Microcircuits,Inc.
525 Los Coches St., Milpitas, 95035 408-263-6300, FAX [phone]
http:/www.imicorp.com
8/31/98
Rev. 1.4
Page 9 of 14