SM532 Datasheet, PDF (11/14 Pages) List of Unclassifed Manufacturers – Low EMI Spectrum Spread Clock

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Approved Product
SM532
Low EMI Sprectum Spread Clock
Modulation Profile
The SM532 moves from max. to min. frequencies of its bandwidth at a pre-determined rate and profile. The
modulation frequency is determined by the input frequency and an internal divider. All 3 operating ranges
modulate the Fout clock at from 20 to 40 kHz. The three operating ranges are 14 - 30 MHz, 30 - 60 MHz and 60
to 120 MHz. If OSCin = 15 MHz, the modulation rate is 20 kHz. If OSCin is 60 MHz, in mid-range, the modulation
rate would be 40 kHz. To provide the proper modulation rate the input reference frequency is divided by a fixed
number in each range. The input reference frequency is divided by 750 in Low Range, 1500 in Mid Range and
3000 in the High Range. From these numbers, the modulation rate can be determined for any input frequency.
Example:
OSCin = 45.378 MHz, Input Range = Mid, Input divisor = 1500
Fmod = OSCin /1500
Fmod = 30.252 kHz
If you have a clock frequency that was on the boundary of the Mid-range and the High range of operation, the
choice of selecting which range to use would be determined by which modulation rate is desired. If you choose
the Mid-range, the modulation rate would be 40 kHz, while choosing the High range would yield a 20 kHz
modulation rate. There is some operational overlap between ranges, such that 58 MHz in the Mid-Range would
give the same results as 58 MHz in the High-Range, except for the modulation rate. This type of operation is not
recommended unless it is thoroughly tested.
The modulation profile of the SM532 is not a linear sweep from max to min and back. The OSCin reference clock
determines the modulation frequency but the internal SSCG control logic determines the actual modulation profile.
The modulation profile can best be described by comparing the instantaneous frequency at Fout with time. The
illustration in figure 9 below is a representation of the modulation profile of the SM532 as displayed on a Time
Domain Analyzer.
50.625
50.468
+1.25%
50.312
2.5%
50.156
50.000
49.844
49.687
-1.25%
49.531
49.375
Fmax
Fcenter
Fmin
0
5
10
15
20
25
30
35
Time (microseconds)
Figure 8. Frequency Profile in Time Domain
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 11 of 14