Programmable Spread Spectrum; Table 4: Emi Reduction With Spectrum Spreading - Semtech SH3000 User Manual

3.2 Programmable Spread Spectrum

The SH3000 offers a technique for reducing electromagnetic interference (EMI). It can be a part of the
initial design strategy, or it can be applied in the prototype stage to fix problems identified during
compliance testing. This feature of the SH3000 may greatly reduce the requirements for radio frequency
(RF) shielding, and permits the use of simple plastic casings in place of expensive RFI-coated or metal
casings.
The SH3000 employs programmable spectrum spreading to reduce RF emissions from the processor's
clock. There are five possible settings; see Table 4 for operating and performance figures in the 8–16
MHz range.
Spectrum spreading is implemented by varying the frequency of the HF oscillator with a pseudo-random
sequence (with a zero-average DC component). The Maximum-Length Sequence (MLS) 8-bit random
number generator, clocked by 32 kHz, is used. Only 4, 5, 6, or 7 bits of the generated 8-bit random
number are used, according to the configuration setting.
Maximum fluctuations of the frequency depend on the selected frequency range and the position within
the range. Selecting the HF oscillator frequency near the high end of the range limits the peak variations
to ±0.1%, ±0.2%, ±0.4%, or ±0.8%.
The spread spectrum values are set in the SS_FreqSet register (R0x15). Bit 5 enables spectrum
spreading, and bits 7:6 specify the spreading bandwidth.
SH3000UM version 0.95
Copyright ©2002 Semtech Corporation

Table 4: EMI reduction with Spectrum Spreading

Spreading
Setting
Bandwidth
En CFG1 CFG0
b5 b7 b6
0 X X
1 0 0
1 0 1
1 1 0
1 1 1
2002-08
Peak EMI Peak EMI
Reduction Reduction
(guaranteed) (measured)
kHz db db
Off 0 0
32 -3 -3
64 -6 -7
128 -9 -10
256 -12 -15
SH3000 User Manual
Preliminary
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