DSC6311ME2AA-T datasheet(8/28 Pages) MICROCHIP

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DSC63XX

DS20005808A-page 8

 2017 Microchip Technology Inc.

4.0

SPREAD SPECTRUM

Spread spectrum is a slow modulation of the clock

frequency over time. The PLL inside the MEMS

oscillator is modulated with a triangular wave at

33 kHz. With such a slow modulation, the peak spectral

energy of both the fundamental and all the harmonics

is spread over a wider frequency range and such an

energy is significantly reduced, thus providing an EMI

reduction. The triangular wave is chosen because of its

flat spectral density.

The DSC63xx MEMS oscillator family offers several

modulation options: the spreading is either center

spread or down spread with respect to the clock

frequency. Center spreading ranges from ±0.25% to

±2.5%, while down spreading ranges from –0.25% to –

3%.

If the clock frequency is 100 MHz and center spreading

with ±1% is chosen, the output clock will range from

99 MHz to 101 MHz. If down spreading with –2% is

chosen, the output clock will range from 98 MHz to

100 MHz.

Figure 4-1 and Figure 4-2 show a spectrum example of

the DSC6331 with a 33.333 MHz clock, modulated with

central spread of ±1%.

FIGURE 4-1:

DSC6331 Spectrum at

33.333 MHz with Modulation Turned Off.

FIGURE 4-2:

DSC6331 Spectrum at

33.333 MHz with Modulation Turned On.

It is noticeable that the spread spectrum provides a

reduction of about 10 dB from the peak power. Such a

reduction may also be estimated by the following

equation:

EQUATION 4-1:

The theoretical calculation for this example provides

10.45 dB, which is consistent with the measurement.

Similarly to the fundamental frequency, all the

harmonics are spread and attenuated in similar

fashion. Figure 4-3

shows how the

DSC6331

fundamental at 33.333 MHz and its odd harmonics are

attenuated when various types of modulations are

selected. For picture clarity, only the center spread

options are shown. However, down spread with

corresponding percentage provides the same level of

harmonic attenuation (e.g. central spread of ±1%

provides the same harmonics attenuation of down

spread with –2%).

EMI Reduction

Log10 Sfc

RBW









Where:

Peak-to-peak spread percentage (0.01,

this example).

Carrier frequency (33.333 Mhz, this

example).

RBW

Resolution bandwidth of the spectrum

analyzer (30 kHz, this example).

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