Datasheet
( )
( )
m c m
B = 2 1 + m = 2 +
¦
´ ¦ ´ ´ D ¦ ¦
c
c
ƒ
=
ƒ
D
d
c
ƒ
m
δ ƒ
m =
ƒ
´
( )
( )
hmfB
ffmfB
fmh
mcfm
×+××=
+D×=+××=
12
)(212
( )
)(212
mcfm
ffmfB +D×=+××=
0dBV
0dBVref
F
1
F
ENV,PEAK
Dfc
Dfc
Non-modulatedharmonic
Side-bandharmonics
windowaftermodulation
TPS82670
,
TPS82671
,
TPS82672
,
TPS82673
,
TPS82674
,
TPS82675
TPS82676
,
TPS82677
,
TPS826711
,
TPS826721
,
TPS826745
,
TPS826765
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SLVSAI0H –OCTOBER 2010–REVISED OCTOBER 2014
Feature Description (continued)
The spread spectrum architecture varies the switching frequency by approximately ±10% of the nominal
switching frequency, thereby significantly reduces the peak radiated and conducting noise on both the input and
output supplies. The frequency dithering scheme is modulated with a triangle profile and a modulation frequency
f
m
.
Figure 8. Spectrum Of A Frequency Modulated Sin. Figure 9. Spread Bands Of Harmonics In
Wave With Sinusoidal Variation In Time Modulated Square Signals
(1)
Figure 8 and Figure 9 show that after modulation the sideband harmonic is attenuated when compared to the
non-modulated harmonic, and when the harmonic energy is spread into a certain frequency band. The higher the
modulation index (mf) the larger the attenuation.
(1)
With:
f
c
is the carrier frequency (i.e. nominal switching frequency)
f
m
is the modulating frequency (approx. 0.016*f
c
)
δ is the modulation ratio (approx 0.1)
(2)
The maximum switching frequency is limited by the process and by the parameter modulation ratio ( δ), together
with f
m
, which is the bandwidth of the side-band harmonics around the carrier frequency f
c
. The bandwidth of a
frequency modulated waveform is approximately given by the Carson’s rule and can be summarized as:
(3)
f
m
< RBW: The receiver is not able to distinguish individual side-band harmonics; so, several harmonics are
added in the input filter and the measured value is higher than expected in theoretical calculations.
f
m
> RBW: The receiver is able to properly measure each individual side-band harmonic separately, so that the
measurements match the theoretical calculations.
(1) Spectrum illustrations and formulae (Figure 8 and Figure 9) copyright IEEE TRANSACTIONS ON ELECTROMAGNETIC
COMPATIBILITY, VOL. 47, NO.3, AUGUST 2005. See References Section for full citation.
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