FS791/92/94
Document #: 38-07343 Rev *A
Page 6 of 9
We see that the original 100-MHz reference clock is at the
center Frequency, Cf, and the minimum and maximum ex-
tremes are positioned symmetrically about the center frequen-
cy. This type of modulation is called Center-Spread. Figure 5
shows a 100-MHz clock, as it would be seen on an oscillo-
scope. The top trace is the non-modulated reference clock.
The bottom trace is the modulated clock at pin 6. From this
comparison chart you can see that the frequency is decreasing
and the period of each successive clock is increasing. The Tc
measurements on the left and right of the bottom trace indicate
the max. and min. extremes of the clock. Intermediate clock
changes are small and accumulate to achieve the total period
deviation. The reverse of this figure would show the clock go-
ing from min. extreme back to the high extreme.
Looking at Figure 4, you will note that the peak amplitude of
the 100-MHz non-modulated clock is higher than the wide-
band modulated clock. This difference in peak amplitudes be-
tween modulated and unmodulated clocks is the reason why
SSCG clocks are so effective in digital systems. This figure
refers to the fundamental frequency of a clock. A very impor-
tant characteristic of the SSCG clock is that the bandwidth of
the fundamental frequency is multiplied by the harmonic num-
ber. In other words, if the bandwidth of a 100-MHz clock is 2.0
MHz, the bandwidth of the 3rd harmonic will be 3 times 2.0, or
6.0 MHz. The amount of bandwidth is relative to the amount of
energy in the clock. Consequently, the wider the bandwidth,
the greater the energy reduction of the clock.
Most applications will not have a problem meeting agency
specifications at the fundamental frequency. It is the higher
harmonics that usually cause the most problems. With an
SSCG clock, the bandwidth and peak energy reduction in-
creases with the harmonic number. Consider that the 9th har-
monic of a 100-MHz clock is 900 MHz. With a total spread of
2.0 MHz at 100 MHz, the spread at the 9th harmonic would be
18.0 MHz which greatly reduces the peak energy content. It is
typical to see as much as 12 to 20 dB reduction at the higher
harmonics, due to a modulated clock.
The difference in the peak energy of the modulated clock and
the non-modulated clock in typical applications will see a 2–3
dB reduction at the fundamental and as much as 8–10 dB
reduction at the intermediate harmonics, 3rd, 5th, 7th etc. At the
higher harmonics, it is quite possible to reduce the peak har-
monic energy, compared to the unmodulated clock, by as
much as 12 to 20 dB.
Fc = 100 MHz
Fmin =
99.0 MHz
Fmax =
101.0 MHz
Figure 4. 100-MHz Spectrum
Tc = 50.50 ns
Tc =49.50 ns.
Figure 5. Period Comparison Chart
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