ICS843301I-108 Datasheet, PDF (7/15 Pages) Integrated Device Technology – FEMTOCLOCKS™ CRYSTAL-TO-3.3V, 2.5V LVPECL 75MHZ FREQUENCY SYNTHESIZER W/SSC

ICS843301I-108
FEMTOCLOCKS™ CRYSTAL-TO-3.3V, 2.5V LVPECL FREQUENCY SYNTHESIZER W/SSC
PRELIMINARY
LVCMOS TO XTAL INTERFACE
The XTAL_IN input can accept a single-ended LVCMOS signal
through an AC coupling capacitor. A general interface diagram is
shown in Figure 3. The XTAL_OUT pin can be left floating. The
input edge rate can be as slow as 10ns. For LVCMOS inputs, it is
recommended that the amplitude be reduced from full swing to
half swing in order to prevent signal interference with the power
rail and to reduce noise. This configuration requires that the output
impedance of the driver (Ro) plus the series resistance (Rs) equals
the transmission line impedance. In addition, matched termination
at the crystal input will attenuate the signal in half. This can be
done in one of two ways. First, R1 and R2 in parallel should equal
the transmission line impedance. For most 50Ω applications, R1
and R2 can be 100Ω. This can also be accomplished by removing
R1 and making R2 50Ω.
VDD
VDD
R1
Ro
Rs
.1uf
Zo = 50
XTAL_IN
Zo = Ro + Rs
R2
XTAL_OUT
FIGURE 3. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL INPUT INTERFACE
SPREAD SPECTRUM
Spread-spectrum clocking is a frequency modulation technique
for EMI reduction. When spread-spectrum is enabled, a 32kHz
triangle waveform is used with 0.5% down-spread (+0.0% /
TBD%) from the nominal 75MHz clock frequency. An example
of a triangle frequency modulation profile is shown in Figure
4A below.
The ICS843301I-108 triangle modulation frequency deviation
will not exceed TBD% down-spread from the nominal clock fre-
quency (+0.0% / TBD%). An example of the amount of down
spread relative to the nominal clock frequency can be seen in
the frequency domain, as shown in Figure 4B. The ratio of this
difference to the fundamental frequency is typically 0.5%, and
will not exceed TBD%. The resulting spectral reduction will be
greater than 7dB, as shown in Figure 4B. It is important to note
the ICS843301I-108 7dB minimum spectral reduction is the
component-specific EMI reduction, and will not necessarily be
the same as the system EMI reduction.
Δ − 8 dBm
Fnom
B
A
(1 - δ) Fnom
0.5/fm
➤
1/fm
FIGURE 4A. TRIANGLE FREQUENCY MODULATION
δ = 0.5% ➤
FIGURE 4B. 75MHZ CLOCK OUTPUT IN FREQUENCY DOMAIN
(A) SPREAD-SPECTRUM OFF
(B) SPREAD-SPECTRUM ON
IDT^™ / ICS^™ 3.3V, 2.5V LVPECL FREQUENCY SYNTHESIZER W/SSC 7
ICS843301BGI-108 REV. A OCTOBER 9, 2007