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ICS8431AM-21LF Datasheet(PDF) 9 Page - Integrated Circuit Systems |
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ICS8431AM-21LF Datasheet(HTML) 9 Page - Integrated Circuit Systems |
9 / 17 page 8431AM-21 www.icst.com/products/hiperclocks.html REV. A APRIL 27, 2005 9 Integrated Circuit Systems, Inc. ICS8431-21 350MHZ, LOW JITTER, CRYSTAL OSCILLATOR- TO -3.3V LVPECL FREQUENCY SYNTHESIZER SPREAD SPECTRUM Spread-spectrum clocking is a frequency modulation tech- nique for EMI reduction. When spread-spectrum is enabled, a 30kHz triangle waveform is used with 0.5% down-spread (+0.0% / -0.5%) from the nominal 200MHz clock frequency. An example of a triangle frequency modulation profile is shown in Figure 4A below. The ramp profile can be expressed as: • Fnom = Nominal Clock Frequency in Spread OFF mode (200MHz with 16MHz IN) • Fm = Nominal Modulation Frequency (30kHz) • δ = Modulation Factor (0.5% down spread) (1 - δ) fnom + 2 fm x δ x fnom x t when 0 < t < , (1 - δ) fnom - 2 fm x δ x fnom x t when < t < 1 2 fm 1 2 fm 1 fm The ICS8431-21 triangle modulation frequency deviation will not exceed 0.6% down-spread from the nominal clock fre- quency (+0.0% / -0.5%). 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 width to the fundamental frequency is typically 0.4%, and will not exceed 0.6%. The resulting spectral reduction will be greater than 7dB, as shown in Figure 4B. It is important to note the ICS8431-21 7dB minimum spectral reduction is the component-specific EMI reduction, and will not necessarily be the same as the system EMI reduction. FIGURE 4B. 200MHZ CLOCK OUTPUT IN FREQUENCY DOMAIN (A) SPREAD-SPECTRUM OFF (B) SPREAD-SPECTRUM ON FIGURE 4A. TRIANGLE FREQUENCY MODULATION Δ − 10 dBm δ = .4% B A ➤ Fnom (1 - δ) Fnom 0.5/fm 1/fm ➤ CRYSTAL INPUT INTERFACE The ICS8431-21 has been characterized with 18pF parallel resonant crystals. The capacitor values, C1 and C2, shown in Figure 3 below were determined using a 25MHz, 18pF parallel resonant crystal and Figure 3. CRYSTAL INPUT INTERFACE were chosen to minimize the ppm error. The optimum C1 and C2 values can be slightly adjusted for different board layouts. C1 22p X1 18pF Parallel Crystal C2 22p XTAL_OUT XTAL_IN |
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