ICS8431-11 Datasheet, PDF (8/13 Pages) Integrated Circuit Systems – 255MHZ, LOW JITTER, LVPECL FREQUENCY SYNTHESIZER

Integrated
Circuit
Systems, Inc.
CRYSTAL INPUT AND OSCILLATOR INTERFACE
The ICS8431-11 features an internal oscillator that uses an
external quartz crystal as the source of its reference frequency.
A 16MHz crystal divided by 16 before being sent to the phase
detector provides the reference frequency. The oscillator is a
series resonant, multi-vibrator type design. This design provides
better stability and eliminates the need for large on chip capacitors.
Though a series resonant crystal is preferred, a parallel resonant
crystal can be used. A parallel resonant mode crystal used in a
series resonant circuit will exhibit a frequency of oscillation a few
hundred ppm lower than specified. A few hundred ppm translates
to KHz inaccuracy. In general computing applications this level
of inaccuracy is irrelevant. If better ppm accuracy is required, an
external capacitor can be added to a parallel resonant crystal in
series to pin 25. Figure 1A shows how to interface with a crystal.
Figures 1A, 1B, and 1C show various crystal parameters
which are recommended only as guidelines. Figure 1A shows
how to interface a capacitor with a parallel resonant crystal.
Figure 1B shows the capacitor value needed for the optimum
PPM performance over various parallel resonant crystals.
Figure 1C shows the recommended tuning capacitance for a
various parallel resonant crystal.
ICS8431-11
255MHZ, LOW JITTER,
LVPECL FREQUENCY SYNTHESIZER
ICS8431-11
XTAL2
(Pin 26, SOIC)
XTAL1
(Pin 25, SOIC)
Optional
FIGURE 1A. CRYSTAL INTERFACE
FIGURE 1B. Recommended tuning capacitance for various parallel FIGURE 1C. Recommended tuning capacitance for various
resonant crystals.
parallel resonant crystal.
60
14.318
50
40
15.000
16.667
30
19.440
20.000
20
10
24.000
0
14 15 16 17 18 19 20 21 22 23 24 25
Crystal Frequency (MHz)
100
80
60
40
20
0
-20 0
-40
-60
-80
-100
10
20
30
40
Series Capacitor, C1 (pF)
50
60
19.44MHz
16MHz
15.00MHz
ICS8431CM-11
www.icst.com/products/hiperclocks.html
8
REV. A JULY 11, 2001