ICS8535-11 Datasheet, PDF (8/14 Pages) Integrated Circuit Systems – LOW SKEW, 1-TO-4, CRYSTAL OSCILLATOR/ LVCMOS-TO-3.3V LVPECL FANOUT BUFFER

Integrated
Circuit
Systems, Inc.
ICS8535-11
LOW SKEW, 1-TO-4, CRYSTAL OSCILLATOR/
LVCMOS-TO-3.3V LVPECL FANOUT BUFFER
APPLICATION INFORMATION
CRYSTAL OSCILLATOR CIRCUIT FREQUENCY FINE TUNING
A crystal can be characterized for either series or parallel mode operation. The ICS8535-11 and ICS8533-11 fanout buffers have
built-in crystal oscillator circuits that can accept either a series or parallel crystal without additional components. The frequency
accuracy provided by this configuration is sufficient for most computer applications.
For applications requiring highly accurate clock frequencies, the output frequency can be fine tuned by inserting a small series
capacitor C1 at the XTAL1 input (Pin 6 for ICS8535-11) as shown in Figure 7. This fine tuning approach can be applied in either
parallel or series crystal. The C1 value depends on the crystal type, frequency and the board layout. The parallel crystal fine tuning
results in smaller ppm and better performance. It is difficult to provide the precise value of C1. This section provides recommended
series capacitor C1 values to start with. This example uses 18pF parallel crystals.
Figure 7 shows the suggested series capacitor value for a parallel crystal. For a 16.666 MHz crystal, the recommended C1 value is
about 33pF.
Figure 8 shows frequency accuracy versus series capacitance for 19.44MHz, 16.666MHz and 15MHz crystals. As seen from this
figure, a 24pF, 33pF and 43pF series capacitor is used to achieve the lowest ppm error for 19.44MHz, 16.666MHz and 15MHz
respectively.
Figure 9 shows the experiment results of crystal oscillator frequency drift due to temperature variation.
U1
XTAL2
X1
C1
XTAL1
FIGURE 7 - CRYSTAL INTERFACE WITH SERIES CAPACITOR C1.
8535AG-11
www.icst.com/products/hiperclocks.html
8
REV. B JULY 27, 2001