8535AG-11

www.icst.com/products/hiperclocks.html

REV. B JULY 27, 2001

8

Integrated

Circuit

Systems, Inc.

ICS8535-11

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.

X1

U1

XTAL2

XTAL1

C1