DAN-108 datasheet(7/15 Pages) EXAR

7 / 15 page

DATA COMMUNICATIONS APPLICATION NOTE

DAN108

EXAR Corporation, 48720 Kato Road, Fremont, CA 94538 Tel 510-668-7000 - Fax 510-668-7017 www.exar.com

7. Crystal Oscillator Circuits

The attraction of a quartz crystal oscillator is its tightly controlled impedance characteristic. A

crystal with a desired series resonant frequency and parallel resonance curve can be

manufactured in mass production. Also, crystals have an extremely high Q factor. For instance a

4 MHz crystal commonly has a Q of about 26000.

By realizing that the impedance of a quartz crystal is solely resistive at Fs, an oscillator as shown

in Figure 7 can be made. Here a second inverter creates the required total 360

° phase shift.

Figure 7: Series Resonant Oscillator

The oscillation frequency is strongly determined by the crystal, since only at Fs does the crystal

add no phase shift. Since the crystal’s resistance is very small at Fs, the oscillator’s energy may

damage the crystal. Hence special crystals need be selected for series resonance use.

At frequencies slightly higher than Fs, the crystal’s impedance is inductive. Hence inserting the

crystal in a

π (pi) configuration with capacitors can achieve the additional 180° phase delay

necessary when used with a single inverter. Figure 8 shows the Parallel Resonant Oscillator.

DAN-108 Datasheet(PDF) 7 Page - Exar Corporation