AN1007 Crystal Oscillators HIGH-PERFORMANCE PRODUCTS Voltage Controlled and Overtone Crystal Oscillators These reference clocks can also be obtained with a PLL based IC devices. Some of the most popular reference frequencies are 155.52 MHz (OC-3) and 622.08 MHz (OC-12) which can be obtained using Semtech’s PLL based devices mentioned in Table 1. Crystal oscillators are used to provide reference frequency clocks for applications such as SONET, Gigabit Ethernet, or fiber channel. Crystal oscillator manufacturers can design oscillators with accurate signal waveforms of ±10 ppm or better of the reference frequency. In this application note only examples of overtone and voltage controlled crystal oscillators are provided in figures 1, 2, 3, and 4, respectively. Device Function Package Type Operating Voltage SK1801W SONET OC - 3 Frequency Synthesizer 16 Pin Wide Body SOIC 3.0V to 5.5V SK1802W SONET OC - 12 Frequency Synthesizer 16 Pin Wide Body SOIC 3.0V to 5.5V Table 1 L2 VCC + C4 0.1 µF Y1 C1 C5 22 µF R1 510 Ω 1 Qm VCC J1 8 20pF 2 D Q 7 SK10/100EL16B 3 D* Q* 6 C2 L1 R4 4 VBB VEE 5 R2 510 Ω 50 Ω C3 0.001µF R3 510 Ω J2 VEE Figure 1: Overtone Crystal Oscillator Revision 1/ November 27, 2001 1 www.semtech.com AN1007 HIGH-PERFORMANCE PRODUCTS Ovelrtone Crystal Oscillators Overtone operation is accomplished by adjusting the tank circuit frequenct at or near the desired frequency. Using formula 1, we can calculate the values of L1 and C1 for a desired frequency. f≅ 1 2 π √ L1C1 L2 VCC R1 510 Ω + C4 0.1 µF R4 510 Ω C5 22 µF U1 Y1 L1 C2 C1 1 Q VCC 10 2 Q* QHG 9 3 D QHG* 8 4 D* VEE 7 55 VBB EN* 6 J3 R6 50 Ω R2 510 Ω SK10/100EL16WXD C3 0.1µF R3 510 Ω J2 VEE 1 JP1 2 1 JP2 2 R5 10K Figure 2: Overtone Crystal Oscillator VCC Revision 1/ November 27, 2001 2 www.semtech.com AN1007 HIGH-PERFORMANCE PRODUCTS Voltage Controlled Crystal Oscillator A voltage-variable capacitance tuning diode is placed in series with the crystal feedback path. Changing the voltage on VR varies the tuning diode capacitance and tunes the oscillator. The 510 KW resistor, R5, establishes a reference voltage for VR and the 100 KW resistor, R6, isolates the tuning voltage from the feedback loop and a 0.1 mF capacitor, C5, provides AC coupling to the tuning diode. The center frequency is set with a trimmer capacitor, C1. Deviation on either side of center is a function of the crystal frequency. VR R6 100 KΩ R5 510KΩ Varactor C1 L1 VCC C5 0.1µF R1 510 Ω Y1 C3 22µF U1 J1 VCC 8 1 Qm R4 1KΩ + C2 0.1 µF 2D Q 7 SK10/100EL16B 3 D* Q* 6 VEE 5 4 VBB R2 510 Ω R3 510 Ω J2 C4 0.1µF Figure 3: Voltage Controlled Crystal Oscillator Revision 1/ November 27, 2001 3 www.semtech.com AN1007 HIGH-PERFORMANCE PRODUCTS Voltage Controlled Crystal Oscillator (con’d) L1 VCC VR R8 510 KΩ R6 100 KΩ C6 Varactor 0.1µF R5 510 Ω C4 0.1 µF R1 510 Ω + C5 22µF U1 C1 Y1 1 Q VCC 10 2 Q* QHG 9 3 D R4 1 KΩ C3 0.1µF J1 QHG* 8 4 D* VEE 7 55 VBB EN* 6 SK10/100EL16WXD R2 510 Ω R3 510 Ω J2 1 JP1 2 1 JP2 2 R7 10 KΩ VCC Figure 4: Voltage Controlled Crystal Oscillator Contact Information Division Headquarters 10021 Willow Creek Road San Diego, CA 92131 Phone: (858) 695-1808 FAX: (858) 695-2633 Revision 1/ November 27, 2001 Semtech Corporation High-Performance Products Division 4 Marketing Group 1111 Comstock Street Santa Clara, CA 95054 Phone: (408) 566-8776 FAX: (408) 727-8994 www.semtech.com