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| AN1007 CRYSTAL Oscillators HIGH-PERFORMANCE PRODUCTS Voltage Controlled and Overtone CRYSTAL Oscillators 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 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. 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 F R1 510 1 Qm 20pF 2D Q7 SK10/100EL16B 3 D* Q* 6 C2 L1 R4 50 C3 0.001F 4 VBB VEE 5 R2 510 R3 510 J2 VCC 8 J1 + C5 22 F Y1 C1 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 L1C1 L2 VCC C4 0.1 F + C5 22 F R1 510 R4 510 U1 Y1 C1 1Q 2 Q* 3D 4 D* 5 5 VBB VCC 10 J3 QHG 9 QHG* 8 VEE 7 EN* 6 R2 510 VEE R3 510 J2 L1 C2 R6 50 SK10/100EL16WXD C3 0.1F 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 K Varactor C1 L1 C5 C2 0.1 F U1 1 Qm VCC 8 J1 + C3 22F VCC R5 510K 0.1F R1 510 Y1 R4 1K 2D Q7 SK10/100EL16B 3 D* Q* 6 4 VBB VEE 5 R2 510 R3 510 J2 C4 0.1F 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 VR R8 510 K R6 100 K C6 Varactor C1 Y1 0.1F R5 510 R1 510 U1 1Q 2 Q* 3D R4 1 K 4 D* 5 5 VBB C3 0.1F VCC 10 QHG 9 QHG* 8 VEE 7 EN* 6 R2 510 R3 510 J2 J1 C4 0.1 F + C5 22F VCC SK10/100EL16WXD 1 JP1 2 1 JP2 2 R7 10 K 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 Semtech Corporation High-Performance Products Division Marketing Group 1111 Comstock Street Santa Clara, CA 95054 Phone: (408) 566-8776 FAX: (408) 727-8994 Revision 1/ November 27, 2001 4 www.semtech.com |
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