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MC13175D Scheda tecnica(PDF) 11 Page - LANSDALE Semiconductor Inc. |
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MC13175D Scheda tecnica(HTML) 11 Page - LANSDALE Semiconductor Inc. |
11 / 16 page LANSDALE Semiconductor, Inc. ML13175/ML13176 REFERENCE CRYSTAL OSCILLATOR (Pins 8 and 9) Selection of Proper Crystal: A crystal can operate in a num- ber of mechanical modes. The lowest resonant frequency mode is its fundamental while higher order modes are called overtones. At each mechanical resonance, a crystal behaves like a RLC series–tuned circuit having a large inductor and a high Q. The inductor Ls is series resonance with a dynamic capacitor, Cs determined by the elasticity of the crystal lattice and a series resistance Rs, which accounts for the power dissi- pated in heating the crystal. This series RLC circuit is in par- allel with a static capacitance, Cp which is created by the crystal block and by the metal plates and leads that make con- tact with it. Figure 20 is the equivalent circuit for a crystal in a signal res- onant mode. It is assumed that other modes of resonance are so far off frequency that their effects are negligible. Series resonant frequency, fs is given by; fs = 1/2π(LsCs)1/2 and parallel resonant frequency, fp is given by; fp = fs(1 + Cs/Cp)1/2 the frequency separation at resonance is given by; ∆f = fp–fs = fs[1 – (1+ Cs/Cp)1/2] Usually fp is less than 1% higher than fs, and a crystal exhibits an extremely wide variation of the reactance with frequency between fp and fs. A crystal oscillator circuit is very stable with frequency. This high rate of change of impedance with frequency stabilizes the oscillator, because any significant change in oscillator frequency will cause a large phase shift in the feedback loop keeping the oscillator on frequency. Legacy Applications Information Figure 20. Crystal Equivalent Circuit C3 R3 L3 Cp www.lansdale.com Page 11 of 16 Issue c |
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