MC13175D データシートの表示（PDF） - LANSDALE Semiconductor Inc.

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MC13175D

UHF FM/AM Transmitter

LANSDALE Semiconductor Inc. 

LANSDALE Semiconductor, Inc.

ML13175/ML13176

Legacy Applications Information

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

Figure 20. Crystal Equivalent Circuit

L3

Cp

R3

C3

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.

Page 11 of 16

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