Crystal Oscillator Working And Resonant Frequency

What Is Crystal Oscillator 

To main­tain con­stant fre­quen­cy, piezo­elec­tric crys­tals are used with the LC or RC oscil­la­tors (or) in the place of LC or RC cir­cuits. Oscil­la­tors of this type are called crys­tal oscillator 

Effects Of Crystal Oscillator 

Due to the fol­low­ing effects, the LC and RC oscil­la­tors do not pro­duce more sta­ble oscillations 

1) The val­ue of R, L, and C which are fre­quen­cy­de­ter­min­ing fac­tors in these cir­cuits, will change­with temperature. 

(2) If any com­po­nent in the feed­back net­work ischanged, it will shift the oper­at­ing fre­quen­cy of the oscillator.

Piezo Electric Effect

The crys­tal has a pecu­liar prop­er­ty known as piezo elec­tric effect, which is expressed as 

(1) when mechan­i­cal stress is applied across its twofaces, the crys­tal vibrates and an ac volt­age is produced.

(2) when an ac volt­age is applied across its crystal,the crys­tal vibrates at the fre­quen­cy of the applied voltage.

The fre­quen­cy of the vibra­tion is equal to the res­o­nance fre­quen­cy of the crystal 

The equiv­a­lent elec­tri­cal cir­cuit of the crys­tal is men­tioned as 

1)when the crys­tal is not vibrat­ing it is equiv­a­lent to the capac­i­tance Cm known as mount­ing capacitance 

2) when a crys­tal vibrates it is equiv­a­lent to RLC series circuit 

It is actu­al­ly a col­pitts Oscil­la­tor mod­i­fied into a crys­tal Oscil­la­tor. The crys­tal replaces the induc­tor in the col­pitts Oscil­la­tor the resis­tance R1 and R2 pro­vide base bias and RE with CE pro­vide stabilization 

Crystal Oscillator Working 

The nat­ur­al fre­quen­cy of the LC cir­cuit is made near­ly equal to the nat­ur­al fre­quen­cy of the crystal 

When the sup­ply is switched ‘ON’, the capac­i­tor C1 is going to charge. When the capac­i­tor C1 is ful­ly charged, it dis­charges through crys­tal which pro­duces oscillation. 

The fre­quen­cy of the oscil­la­tion depends upon the val­ues of C1, C2, and the RLC equiv­a­lent val­ues of crys­tal. If the fre­quen­cy of the oscil­la­tion is equal to its crys­tal res­o­nant fre­quen­cy, the cir­cuit pro­duces more sta­ble oscillation. 

The crys­tal fre­quen­cy is inde­pen­dent of temperature.The C, feed­back net­work, pro­vides 180° phase shift, and also the CE ampli­fi­er pro­vides anoth­er 180° phase shift. Hence the total phase shift is 360°, which pro­vides pos­i­tive feedback. 

There­fore, con­tin­u­ous undamped oscil­la­tion is produced.A crys­tal oscil­la­tor always gen­er­ates high fre­quen­cy oscil­la­tions, range from 20KHz to 20MHz 

Crystal Oscillator Resonant Frequency 

Series Resonant Frequency 

When the XL is equal to XC of the RLC cir­cuit the imped­ance is equal to the resis­tance R [small­er]

Fs=1/2π√LCs

Parallel Resonant Frequency 

When the reac­tance of RLC series cir­cuit is equal to the reac­tance of capac­i­tor Cp the crys­tal offers a very high imped­ance to the exter­nal circuit 

Fp=1/2π√LCeq

Ceq=Cs Cp/Cs+Cp

The crys­tal oscil­la­tor has a very high val­ue of Q — factor 

The extreme­ly high val­ues of the Q fac­tor of a crys­tal pro­duces oscil­la­tions with high fre­quen­cy values 

Qual­i­ty fac­tor of a crystal 

Q=wL/R

Crystal Oscillator Symbol 

Advantages of crystal Oscillator 

• Excel­lent fre­quen­cy stability 
• sim­ple circuit 
• High qual­i­ty factor 

Disadvantages of crystal Oscillator

• Crys­tal is frag­ile type hence it is used only in low pow­er circuits 
• Not used as tuned Oscillator 

Crystal Oscillator Applications 

• Phase con­trol and tim­ing circuit 
• Trig­ger Circuit