crystaloscillator

1. Established as per the Section 2(f) of the UGC Act, 1956
Approved by AICTE, COA and BCI, New Delhi
Lecture 3.5
Crystal Oscillator
S c h o o l o f C o m p u t i n g a n d I n f o r m a t i o n Te c h n o l o g y

2. OUTLINE
Lecture Discussion: Types- Crystal Oscillators
Introduction and Circuit
Frequency
Condition for oscillations

3. RECAP OF PREVIOUS LECTURE
LC Oscillators:
Colpitts oscillator
Hartley oscillator

4. Crystal Oscillator
In trodu ction

5. CRYSTAL OSCILLATOR
Introduction
 Crystals used in electronic applications consist of a quartz wafer held between two metal
plates and housed in a package.
 Most communications and digital applications require the use of oscillators with extremely
stable output. Crystal oscillators are invented to overcome the output fluctuation
experienced by conventional oscillators.

6. CRYSTAL OSCILLATOR
Piezoelectric effect
 The quartz crystal is made of silicon oxide (SiO2) and exhibits a property called the
piezoelectric.
 When a changing alternating voltage is applied across the crystal, it vibrates at the
frequency of the applied voltage. In the other word, the frequency of the applied ac
voltage is equal to the natural resonant frequency of the crystal.
 The thinner the crystal, higher its frequency of vibration. This phenomenon is called
piezoelectric effect.

7. Crystal Oscillator
Circu it Diagram

8. CRYSTAL OSCILLATOR
Circuit Diagram (AC equivalent circuit of Crystal)
 The crystal can have two resonant frequencies:
 One is the series resonance frequency fS which
occurs when 𝑋𝐿 =
𝑋𝐶𝑠
.
𝑓𝑠 =
1
2𝜋 𝐿𝐶𝑠
 The other is the parallel resonance frequency fP
which occurs when the inductive reactance equals
the overall reactance of the equivalent circuit CM.
𝑓𝑃 =
1
2𝜋 𝐿𝐶𝑃
Where, 𝐶𝑃 =
𝐶𝑀×𝐶𝑆
𝐶𝑀+𝐶𝑆
and 𝐶𝑆 ≪ 𝐶𝑀, thus 𝐶𝑃 ≈ 𝐶𝑠
⟹ 𝑓𝑃 = 𝑓𝑠

9. CRYSTAL OSCILLATOR
Circuit diagram & Working
 The basic components required for crystal
oscillator are:
 Amplifier circuit &
 Tank circuit with quartz crystal
• The circuit operates as follows:
 The LC tank circuit is tuned to series
resonant frequency of the crystal.
 The crystal offers minimum impedance at the series resonant frequency and thus
allows the feedback signal to reach the input.
 A slight variation in frequency introduces very high impedance. The feedback signal
further attenuated to an extent that loop gain criteria is not met, and oscillations
stops.
 Thus, the oscillator can oscillate only at resonant frequency of the crystal,
significantly improving the frequency stability of the oscillator
Fig. Crystal controlled Colpitts oscillator

10. QUIZ TIME
10 MINUTES

11. THANK YOU