Crystal Oscillator Explained

Crystal Oscillator Explained


Hey friends, welcome to the YouTube channel
ALL ABOUT ELECTRONICS. In this video, we will learn about crystal
oscillator. Now, in the previous videos, we had seen the
different types of RC as well as the LC oscillators, which can be used for generating the frequencies
from audio to RF range. But in this type of oscillator, the frequency
can drift due to the change in temperature or the change in the power supply voltage. And the frequency of the oscillation will
change even if there is a slight variation in the component values. So, in applications where the high level of
stability is required, the crystal oscillators are the obvious choice. So, not only this crystal provides a very
high level of stability, but it also provides good selectivity. Because the crystal which is used in this
oscillator has a very high-quality factor. Typically something like 10000 to 20000. And some crystals have an even higher quality
factor. So, because of this desired properties, these
crystal oscillators are used in radio and telecommunications. As well as they are part of many digital circuits. And they are used in smartphone and desktop
computers for generating the stable clock frequency. Similarly, it is the essential part of the
microcontrollers for generating the clock signals. So, this crystal can generate the stable frequencies
from 100s of kHz to even 100s of MHz. So, in this video, let’s find out how this
crystal oscillator works and let’s also see the different parameters related to this crystal
oscillator. So, the crystal oscillator works on the principle
of inverse piezo-electric effect. And it is made up of piezo-electric material. So, first of all, let’s understand this piezo-electric
effect. So, whenever some external voltage is applied
to certain materials then they produce the mechanical deformation. So, suppose if we apply the AC signal of particular
frequency, then this material starts vibrating at the same frequency. And this effect is known as the inverse piezoelectric
effect. On the other end, whenever we apply the external
force to this piezo-electric material, then they generate the voltage across the two terminals. So, somehow if we mechanically force them
to vibrate a the certain frequency, then they can generate the AC signal of the same frequency. And this effect is known as the piezoelectric
effect. And the materials which show this effect are
known as the piezoelectric materials. So, Rochelle salt, Quartz, and tourmaline
are the few examples of naturally occurring crystals which posses this piezoelectricity. And among this materials, the Rochelle salt
has the maximum piezoelectric activity. Meaning that for the given applied voltage,
it generates the maximum vibration. But mechanically it is weakest and it can
break very easily. While on the other end, the Tourmaline has
the least piezoelectric activity, but it is strongest among the given list. While the quartz is the compromise between
the piezoelectricity of the Rochelle salt and the strength of the Tourmaline. And as it is very inexpensive and the easily
available, it is the most preferred material in the crystal design. Now, you might have seen this quartz crystal
which is often used in the electronic circuits. And here is the electronic symbol of this
quartz crystal. So, in the center, there is a quartz crystal,
and these two plates are the metallized electrodes which provide the electrical contact. So, if you see the electrical equivalent circuit
of this crystal, then it is nothing but the RLC circuit. So, here this Cs is nothing but the motional
capacitance. So, this capacitance depends on the elasticity
of this quartz material. Apart from that, it also depends on the area
of the plates as well as the thickness of this quartz material. So, the crystals which are used in the oscillator
are fabricated in the form of the wafer. And if you look inside this crystal, then
it looks like this. Then the next component in the equivalent
circuit is the series inductance, which is known as the motional inductance. So, basically, it defines the mechanical mass
of the quartz when it is vibrating. So, the value of this motional inductance
ranges from few Heneries to the milli-Henrey. and it also depends on the thickness of this
quartz material. Then the next parameter in this equivalent
circuit is the series resistance. which is also known as the equivalent series
resistance. And it defines the real resistive loss which
happens in the crystal. So, the typical value of this series resistance
varies from few ohms to the 100s of kilo-ohms. And it is a function of crystal frequency. Then the next component in the equivalent
circuit is the shunt capacitance. And this capacitance exists because of the
electrode plates which is used for the electrical contact. So, this is the electrical equivalent circuit
of the quartz crystal. So, as you can see, this quartz crystal acts
as an LC tank circuit. And because of that, it provides the frequency
selectivity whenever it is used with the amplifier in the feedback circuit. And using this we can generate the oscillations
at the specific frequency. Now, the resonating crystal which is used
in the feedback of the oscillator has two resonant frequencies. The one is series resonant frequency and the
second is parallel resonant frequency. So, while selecting the quartz crystal for
the specific application, we need to decide at which resonant frequency we are going to
operate this quartz crystal. So, as you can see from the graph, at the
series resonant frequency, the impedance offered by the crystal will be minimum. While at the parallel resonant frequency,
the impedance will be maximum. and if you observe over her between this series
and the parallel resonant frequency, the impedance of this crystal will be inductive. While if you go above or below this series
and the parallel resonant frequency, it will be capacitive. So, the series resonant frequency can be given
by this expression. That is fs is equal to 1/ (2*pi*√Ls*Cs)
While the parallel resonant frequency fp can be given by the expression, 1/ (2*Pi*√Ls*Ceq)
Where the Ceq is the parallel combination of this Cp and Cs. Now, one more thing if you observe in the
graph, the parallel resonant frequency is always above this series resonant frequency. And this resonant frequency depends on how
this crystal is cut during the fabrication. And it also depends on the thickness of this
crystal. So, smaller the thickness of this crystal,
the larger will be the resonating frequency. But if we go above a certain frequency, then
it is not possible to reduce the thickness of this crystal. So, in such case, the crystal is operated
at the overtone frequencies instead of the fundamental frequency. So, in simple terms, this overtone is approximately
the odd harmonics of the fundamental frequency. So, by operating this crystal at the overtones
it is possible to generate the frequencies in the range of 100s of Mhz. So, now let’s see using this crystal, how
we can design the crystal oscillator. Now, whenever this crystal is used in the
feedback of this amplifier, then it provides the 180 degrees of phase shift. And the remaining 180 degrees of phase shift
is introduced by the amplifier circuit so that the overall phase shift is equal to 360
degrees. And the loop gain of this crystal oscillator
is set in a such a way that we can get the sustained oscillations. Now, as I said, the crystal can be operated
at either series or parallel resonant frequency. So, in this first circuit, the crystal is
operated at the series resonant frequency. So, in this circuit, the transistor is used
as an amplifier. And through this crystal, the feedback is
provided in the circuit from the collector to the base terminal. So, at series resonance, the impedance that
is offered by the crystal will be minimum. Or we can say that the feedback which is provided
from the output to the input side will be maximum. And by setting the gain at this resonant frequency,
we can use this circuit for generating the sustained oscillations. So, now let’s see few circuits in which this
crystal can be operated at the parallel resonance. So, whenever the crystal is used in the parallel
resonance mode, then it is operated between the series and the parallel resonance frequency. And it will act as an inductor in the given
circuit. So, this circuit which is shown over here
is the Colpitts oscillator. And we have already discussed this circuit
in the earlier video. But here instead of using the separate inductor,
the crystal is used as an inductor. So, by using this crystal at the parallel
resonance, this feedback combination of C1, C2 and the crystal will act as a tank circuit. And it provides the frequency selectivity
which is required for the given circuit. So, this is one of the circuits in which the
crystal is operated at the parallel resonance. Then the next circuit which we are going to
see the Pierce oscillator. So, here this circuit is designed using the
CMOS inverter. And in this circuit, the crystal is used in
the parallel resonance mode. So, because of that, it acts as an inductor. And the combination of this C1, C2, and the
crystal will act as an LC tank circuit. And it provides the necessary frequency selectivity
for the given circuit. Now, in this circuit, if you observe, the
feedback resistor Rf is connected between the input and output of this CMOS inverter. And due to that, this CMOS inverter operates
in the linear range of this voltage transfer curve. So, due to this resistor, this CMOS inverter
acts as an amplifier. Now, the value of this feedback resistor Rf
depends on the operating frequency. But usually, it used to be more than 1 Mega
ohm. Now, in this circuit, this series resistor
Rs reduces the overtone oscillations and it also improves the start-up response of this
oscillator. So, using this circuit with this crystal,
we can generate the oscillations at the desired frequency. Now, whenever this Pierce oscillator is designed
using the CMOS inverter, then the output of this oscillator will be a square wave. And that’s why this Pierce oscillator is most
often used in digital circuits. And the same oscillator is also used in the
microcontroller and the processors. But in this oscillator, the series resistor
Rs and the feedback resistor Rf are internal to the microcontroller. So, just by connecting the crystal and the
external capacitors, we can generate a stable clock of the desired frequency. Now, whenever the crystal is used in the parallel
resonant mode, then manufacturers used to provide the load capacitance for the specific
frequency. And this load capacitance is the equivalent
capacitance which is seen across the crystal terminals. So, while designing the crystal oscillator
for the specific frequency, this parameter is a very important parameter. so, the equivalent capacitance which is seen
by the crystal should almost match which this load capacitance, so that the crystal can
operate at the specific frequency. So, if the equivalent capacitance which is
seen by the crystal just above or below this load capacitance, in that case, this crystal
will not operate at the desired frequency. So, for the given pierce oscillator, if we
assume that, the input and the output capacitance are zero, in that case, the equivalent capacitance
which is seen across this crystal will be equal to the parallel combination of this
C1 and C2. which is C1*C2 /(C1 + C2). And if we assume we have input and output
capacitance across the CMOS inverter, in that case, the equivalent capacitance will be equal
to (Cin + C1)(Co + C2) / (C1 +C2 +Cin +Co)
And if there is a stray capacitance in the circuit, then it will also get added with
this combination. So, this will be the equivalent capacitance
which is seen by this crystal. And to operate this crystal at the desired
frequency, this equivalent capacitance should be matched with this load capacitance. So, this is all about the different circuits
using which we can design the crystal oscillator. Now, before ending this video, let’s see few
parameters which needs to be considered while selecting the crystal for the specific oscillator. So, the first thing is the crystal frequency. And it defines the frequency at which the
crystal is going to get operated. then the next parameter which needs to be
considered is the drift in the frequency. And it defines the over the period of time
how the crystal frequency is going to change. And we need to see for the given application,
how much drift in the frequency we can tolerate. Then the next thing which we need to consider
is the mode of operation. That means whether the crystal is going to
operate at the fundamental frequency mode or at the overtones. So, depending on the mode of operation, the
complexity of the circuit will get decided. Then we need to see the temperature stability
of the oscillator. That means over the temperature range how
the crystal frequency is going to change. then the next parameter which we need to consider
is the drive level. Or the power dissipation across the crystal. So, we need to select a crystal in such a
way that the drive level or the power dissipation is less than the rated value. So, these are the few parameters which we
need to consider while selecting the crystal for the specific application. So, I hope in this video, you understood the
working of the crystal as well as using this crystal how we can design the crystal oscillator. So, if you have any question or suggestion,
do let me know here in the comment section below. If you like this video, hit the like button
and subscribe to the channel for more such videos.

31 thoughts to “Crystal Oscillator Explained”

  1. The timestamps for the different topics covered in the video:

    0:21 why crystal oscillator is used in certain applications

    1:58 Working principle of crystal and different piezo-electric materials

    4:05 The equivalent circuit of crystal and discussion on series and parallel resonant frequencies in the crystal

    8:38 Crystal oscillator design using series resonance of the crystal

    9:30 Colpitts oscillator using crystal

    10:20 Pierce Oscillator using crystal

    14:12 Things to consider while selecting crystal for crystal oscillator / for a particular application

  2. Can you reduce your Indian accent a little bit to the General English because your content is very useful but the accent makes me really hard to understand what you want to deliver

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  4. You have one of the most annoying Indian accents I've ever heard. All Indian accents are annoying but you sir, take the cake

  5. Good info but I need advice to change a crystal frequency of a AM transmitter from 1620 khz to 1000 khz …I have found 1MHZ crystal, what capacitor i need to change it's a 30-50 watts with lamp final

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