Crystal Oscillator Hey friends welcome to the kohiki In this article, we will learn about crystal oscillators. Now, in the previous articles, 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.
- why crystal oscillator is used in certain applications
- Working principle of crystal and different piezo-electric materials
- The equivalent circuit of crystal
- Crystal oscillator design using the series resonance of the crystal
- Colpitts oscillator using crystal
- Pierce Oscillator using crystal
- Selecting crystal for crystal oscillator / for a particular application
- YouTube Video
why crystal oscillator is used in certain applications
So, in applications where a 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 these 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 stable clock frequency.
Similarly, it is the essential part of the microcontrollers for generating the clock signals.
So, this crystal can generate stable frequencies from 100s of kHz to even 100s of MHz.
So, in this article, let’s find out how this crystal oscillator works, and let’s also see the different parameters related to this crystal oscillator.
- Why do we use a crystal oscillator: In general, we know that crystal oscillators are used to provide clock signals in microprocessors and microcontrollers This crystal oscillator is used to generate the clock pulses needed for synchronization of all internal operations.
- Why crystal oscillator is best among all: Virtually all microprocessors, micro-controllers, PICs, and CPUs typically operate using a quartz crystal oscillator as a device to determine its frequency to generate its clock wave as we already know. , Crystal oscillators provide the highest accuracy and frequency stability compared to a resistance-capacitor.
Why are crystal oscillators preferred over other types of oscillators?
- A crystal oscillator is compact and inexpensive
The compact size and low cost of crystals are due to being used in various industries. Any product needs precise timing and measurement that can take advantage of these oscillators.
Working principle of crystal and different piezo-electric materials
So, the crystal oscillator works on the principle of the inverse piezoelectric effect. And it is made up of piezoelectric 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 mechanical deformation.
So, suppose if we apply the AC signal of a 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 piezoelectric material, then they generate the voltage across the two terminals.
So, somehow if we mechanically force them to vibrate a 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 piezoelectric materials.
So, Rochelle salt, Quartz, and tourmaline are the few examples of naturally occurring crystals that possess this piezoelectricity.
And among these materials, the Rochelle salt has the maximum piezoelectric activity. Meaning that the given applied voltage 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 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 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 metalized electrodes that provide the electrical contact.
- What is the working principle of piezoelectric transducer: The piezoelectric transducer works with the principle of piezoelectric. The face of the piezoelectric material is coated with a thin layer of conducting material such as ordinary quartz, silver. When the stress moves the ions into the material, it moves towards the surface of the conductor, moving away from the other.
- What is the utility of piezoelectric crystals: Piezoelectric ignitors are commonly used for butane lighters, gas grills, gas stoves, blowtorches, and instant potato cannons. Power generation – Some applications require energy harvesting from pressure changes, vibrations, or mechanical impulses.
- What is true about piezoelectric crystal: What is a piezoelectric crystal? The piezoelectric crystal is one of the small-scale energy resources. When these crystals deform automatically they produce a small voltage known as piezoelectricity. Such renewable energy may not be suitable for industrial conditions.
- What is the operating: mechanism of the piezoelectric pressure sensor: Piezoelectricity is the charge produced in some materials when mechanical stress is applied. Piezoelectric pressure sensors exploit this effect by measuring the voltage in the piezoelectric element generated by the applied pressure. They are very strong and are used in a wide range of industrial applications.
- Is piezoelectric AC or DC: A vibrating piezoelectric device generates an AC voltage, while electrochemical batteries require a DC voltage, so the first step required in an energy harvesting circuit is an AC-DC rectifier connected to the output of the piezoelectric device.
The equivalent circuit of crystal
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 dx, which is known as the emotional 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 Henries 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 the 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 a specific frequency.
Now, the resonating crystal which is used in the feedback of the oscillator has two resonant frequencies. The one is the series resonant frequency and the second is the 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, the 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 a 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 360degrees.
And the loop gain of this crystal oscillator is set in such a way that we can get the sustained oscillations.
- What is crystal in the circuit: Crystal is the general term used in electronics for the frequency-determining component, which is associated with quartz crystals or ceramics. A more accurate term for this is a piezoelectric resonator. Crystals are also used in other types of electronic circuits, such as crystal filters.
- What is the output of the crystal oscillator: The 16 MHz crystal oscillator module is designed to handle off-chip crystals with a frequency of 4œ16 MHz. The output system of the crystal oscillator is fed to the PLL as an input reference. The oscillator design produces low frequency and phase jitter, which are recommended for USB operation.
- What frequency do crystals vibrate at: As with molecules, quantum mechanics requires that the vibrational energy in a crystal is gained or lost in discrete packets, that of energy, which is related to hν, where h is the Planck constant (6.626×10- 34 J • seconds) and ν is the frequency of vibration.
Crystal oscillator design using the series resonance of the crystal
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.
- How do you make a crystal oscillator
The Following Factors Is necessary For crystal oscillator design
- Series circuit.
- Load Capacitance.
- Parallel Circuit.
- Drive Level.
- Frequency vs mode.
- Design Considerations.
- Negative Resistance.
- What is a crystal oscillator with a circuit diagram: The crystal oscillator circuit works on the principle of inverse piezoelectric effect, namely, that in some materials a mechanical deformation is produced by applying an electric field. quartz crystal oscillator circuit diagram consists of series resonance and parallel resonance.
Colpitts oscillator using crystal
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 article.
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.
- What is the difference between crystal and oscillator: An oscillator is any device or circuit that periodically generates oscillating electrical signals typically sine wave or square wave, A crystal is a piece of piezoelectric material that produces an oscillating sinusoidal electric signal due to the mechanical vibration of its structure.
Pierce Oscillator using crystal
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 Rfdepends on the operating frequency. But usually, it used to be more than 1 Megaohm.
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 that 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 this load capacitance, so that the crystal can operate at the specific frequency.
So, if the equivalent capacitance 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 thisC1 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 that is seen by this crystal. And to operate this crystal at the desired frequency, this equivalent capacitance should be matched with this load capacitance.
- How does a Pierce oscillator work: In this simple circuit, the crystal determines the frequency of oscillations and operates at its series resonant frequency, giving a low impedance path between output and input, These two capacitors determine the value of the crystal load capacitance.
- How do you probe the crystal oscillator: Bring the multimeter’s measurement probe into contact with the metal leg of the crystal oscillator. A probe should touch each leg. The multimeter should now read a frequency that corresponds to the one written on the crystal oscillator casing.
Selecting crystal for crystal oscillator / for a particular application
So, this is all about the different circuits using which we can design the crystal oscillator. Now, before ending this article, let’s see few parameters which need 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 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.
What is the use of a crystal oscillator?
What is the principle of crystal oscillator?
What are the advantages of crystal oscillator?
Stability. Stability is one of the most important requirements of any oscillator. …
High Q. The Q factor or quality factor describes how ‘underdamped’ oscillators are. …
Frequency Customization and Range. …
Low Phase Noise. …
A Crystal Oscillator Is Compact and Inexpensive.
What is 16 MHz crystal oscillator?
How do I choose a crystal oscillator?
Frequency Stability and Temperature Range. The required frequency stability is determined by the requirements of the system.
So Here a youtube video based on crystal oscillator, Which was uploaded by Ekeeda
So, I hope in this article, 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 questions or suggestions, do let me know here in the comment section below. If you like this article, hit the like button and subscribe to the channel for more such articles.