In our modern era, digital logic has become the core of all the electronics circuits either in the form of an FPGA, microcontroller, microprocessor, or discreet logic. Digital systems use many components that must be interconnected to perform the required functions. The vital element for proper operation of such a digital system is a CLOCK signal that enables all these digital components to communicate and establish synchronization between them. Hence, we always need a source to generate this clock signal.
This source comes in the form of an oscillator. Although most of today’s microcontrollers have an integrated RC oscillator, the clock generated by such an internal RC oscillator is typically not good enough to support the precision required for communication with other modules in the system. Thus, an external oscillator is required that can provide a clock signal to the complete system and yet meet all the requirements for precision, signal integrity and stability.
This blog gives an idea on the various aspects of a Crystal Oscillator for accurate clock generation over temperature and time. But before we go about explaining the Crystal Oscillator, we need to know about Oscillators and its function
Basics of an Oscillator
In electronics, any circuit which is capable of generating a repetitive signal without any input can be termed an Oscillator. In simple words, an Oscillator converts DC energy into AC energy of the desired frequency. This oscillation frequency is determined by the constants of those elements which were used in designing of an oscillator.
For something to oscillate, energy needs to move back and forth between two forms. For example, in a Pendulum, energy moves between potential energy and kinetic energy. When the pendulum is at one end of its travel, its energy is all potential energy and it is ready to fall. When the pendulum is in the middle of its cycle, all of its potential energy turns into kinetic energy and the pendulum is moving as fast as it can. As the pendulum moves toward the other end of its swing, all the kinetic energy turns back into potential energy. This movement of energy between the two forms is what causes the oscillation.
Eventually, any physical oscillator stops moving because of friction. To keep it going, you have to add a little bit of energy on each cycle. In a pendulum clock, the energy that keeps the pendulum moving comes from the spring. The pendulum gets a little push on each stroke to make up for the energy it loses to friction.
An electronic oscillator works on the same principle.
There are various types of Oscillators available that can be used to build up oscillations, including RC oscillators, LC oscillators, and Crystal Oscillators. But when it comes to precision and accuracy over temperature and time, Crystal Oscillator are preferred because of their high Q (in the range of 104 to 106 as compared to 102 for LC) which aids in achieving better stability across temperature and time.
Let’s get to know more about the Crystal Oscillator
A Crystal Oscillator is an electronic oscillator circuit which uses inverse piezoelectric effect, ie when electric field is applied across certain materials it produces mechanical deformation. Thus it uses mechanical resonance of a vibrating crystal of piezoelectric materiel to create an electric signal with very precise frequency. They have high stability, quality factor, small size and low cost and this makes them superior over other resonators like LC circuit, ceramic resonator, turning forks etc.
This image show a Crystal oscillator commonly used in microcontrollers and microprocessors. Although the crystal has electromechanical resonance, we can represent the crystal action by an equivalent electrical resonant circuit as show below. The inductance and capacitance L1 and C1 represents electrical equivalents of crystal mass and compliance, while the resistance R1 represents the friction of crystal’s internal structure and C0 represents the capacitance formed due to mechanical moulding of the crystal.
From the circuit we can find that it can have two resonant frequencies, series resonance and parallel resonance. Series resonance occurs when the reactance produced by capacitance C1 and inductance L1 becomes equal and opposite. Thus during this condition impedance is very low, equal to resistance R1. Parallel resonance occurs when the reactance of series resonant leg becomes equal to reactance produced by capacitance C0. During this condition the crystal offers very high impedance to the external circuit.
Impedance versus frequency graph is shown below…..
Oscillator and stability
When it comes to Oscillators, there are many factors that can affect the frequency stability of the system such as aging, noise, temperature, sustaining circuit, tenability, magnetic field, humidity, power supply voltage, and shock. Some of these important factors are discussed below:
- Instabilities because of time
Instabilities because of time can be subdivided into two categories – Aging and Short-term instability. Aging is a systematic change in frequency observed over long periods of time due to internal changes in the oscillator. However, while this change in frequency is just a few PPM, it can be very important when dealing with systems with precise frequency requirements such as DTV, set-top boxes, etc. In contrast, short-term instabilities are random in nature and often can be termed as noise.
- Aging
There are multiple factors that can contribute to aging like mass transfer, stress on the crystal, thermal expansion, mounting force, bonding elements, drive level of the crystal, and DC bias.
- Short-term noise
The output of an ideal oscillator is a perfect sine wave. In an ideal system, however, because of random noise or flicker noise, deviations in the phase of the signal occur which cause the frequency to change in order to maintain the 2nπ phase condition. The phase slope dφ/df is directly proportional to QL and must be high for maximum frequency stability. For the phase slope to be high, Cm should be minimized. Thus, the steeper the slope of reactance v/s frequency in between fs and fp, the better the frequency stability.
- Instabilities because of temperature
The variation from the resonant frequency of a crystal is minimal at room temperature. However, as the temperature changes towards the extremes, the variation from the nominal frequency starts to increase and can go as high as a few tenths of ppm. This is acceptable for applications like computing. Where accuracy and precision is of vital concern in applications like navigation, radar, radio communication, satellite communication, etc., such a huge variation is not acceptable.
- Instabilities because of tunability
Making an oscillator tunable over a wide range of frequency can lead to instabilities. In order to achieve tunability, filters are used to reject unwanted modes of frequencies.
Types of Crystal Oscillators
There are different types of crystal oscillators as explained below. Based on the compensation techniques used to achieve higher precision and accuracy, crystals can be divided into further sub-categories. Some of the most commonly used are:
- Voltage Controlled Crystal Oscillator (VCXO)
A Voltage Controlled Crystal Oscillator uses the very basic characteristic of a crystal that it will resonate at its specified frequency only if the Load Capacitance (CL) at oscillator terminal matches a certain value normally known as CL_NOM (usually provided by the crystal manufacturer).
This feature of crystal oscillators is implemented in VCXOs where accurate tracking of the frequency is required within a very fine range, such for a digital set-top box, DTV, etc.
- Temperature Controlled Oscillator (TCXO)
A TCXO works on the same principle as that of a VCXO. A TCXO uses a temperature sensor to measure the temperature and applies a correcting signal to the Varactor diode to compensate for the change in frequency.
- Oven Controlled Oscillator
In this configuration, crystal and other temperature sensitive components are placed in a temperature-controlled chamber (oven), which is adjusted to the temperature where the crystal’s frequency vs. temperature has zero slope. Such oscillators can get the best possible stability in terms of temperature, on the order of 0.001ppm.
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