“Most Electronic systems you design or use have one or more oscillators that provide frequency for synchronized operation, as a frequency reference or for accurate timing. This article will discuss the advantages of quartz crystal oscillators, as well as some of the options available.
Most electronic systems you design or use have one or more oscillators that provide frequency for synchronized operation, as a frequency reference or for accurate timing. This article will discuss the advantages of quartz crystal oscillators, as well as some of the options available.
In microprocessor-based systems, there are several different clock signals used to execute instructions, move data in and out of memory, and interface with external communications.
A simple embedded controller might have a clock frequency of a few MHz, while a microprocessor in a personal computer typically expects an input frequency of 15 MHz. This will be multiplied internally to provide the frequency of the CPU and other subsystems. Other components in the system may each have their own frequency requirements. For example, an Ethernet controller requires a frequency of 25 MHz, or a real-time frequency (RTC) of 32.768 kHz.
Radio frequency (RF) systems require accurate frequency references for end-to-end communication and filtering of unwanted signals and noise.
Key Oscillator Features
In addition to the basic need to provide a specified frequency, depending on your product application, the oscillator may have to meet other needs.
For example, many product applications require extremely precisely defined frequencies. This is especially important for systems that need to communicate with other devices via serial or wireless interfaces. Accuracy is usually measured in parts per million (ppm).
It is important to have low power consumption for handheld or battery powered devices. This is especially true for RTCs as this part of the circuit will always be active even in low power or standby mode.
Finally, you may need to consider various factors such as operating environment, cost and form factor.
Any oscillator uses some kind of resonant or trim circuit, with amplification and feedback to produce an output at a specific frequency.
Trimming circuits can be based on resistor-capacitor (RC) or Inductor-capacitor (LC) networks. These devices are relatively simple and can vary the frequency over a wide range. However, designing an accurate RC or LC oscillator requires the use of expensive precision components. Even so, they cannot meet the highest accuracy and stability required for many product applications.
A crystal (usually quartz) can also be used as a resonant component. The crystal was cut into two parallel crystal planes and metal contacts were deposited on them. Quartz has a piezoelectric effect, which means that when the crystal is placed under pressure, a voltage is generated on its facets. Conversely, when a voltage is applied to the crystal, the crystal also changes shape.
This feedback causes the crystal to oscillate at its natural resonant frequency. This will be determined by the size of the crystal and how it is cut. The most common cropping method is called AT. This can be used over a wide frequency range and has good thermal stability.
Crystal resonators have a high quality (Q) factor, which means that the frequency is well defined and very stable, so crystals can be used as the basis for low-cost, high-accuracy oscillators.
Figure 1 Crystal structure and equivalent circuit
The crystal resonator structure and equivalent circuit are shown in Figure 1. The value Cp represents the capacitance of the two parallel electrodes. The components Ls, Rs and Cs represent the mechanical properties (mass, internal friction and elasticity) of the crystal.
The equivalent circuit shows that there are two possible resonant frequencies: one is due to the series connection of Ls and Cs, and the other is due to the parallel connection of Cp and the inductor.
Series resonance definition formula:
Parallel resonance frequency formula:
The distance between these two frequencies is typically less than 1%, and the oscillator circuit
Defines which resonant mode to use. Most oscillators use parallel mode.
For high frequencies above about 75 MHz, the crystal can oscillate at multiples of the fundamental frequency or overtones.
Oscillator circuits are usually integrated into devices that require a frequency signal. For example, many microcontrollers and similar devices have two pins on which you can simply connect a crystal and a pair of ceramic capacitors to complete the circuit.
Figure 2 Oscillator Components and Stray Capacitance
The total load capacitance (CL) of the circuit needs to match the specified CL of the crystal. This consists of the ceramic capacitor plus any stray capacitance from the crystal package, oscillator input pins, and circuit board traces.
It is not easy to accurately calculate all the stray and parasitic capacitances in a circuit, so you can make an estimate (usually around 4 to 6 pF) and measure the output frequency to see if the capacitor value needs to be adjusted.
If the total CL is greater than the specified CL, the oscillation frequency will be reduced. If CL is too low, the frequency will be higher.
If CL is too high and low, the oscillator may not start at all.
Figure 3 Oscillator circuit
You can also use transistors or inverted logic gates as feedback amplifiers to build an external oscillator circuit, as shown in Figure 3. However, even though most crystal manufacturers provide design guidelines, designing a high-quality oscillator can be challenging; therefore, it may be simpler to purchase an off-the-shelf oscillator module. The oscillator module contains a crystal and all required components including load capacitors. This guarantees you a high-performance oscillator at a reasonable price. All you need to do is provide a suitable power supply.
Figure 4 Crystal oscillator module
Crystal oscillator modules are a good choice for product applications that require accurate and stable frequencies, such as Ethernet network interfaces or wireless communication systems.
change in theme
Since the frequency varies with the external CL, it is possible to construct a crystal oscillator that can adjust the output within a small range. This is useful, for example, in RF applications where the receiver needs to adjust its frequency to match the received signal.
Voltage Controlled Crystal Oscillators (VCXOs) use devices called varactors (or varactors) as load capacitors. The capacitance of the varactor varies with the applied control voltage, which in turn changes the frequency of oscillation.
The key parameter of the VCXO is the “pull rate”, which controls the voltage range and frequency jitter.
• Traction rate defines the frequency change for a given control voltage change. Larger values indicate that the oscillator can operate over a larger range, but smaller values indicate better stability and lower phase noise. The maximum adjustment range is typically around +/-200 ppm.
• The control voltage is typically 0 V to 2 or 3V.
• Frequency jitter can be higher than for fixed frequency oscillators, especially at the extremes of the adjustment range when operating in a limited state.
If you require more stability over the operating temperature range than a normal crystal oscillator can provide, you may want to use a temperature compensated crystal oscillator (TCXO). These are also available as ready-made modules with extensive parameters.
The TCXO contains a circuit that measures the ambient temperature and then generates a control voltage to adjust the frequency of the VCXO to compensate for the effects of temperature changes. The TCXO calculates the required control voltage based on the temperature frequency response curve of the crystal.
The TCXO module also usually includes its own voltage regulator, so the oscillator is not affected by changes in the external supply voltage.
Quartz crystals provide a highly accurate, stable, and low-cost frequency reference.
Crystals and crystal oscillators are available in a wide range of parameters and implementations to suit your product application needs.
Many devices incorporate oscillator circuits, making the design process very simple.
As an alternative, especially if higher quality frequencies are required, a crystal oscillator module can be used. They are generally more accurate and stable than integrated oscillators. Modules are also available with voltage controlled frequency or temperature compensation.