What is an oscillator and how can you design a circuit using it?

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What is an oscillator and how can you design a circuit using it? [Copy link]

The advent of synchronous digital systems has made the humble oscillator the heart of modern microprocessor-based digital systems. The thousands of applications for oscillators have created an extremely wide range of oscillator sources and configurations, using manifold resonator structures.
However, due to the wide variety of resonators and internal amplifiers, as well as several different temperature stabilization schemes, a full understanding of their purpose is often overlooked when selecting an oscillator. All of these factors affect the size, accuracy, stability, and cost of the device, as well as how they are used in a design.
This article will help designers better understand the operation and construction of oscillators, key specifications, and how to match them to design requirements.
It will also explore output waveforms, frequency accuracy and stability, phase noise, jitter, load and temperature variations, and cost, as well as how to best use oscillators for design success.

Oscillator Basics

An oscillator is an electronic circuit that produces a periodic waveform at a desired frequency. The functional block diagram of a general-purpose oscillator consists of an amplifier and a feedback path with a frequency-selective feedback network (Figure 1). Oscillations can be initiated and sustained if the loop gain is equal to or greater than unity at the desired oscillation frequency and the phase shift of the loop is a multiple of 2p radians. This is a positive feedback condition. The
frequency-dependent network can be an inductor-capacitor (LC) network or a resistor-capacitor (RC) network, but precision oscillators usually require a resonator. Resonator selection is one of the specifications that needs to be addressed, as each resonator has its own advantages and disadvantages.

Figure 1: The functional diagram of a basic oscillator consists of an amplifier and a frequency selective network or resonator in a positive feedback configuration.
Common resonators include quartz crystals, surface acoustic wave (SAW) filters, or microelectromechanical systems (MEMS).
When such an oscillator is first powered up, the only signal in the circuit is noise. At frequencies that satisfy the oscillation gain and phase conditions, the noise element circulates around the circuit loop and the amplitude gradually increases due to the circuit's positive feedback. The signal amplitude continues to increase until it is limited by the amplifier characteristics or an external automatic gain control (AGC) unit. At this point, the waveform of the oscillator output can be controlled, and common waveform choices include sine wave, clipped sine wave, or logic ("0" or "1") output. If a logic output is selected, the logic family (HCMOS, TTL, ECL, LVDS...) must also be selected.
Sine outputs are primarily used for carrier and local oscillator signal generation in communication-related applications, where spectral purity is a key consideration. Sine waveforms have significant power only at the fundamental frequency and almost no power at harmonic frequencies.
The key specification for an oscillator is frequency stability, which defines how well the oscillator maintains its frequency. A related specification is aging, which details how the oscillator frequency drifts over a relatively long time interval (usually a year). As application speeds increase, short-term variations in the oscillator phase have become a significant issue. This short-term phase variation is called the oscillator's phase noise. Phase noise is a frequency domain specification. The equivalent time domain specification includes phase jitter or time interval error.

Resonator
In a basic oscillator, the feedback network can be any of several resonant structures. The most common is a quartz crystal. A quartz crystal resonator uses the piezoelectric effect. A small voltage applied to the crystal causes it to deform, while forces applied to the crystal induce an electrical charge. This series of electromechanical interactions forms the basis for a very stable oscillator. This effect produces oscillations at a specific frequency that is related to the type of crystal, the geometric orientation in which the crystal is cut, and the size of the crystal.
The crystal is mounted between two electrodes, which form the input and output of the crystal resonator. Under these conditions, the crystal behaves like a highly selective LC circuit (Figure 2). It can be observed that the crystal in the mount can be represented by a series RLC circuit, indicating that the series resonant frequency of the crystal is controlled by the model components LS and CS. The parallel capacitor represents the capacitance of the mount and associated wiring. The parallel capacitance CP reacts with the series inductance LS to produce the parallel resonant frequency. In operation, the series resonance controls the resonator operation. The fundamental frequency of the crystal ranges from kilohertz (kHz) to about 200 megahertz (MHz).

Figure 2: Equivalent circuit model of a quartz crystal. The model components LS and CS determine the series resonant frequency, while LS, CS, and CP are used to determine the parallel resonance. (Image source: Digi-Key Electronics)
Another common resonator is the surface acoustic wave (SAW) resonator (Figure 3).

Figure 3: SAW filters/resonators use interdigital transducers mounted on a piezoelectric substrate to generate surface acoustic waves in the gap between the transducers, resulting in a frequency-dependent response at the output. (Image source: Digi-Key Electronics) A
SAW filter is a frequency-selective device that uses surface acoustic waves propagating along the surface of an elastic substrate. As shown in the figure, interdigital transducers (IDTs) formed by conductive pathways on the substrate generate and detect the SAWs. SAW filters/resonators operate in the 10 MHz to 2 GHz frequency range. The frequency depends on the size of the IDT element and the properties of the substrate material. The circuit model of a SAW device is similar to that of a quartz crystal. SAW resonators are inexpensive to manufacture and can be fabricated in small packages using photolithography. These oscillators are called SAW oscillators (SOs for short). The
last type of resonator technology to be discussed in this article is resonators based on microelectromechanical systems (MEMS). MEMS use standard semiconductor manufacturing processes to produce tiny mechanical components. The size of these devices ranges from microns to millimeters. Resonators similar to high-frequency tuning forks are designed to vibrate under electrostatic excitation. The chip structure of these resonators is combined with a programmable oscillator/controller integrated circuit (Figure 4).

Figure 4: MEMS oscillator modules combine a MEMS mechanical structure with an oscillator/controller IC in a single package. (Image source: SiTime)
The oscillator/driver provides excitation to the MEMS structure and feeds its output to a fractional-N phase-locked loop (PLL), which multiplies the output frequency of the MEMS device by a programmable factor “N.” A one-time programmable (OTP) memory is used to store module configuration parameters. Temperature compensation can be achieved by adjusting the output frequency within the PLL. The PLL can also be programmed to provide a digitally controlled frequency output for the oscillator. The
biggest advantage of MEMS oscillators is that they are not affected by mechanical shock and vibration, an important factor in mobile applications such as cell phones, cameras, and watches.

Oscillator Circuit Types
The circuit topologies of modular oscillators have evolved over decades, and there are many technologies available today. In almost all cases, the circuit improvements have been made to improve the accuracy and stability of the oscillator output frequency. The examples in the previous paragraph include non-quartz SAW and MEMS oscillators. The techniques applied to quartz oscillators can also be applied to any type of oscillator. These oscillators all operate with a nominal load capacitance of 15 picofarads (pF). Changes in load capacitance can affect the operating frequency.
The comparison of these topologies is based on a bare quartz crystal oscillator (XO) (Figure 5). This example is implemented using logic gates and includes varactor diodes to allow for adjustment. These simple oscillators exhibit frequency stability of approximately 20 - 100 parts per million (ppm).

Figure 5: A basic crystal oscillator implemented using logic inverters provides a feature that controls the voltage via a varactor diode in series with the quartz crystal. (Image source: Digi-Key Electronics)
Abracon’s ASV-10.000MHZ-LCS-T is a surface-mount crystal clock oscillator with digital outputs at HCMOS logic levels. The main advantages of this oscillator are its low cost and its ±50 ppm frequency stability, although other devices in this family of oscillators have stability specifications of 20 to 100 ppm. The primary source of frequency drift is temperature variation. Another source is crystal aging, or the change in frequency over time. The aging rate is proportional to the basic stability. In the case of this oscillator, the aging rate is ±5 ppm per year. XOs are suitable for general-purpose applications that do not require high frequency stability, including clock sources for microprocessors.
Temperature-compensated crystal oscillators, or “TCXOs” for short, add circuit components to compensate for temperature-related changes in the quartz resonator and amplifier (Figure 6).

Figure 6: Because quartz resonators and amplifiers are sensitive to temperature, TCXOs add temperature sensors and temperature compensation networks to correct for frequency drift. (Image source: Digi-Key Electronics)
Temperature sensors such as thermistors can be used to generate a correction voltage that is applied through an appropriate network to a variable voltage varactor diode in series with the crystal, thereby achieving frequency control. This is performed by varying the capacitive load of the quartz crystal. Temperature compensation can improve frequency stability by a factor of 20 or more.
Abracon's ASTX-H12-10.000MHZ-T is a typical TCXO with HCMOS output levels and a frequency stability specification of ±2 ppm, which costs about three times that of a basic XO.
Another way to stabilize temperature is to package the oscillator module in a temperature-controlled box (Figure 7), a topology known as an OCXO.

Figure 7: An OCXO keeps the oscillator’s temperature stable by enclosing the oscillator in a temperature-controlled chamber that is set to the zero slope of the crystal’s frequency-temperature curve. (Image source: Digi-Key Electronics) A
crystal oscillator is enclosed in a temperature-controlled chamber. The chamber is set to the zero slope of the crystal’s frequency-temperature curve, so that small temperature changes cause little or no change in the oscillator’s frequency. An OCXO can improve the stability of an oscillator by more than a thousand times. Such oscillators are useful in applications that require precise timing, such as navigation systems or high-speed serial data communications.
The Connor-Winfield DOC050F-010.0M is an OCXO with LVCMOS output levels. This oscillator has a specified frequency stability of ±0.05 ppm. The performance improvements come at the expense of higher power consumption (due to the addition of the temperature-controlled chamber), larger size, and higher cost (approximately 30 to 40 times that of an XO) relative to a basic crystal oscillator.
The MEMS oscillator discussed above is an example of a digitally controlled oscillator (DCXO).
SiTime’s SIT3907AC-23-18NH-12.000000X is a MEMS-based DCXO with LVCMOS logic outputs and 10 ppm frequency stability. This DCXO can program frequency changes using an internal PLL with a “pull” range of ±25 to ±1600 ppm.
Microcomputer controlled crystal oscillators (MCXOs) have the same frequency stability as OCXOs, but in a smaller package and with lower power requirements. MCXOs use one of two methods to stabilize their output frequency. The first method is to run the source oscillator at a higher frequency than the desired output and use pulse deletion to achieve the desired output frequency. The second method is to run the internal source oscillator at a frequency slightly lower than the desired output and add a correction frequency generated by an internal direct digital synthesizer (DDS) to the source output frequency.
IQD Frequency Products’ LFMCXO064078BULK is an HCMOS-compatible MCXO with a frequency stability of 0.05 ppm. The product family includes oscillators with critical fixed frequencies between 10 and 50 MHz in a physical footprint of only 88 mm3, requiring only 10 milliamps (mA) at 3.3 V, for a total power consumption of 33 milliwatts (mW).
Some applications require the frequency of the oscillator to be adjusted, which can be done digitally or with analog control. Analog control is accomplished using a voltage-controlled crystal oscillator (VCXO). Figure 5 shows how an oscillator can be adjusted by applying a voltage to a varactor diode in series with the resonator and changing its frequency by varying the load capacitance, which is the basic principle of a VCXO. An
example of a VCXO is the XLH53V010.000000I from Integrated Device Technology Inc., which offers HCMOS output levels and a frequency stability of ±50 ppm. The pull range of a VCXO shows the maximum frequency deviation that can be achieved by varying the control voltage. The pull range of this oscillator is ±50 ppm. For a nominal output frequency of 10 MHz, the pull range is ±500 Hz.
The SAW oscillator described in the resonator section is another low-cost oscillator with high reliability characteristics. EPSON's XG-1000CA 100.0000M-EBL3 is an example of an SO. These devices can be used in fixed frequency applications, such as remote control transmitters. They have good stability and jitter specifications, but the biggest benefit is reliability.

Choosing the Right Oscillator for Your ApplicationIn
general, applications that use an oscillator as a precision timebase require an oscillator with greater frequency stability. Thus, GPS-related applications are well suited to using an OCXO- or MCXO-based oscillator. If an application requires resistance to shock and vibration, an SO oscillator is best suited. Clocks for high-speed serial interfaces require low timing jitter. Cost is a factor in all designs and generally varies with the degree of frequency stability provided. Other factors, such as size or power requirements, depend on the device, depending on the technology used. These factors may require engineering trade-offs. Table 1 compares the key specifications of the oscillators discussed in this article to help you focus on the features and benefits of each oscillator.

Note:
Estimated values calculated based on phase noise
Startup/Steady-
StateTable 1: Typical parameters used to compare various oscillators. The selection of each parameter is based on design requirements and other factors, such as cost and availability at the time of design. (Table source: Digi-Key Electronics)The
oscillators in the table are listed in order of frequency stability. Note that while specific output frequencies are used in this article, all of these oscillators offer a range of output frequencies within their respective model families.

Conclusion
A good understanding of oscillator structure and operation can help designers focus on finding the right oscillator to meet their application needs. When designers select an oscillator for a design project, there are always engineering trade-offs involved, including cost, power, space, stability, and accuracy; but the wide variety of oscillators available today can minimize these trade-offs through off-the-shelf solutions.