Parasitic oscillation
Encyclopedia
Parasitic oscillation is an undesirable oscillation caused by feedback
. The problem occurs notably in RF
, audio
, and other electronic amplifier
s as well as in digital signal processing
. It is one of the fundamental issues addressed by control theory
.
Parasitic oscillation is undesirable for several reasons. The oscillations may radiate from the circuit, causing interference to other devices. The oscillations waste power and may cause undesirable heating. For example, an audio power amplifier that goes into parasitic oscillation may generate enough power to damage connected speakers
. A circuit that is oscillating will not amplify linearly, so desired signals passing through the stage will be distorted. In digital circuits, parasitic oscillations may only occur on particular logic transitions and may result in erratic operation of subsequent stages; for example, a counter stage may see many spurious pulses and count erratically.
or mutual inductance
between input and output. In some solid-state or vacuum electron devices there is sufficient internal capacitance to provide a feedback path. Since the ground
is common to both input and output, output current flowing through the impedance of the ground connection can also couple signals back to the input.
Similarly, impedance in the power supply can couple input to output and cause oscillation.
When a common power supply is used for several stages of amplification, the supply voltage may vary with the changing current in the output stage. The power supply voltage changes will appear in the input stage as positive feedback. An example is a transistor radio
which plays well with a fresh battery, but squeals or "motorboats" when the battery is old.
In audio systems, if a microphone is placed close to a loudspeaker, parasitic oscillations may occur. This is caused by positive feedback, from amplifier's output to loudspeaker to sound waves, and back via the microphone to the amplifier input. See Audio feedback
.
developed to address the problem of parasitic oscillation in servo
control systems – the system oscillated rather than performing their intended function, for example velocity control in engines. The Nyquist stability criterion
for oscillation is that the product of the stage gain multiplied by the transfer function
of the inadvertent feedback network is greater than 1.
In practice, feedback may occur over a range of frequencies (for example the operating range of an amplifier); at various frequencies, the phase of the amplifier may be different. If there is one frequency where the feedback is positive and the amplitude condition is also fulfilled – the system will oscillate at that frequency.
These conditions can be expressed in mathematical terms using the Nyquist plot
. Another method used in control loop theory uses Bode plot
s of gain and phase vs. frequency. Using Bode plots, a design engineer checks whether there is a frequency where both conditions for oscillations are met: the phase is zero (positive feedback
) and the loop gain is 1 or greater.
When parasitic oscillations occur, the designer can use the various tools of control loop engineering to correct the situation – to reduce the gain or to change the phase at problematic frequencies.
signals and prevent inter-stage coupling through the power supply. Where printed circuit board
s are used, high-power stages are separated from low-power stages and ground return traces are arranged so that heavy currents don't flow in mutually shared portions of the ground trace. In some cases the problem may only be solved by introduction of another feedback neutralization network, calculated and adjusted to eliminate the negative feedback within the passband of the amplifying device.
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
. The problem occurs notably in RF
Rf power amplifier
An RF power amplifier is a type of electronic amplifier used to convert a low-power radio-frequency signal into a larger signal of significant power, typically for driving the antenna of a transmitter...
, audio
Audio amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...
, and other electronic amplifier
Electronic amplifier
An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...
s as well as in digital signal processing
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
. It is one of the fundamental issues addressed by control theory
Control theory
Control theory is an interdisciplinary branch of engineering and mathematics that deals with the behavior of dynamical systems. The desired output of a system is called the reference...
.
Parasitic oscillation is undesirable for several reasons. The oscillations may radiate from the circuit, causing interference to other devices. The oscillations waste power and may cause undesirable heating. For example, an audio power amplifier that goes into parasitic oscillation may generate enough power to damage connected speakers
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
. A circuit that is oscillating will not amplify linearly, so desired signals passing through the stage will be distorted. In digital circuits, parasitic oscillations may only occur on particular logic transitions and may result in erratic operation of subsequent stages; for example, a counter stage may see many spurious pulses and count erratically.
Causes of parasitic oscillation
Parasitic oscillation in an amplifier stage occurs when part of the output energy is coupled into the input, with the correct phase and amplitude to provide positive feedback at some frequency. The coupling can occur directly between input and output wiring with stray capacitanceCapacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
or mutual inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
between input and output. In some solid-state or vacuum electron devices there is sufficient internal capacitance to provide a feedback path. Since the ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
is common to both input and output, output current flowing through the impedance of the ground connection can also couple signals back to the input.
Similarly, impedance in the power supply can couple input to output and cause oscillation.
When a common power supply is used for several stages of amplification, the supply voltage may vary with the changing current in the output stage. The power supply voltage changes will appear in the input stage as positive feedback. An example is a transistor radio
Transistor radio
A transistor radio is a small portable radio receiver using transistor-based circuitry. Following their development in 1954 they became the most popular electronic communication device in history, with billions manufactured during the 1960s and 1970s...
which plays well with a fresh battery, but squeals or "motorboats" when the battery is old.
In audio systems, if a microphone is placed close to a loudspeaker, parasitic oscillations may occur. This is caused by positive feedback, from amplifier's output to loudspeaker to sound waves, and back via the microphone to the amplifier input. See Audio feedback
Audio feedback
Audio feedback is a special kind of positive feedback which occurs when a sound loop exists between an audio input and an audio output...
.
Conditions for parasitic oscillations
Feedback control theoryControl theory
Control theory is an interdisciplinary branch of engineering and mathematics that deals with the behavior of dynamical systems. The desired output of a system is called the reference...
developed to address the problem of parasitic oscillation in servo
Servomechanism
thumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...
control systems – the system oscillated rather than performing their intended function, for example velocity control in engines. The Nyquist stability criterion
Nyquist stability criterion
When designing a feedback control system, it is generally necessary to determine whether the closed-loop system will be stable. An example of a destabilizing feedback control system would be a car steering system that overcompensates -- if the car drifts in one direction, the control system...
for oscillation is that the product of the stage gain multiplied by the transfer function
Transfer function
A transfer function is a mathematical representation, in terms of spatial or temporal frequency, of the relation between the input and output of a linear time-invariant system. With optical imaging devices, for example, it is the Fourier transform of the point spread function i.e...
of the inadvertent feedback network is greater than 1.
In practice, feedback may occur over a range of frequencies (for example the operating range of an amplifier); at various frequencies, the phase of the amplifier may be different. If there is one frequency where the feedback is positive and the amplitude condition is also fulfilled – the system will oscillate at that frequency.
These conditions can be expressed in mathematical terms using the Nyquist plot
Nyquist plot
A Nyquist plot is a parametric plot of a transfer function used in automatic control and signal processing. The most common use of Nyquist plots is for assessing the stability of a system with feedback. In Cartesian coordinates, the real part of the transfer function is plotted on the X axis. The...
. Another method used in control loop theory uses Bode plot
Bode plot
A Bode plot is a graph of the transfer function of a linear, time-invariant system versus frequency, plotted with a log-frequency axis, to show the system's frequency response...
s of gain and phase vs. frequency. Using Bode plots, a design engineer checks whether there is a frequency where both conditions for oscillations are met: the phase is zero (positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
) and the loop gain is 1 or greater.
When parasitic oscillations occur, the designer can use the various tools of control loop engineering to correct the situation – to reduce the gain or to change the phase at problematic frequencies.
Mitigation
Several measures are used to prevent parasitic oscillation. Amplifier circuits are laid out so that input and output wiring are not adjacent, preventing capacitive or inductive coupling. A metal shield may be placed over sensitive portions of the circuit. Bypass capacitors may be put at power supply connections, to provide a low-impedance path for ACAlternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
signals and prevent inter-stage coupling through the power supply. Where printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
s are used, high-power stages are separated from low-power stages and ground return traces are arranged so that heavy currents don't flow in mutually shared portions of the ground trace. In some cases the problem may only be solved by introduction of another feedback neutralization network, calculated and adjusted to eliminate the negative feedback within the passband of the amplifying device.