Relaxation oscillator

The blinking turn signal on motor vehicles is generated by a simple relaxation oscillator

In electronics a relaxation oscillator is a nonlinear electronic oscillator circuit that produces a nonsinusoidal repetitive output signal, such as a triangle wave or square wave.[1][2][3][4] The circuit consists of a feedback loop containing a switching device such as a transistor, comparator, relay,[5] op amp, or a negative resistance device like a tunnel diode, that repetitively charges a capacitor or inductor through a resistance until it reaches a threshold level, then discharges it again.[4][6] The period of the oscillator depends on the time constant of the capacitor or inductor circuit.[2] The active device switches abruptly between charging and discharging modes, and thus produces a discontinuously changing repetitive waveform.[2][4] This contrasts with the other type of electronic oscillator, the harmonic or linear oscillator, which uses an amplifier with feedback to excite resonant oscillations in a resonator, producing a sine wave.[7] The difference between the two types is that in a linear oscillator the circuit operates close to linearity, while in a relaxation oscillator one of the components, the switching device, operates in an extremely nonlinear fashion, in a saturated condition, during most of the cycle.[7]

The term relaxation oscillator is also applied to dynamical systems in many diverse areas of science that produce nonlinear oscillations and can be analyzed using the same mathematical model as electronic relaxation oscillators.[8][9][10] For example geothermal geysers,[11][12] networks of firing nerve cells,[10] thermostat controlled heating systems,[13] coupled chemical reactions,[9] the beating human heart,[10][13] earthquakes,[11] the squeaking of chalk on a blackboard,[13] the cyclic populations of predator and prey animals, and gene activation systems[9] have been modeled as relaxation oscillators. Relaxation oscillations are characterized by two alternating processes on different time scales: a long relaxation period during which the system approaches an equilibrium point, alternating with a short impulsive period in which the equilibrium point shifts.[11][10][12][14] The period of a relaxation oscillator is mainly determined by the relaxation time constant.[10] Relaxation oscillations are a type of limit cycle and are studied in nonlinear control theory.[15]

Electronic relaxation oscillators

The first relaxation oscillator circuit, the astable multivibrator, was invented by Henri Abraham and Eugene Bloch using vacuum tubes during World War 1.[16][17] Balthasar van der Pol originated the term, first distinguished relaxation oscillations from harmonic oscillations, and derived the first mathematical model of a relaxation oscillator, the influential Van der Pol oscillator model, in 1920.[18][19][17] Van der Pol borrowed the term relaxation from mechanics; the discharge of the capacitor is analogous to the process of stress relaxation, the gradual disappearance of deformation and return to equilibrium in a inelastic medium.[20]

Relaxation oscillators are generally used to produce low frequency signals for such applications as blinking lights, electronic beepers, horizontal deflection circuits and time bases for CRT oscilloscopes, and clock signals in some digital circuits. They are also used in voltage controlled oscillators (VCOs),[21] inverters and switching power supplies, dual-slope analog to digital converters, and in function generators to produce square and triangle waves. Relaxation oscillators are widely used because they are easier to design than linear oscillators, are easier to fabricate on integrated circuit chips because they do not require inductors like LC oscillators,[21][22] and can be tuned over a wide frequency range.[22] However they have more phase noise[21] and poorer frequency stability than linear oscillators.[2][21] Before the advent of microelectronics, simple relaxation oscillators often used a negative resistance device with hysteresis such as a thyratron tube, neon lamp, or unijunction transistor, however today they are more often built with dedicated integrated circuits such as the 555 timer chip.

Relaxation oscillators can be divided into two classes[12]

• Sawtooth oscillator: In this type the energy storage capacitor is charged slowly but discharged rapidly, essentially instantly, by a short circuit through the switching device. The charging period thus takes up virtually the entire period of the waveform. The voltage across the capacitor is a sawtooth wave, while the current through the switching device is a sequence of short pulses.
• Astable multivibrator: In this type the capacitor is both charged and discharged slowly through a resistor, so both the charge and discharge periods contribute to the period and the output waveforms consist of two parts. The voltage generated by the capacitor is a triangle waveform, while the voltage from the switching device is a square wave.

Pearson–Anson oscillator

Circuit diagram of a capacitive relaxation oscillator with a neon lamp threshold device
Main article: Pearson-Anson effect

This example can be implemented with a capacitive or resistive-capacitive integrating circuit driven respectively by a constant current or voltage source, and a threshold device with hysteresis (neon lamp, thyratron, diac, reverse-biased bipolar transistor,[23] or unijunction transistor) connected in parallel to the capacitor. The capacitor is charged by the input source causing the voltage across the capacitor to rise. The threshold device does not conduct at all until the capacitor voltage reaches its threshold (trigger) voltage. It then increases heavily its conductance in an avalanche-like manner because of the inherent positive feedback, which quickly discharges the capacitor. When the voltage across the capacitor drops to some lower threshold voltage, the device stops conducting and the capacitor begins charging again, and the cycle repeats ad infinitum.

If the threshold element is a neon lamp,[nb 1][nb 2] the circuit also provides a flash of light with each discharge of the capacitor. This lamp example is depicted below in the typical circuit used to describe the Pearson–Anson effect. The discharging duration can be extended by connecting an additional resistor in series to the threshold element. The two resistors form a voltage divider; so, the additional resistor has to have low enough resistance to reach the low threshold.

Alternative implementation with 555 timer

A similar relaxation oscillator can be built with a 555 timer IC (acting in astable mode) that takes the place of the neon bulb above. That is, when a chosen capacitor is charged to a design value, (e.g., 2/3 of the power supply voltage) comparators within the 555 timer flip a transistor switch that gradually discharges that capacitor through a chosen resistor (RC Time Constant) to ground. At the instant the capacitor falls to a sufficiently low value (e.g., 1/3 of the power supply voltage), the switch flips to let the capacitor charge up again. The popular 555's comparator design permits accurate operation with any supply from 5 to 15 volts or even wider.

Other, non-comparator oscillators may have unwanted timing changes if the supply voltage changes.

Comparator–based relaxation oscillator

Alternatively, when the capacitor reaches each threshold, the charging source can be switched from the positive power supply to the negative power supply or vice versa. This case is shown in the comparator-based implementation here.

A comparator-based hysteretic oscillator.

This relaxation oscillator is a hysteretic oscillator, named this way because of the hysteresis created by the positive feedback loop implemented with the comparator (similar to, but different from, an op-amp). A circuit that implements this form of hysteretic switching is known as a Schmitt trigger. Alone, the trigger is a bistable multivibrator. However, the slow negative feedback added to the trigger by the RC circuit causes the circuit to oscillate automatically. That is, the addition of the RC circuit turns the hysteretic bistable multivibrator into an astable multivibrator.

General Concept

The system is in unstable equilibrium if both the inputs and outputs of the comparator are at zero volts. The moment any sort of noise, be it thermal or electromagnetic noise brings the output of the comparator above zero (the case of the comparator output going below zero is also possible, and a similar argument to what follows applies), the positive feedback in the comparator results in the output of the comparator saturating at the positive rail.

In other words, because the output of the comparator is now positive, the non-inverting input to the comparator is also positive, and continues to increase as the output increases, due to the voltage divider. After a short time, the output of the comparator is the positive voltage rail, $V_{DD}$.

Series RC Circuit

The inverting input and the output of the comparator are linked by a series RC circuit. Because of this, the inverting input of the comparator asymptotically approaches the comparator output voltage with a time constant RC. At the point where voltage at the inverting input is greater than the non-inverting input, the output of the comparator falls quickly due to positive feedback.

This is because the non-inverting input is less than the inverting input, and as the output continues to decrease, the difference between the inputs gets more and more negative. Again, the inverting input approaches the comparator's output voltage asymptotically, and the cycle repeats itself once the non-inverting input is greater than the inverting input, hence the system oscillates.

Example: Differential Equation Analysis of comparator-based Relaxation Oscillator

Transient analysis of a comparator-based relaxation oscillator.

$\, \! V_+$ is set by $\, \! V_{out}$ across a resistive voltage divider:

$V_+ = \frac{V_{out}}{2}$

$\, \! V_-$ is obtained using Ohm's law and the capacitor differential equation:

$\frac{V_{out}-V_-}{R}=C\frac{dV_-}{dt}$

Rearranging the $\, \! V_-$ differential equation into standard form results in the following:

$\frac{dV_-}{dt}+\frac{V_-}{RC}=\frac{V_{out}}{RC}$

Notice there are two solutions to the differential equation, the driven or particular solution and the homogeneous solution. Solving for the driven solution, observe that for this particular form, the solution is a constant. In other words, $\, \! V_-=A$ where A is a constant and $\frac{dV_-}{dt}=0$.

$\frac{A}{RC}=\frac{V_{out}}{RC}$
$\, \! A=V_{out}$

Using the Laplace transform to solve the homogeneous equation $\frac{dV_-}{dt}+\frac{V_-}{RC}=0$ results in

$V_-=Be^{\frac{-1}{RC}t}$

$\, \! V_-$ is the sum of the particular and homogeneous solution.

$V_-=A+Be^{\frac{-1}{RC}t}$
$V_-=V_{out}+Be^{\frac{-1}{RC}t}$

Solving for B requires evaluation of the initial conditions. At time 0, $V_{out}=V_{dd}$ and $\, \! V_-=0$. Substituting into our previous equation,

$\, \! 0=V_{dd}+B$
$\, \! B=-V_{dd}$

Frequency of Oscillation

First let's assume that $V_{dd} = -V_{ss}$ for ease of calculation. Ignoring the initial charge up of the capacitor, which is irrelevant for calculations of the frequency, note that charges and discharges oscillate between $\frac{V_{dd}}{2}$ and $\frac{V_{ss}}{2}$. For the circuit above, Vss must be less than 0. Half of the period (T) is the same as time that $V_{out}$ switches from Vdd. This occurs when V- charges up from $-\frac{V_{dd}}{2}$ to $\frac{V_{dd}}{2}$.

$V_-=A+Be^{\frac{-1}{RC}t}$
$\frac{V_{dd}}{2}=V_{dd}(1-\frac{3}{2}e^{\frac{-1}{RC}\frac{T}{2}})$
$\frac{1}{3}=e^{\frac{-1}{RC}\frac{T}{2}}$
$\ln\left(\frac{1}{3}\right)=\frac{-1}{RC}\frac{T}{2}$
$\, \! T=2\ln(3)RC$
$\, \! f=\frac{1}{2\ln(3)RC}$

When Vss is not the inverse of Vdd we need to worry about asymmetric charge up and discharge times. Taking this into account we end up with a formula of the form:

$T = (RC) \left[\ln\left( \frac{2V_{ss}-V_{dd}}{V_{ss}}\right) + \ln\left( \frac{2V_{dd}-V_{ss}}{V_{dd}} \right) \right]$

Which reduces to the above result in the case that $V_{dd} = -V_{ss}$.

Practical examples of the use of the relaxation oscillator

This type of circuit was used as the time base in early oscilloscopes and television receivers. Variants of this circuit find use in stroboscopes used in machine shops and nightclubs. Electronic camera flashes are a monostable version of this circuit, generating one cycle of the sawtooth. The rising edge develops as the flash capacitor is charged, and the rapid falling edge as the capacitor is discharged. The flash is produced upon receiving the firing signal from the shutter button. Use as a timebase in oscilloscopes was discontinued when the much more linear Miller Integrator timebase circuit (invented by Alan Blumlein), using "hard" valves (vacuum tubes) as a constant current source, was developed. [24]

Notes

1. ^ When a (neon) cathode glow lamp or thyratron are used as the trigger devices a second resistor with a value of a few tens to hundreds ohms is often placed in series with the gas trigger device to limit the current from the discharging capacitor and prevent the electrodes of the lamp rapidly sputtering away or the cathode coating of the thyratron being damaged by the repeated pulses of heavy current.
2. ^ Trigger devices with a third control connection, such as the thyratron or unijunction transistor allow the timing of the discharge of the capacitor to be synchronized with a control pulse. Thus the sawtooth output can be synchronized to signals produced by other circuit elements as it is often used as a scan waveform for a display, such as a cathode ray tube.

References

1. ^ Graf, Rudolf F. (1999). Modern Dictionary of Electronics. Newnes. p. 638. ISBN 0750698667.
2. ^ a b c d Edson, William A. (1953). Vacuum Tube Oscillators. New York: John Wiley and Sons. p. 3. on Peter Millet's Tubebooks website
3. ^ Morris, Christopher G. Morris (1992). Academic Press Dictionary of Science and Technology. Gulf Professional Publishing. p. 1829. ISBN 0122004000.
4. ^ a b c Du, Ke-Lin; M. N. S. Swamy (2010). Wireless Communication Systems: From RF Subsystems to 4G Enabling Technologies. Cambridge Univ. Press. p. 443. ISBN 1139485768.
5. ^ Varigonda, Subbarao; Tryphon T. Georgiou (January 2001). "Dynamics of Relay Relaxation Oscillators". IEEE Trans. on Automatic Control (Inst. of Electrical and Electronic Engineers) 46 (1): 65. Retrieved February 22, 2014.
6. ^ Nave, Carl R. (2014). "Relaxation Oscillator Concept". HyperPhysics. Dept. of Physics and Astronomy, Georgia State Univ. Retrieved February 22, 2014.
7. ^ a b Oliveira, Luis B.; et al (2008). Analysis and Design of Quadrature Oscillators. Springer. p. 24. ISBN 1402085168.
8. ^ DeLiang, Wang (1999). "Relaxation oscillators and networks". "Wiley Encyclopedia of Electrical and Electronics Engineering, Vol. 18". Wiley & Sons. pp. 396–405. Retrieved February 2, 2014.
9. ^ a b c Sauro, Herbert M. (2009). "Oscillatory Circuits". Class notes: Systems and Synthetic Biology 492A. Sauro Lab, Center for Synthetic Biology, University of Washington. Retrieved February 2, 2014. p. 10-12, 22, 23
10. Letellier, Christopher (2013). Chaos in Nature. World Scientific. pp. 132–133. ISBN 9814374423.
11. ^ a b c Enns, Richard H.; George C. McGuire (2001). Nonlinear Physics with Mathematica for Scientists and Engineers. Springer. p. 277. ISBN 0817642234.
12. ^ a b c Pippard, A. B. (2007). The Physics of Vibration. Cambridge Univ. Press. pp. 359–361. ISBN 0521033330.
13. ^ a b c Pippard, The Physics of Vibration, p. 41-42
14. ^ Kinoshita, Shuichi (2013). "Introduction to Nonequilibrium Phenomena". "Pattern Formations and Oscillatory Phenomena". Newnes. p. 17. ISBN 012397299X. Retrieved February 24, 2014.
15. ^ see Ch. 9, "Limit cycles and relaxation oscillations" in Leigh, James R. (1983). Essentials of Nonlinear Control Theory. Institute of Electrical Engineers. pp. 66–70. ISBN 0906048966.
16. ^ Abraham, H.; E. Bloch (1919). "Mesure en valeur absolue des périodes des oscillations électriques de haute fréquence (Measurement of the periods of high frequency electrical oscillations)". Annales de Physique (Paris: Société Française de Physique) 9 (1): 237–302. doi:10.1051/jphystap:019190090021100.
17. ^ a b Letellier, Christopher (2013). Chaos in Nature. World Scientific. pp. 113, 116–119. ISBN 9814374423.
18. ^ van der Pol, B. (1920). "A theory of the amplitude of free and forced triode vibrations". Radio Review 1: 701–710, 754–762.
19. ^ van der Pol, Balthasar (1926). "On Relaxation-Oscillations". The London, Edinburgh, and Dublin Philosophical Magazine 2 2: 978–992.
20. ^ Shukla, Jai Karan N. (1965). "Discontinuous Theory of Relaxation Oscillators". Master of Science thesis. Dept. of Electrical Engineering, Kansas State Univ. Retrieved February 23, 2014.
21. ^ a b c d Abidi, Assad A.; Robert J. Meyer (1996). "Noise in Relaxation Oscillators". "Monolithic Phase-Locked Loops and Clock Recovery Circuits: Theory and Design". John Wiley and Sons. p. 182. Retrieved 0780311493. Check date values in: |accessdate= (help)
22. ^ a b van der Tang, J.; Dieter Kasperkovitz, Arthur H.M. van Roermund (2006). High-Frequency Oscillator Design for Integrated Transceivers. Springer. p. 12. ISBN 0306487160.
23. ^ http://members.shaw.ca/roma/twenty-three.html
24. ^ Book: Time Bases, by Owen Standige Puckle, ca. 1946