In electronics, when describing a voltage or current step function, rise time is the time taken by a signal to change from a specified low value to a specified high value. Typically, in analog electronics, these values are 10% and 90% of the step height: in control theory applications, according to Levine (1996, p. 158), rise time is defined as "the time required for the response to rise from x% to y% of its final value", with 0%-100% rise time common for underdamped second order systems, 5%-95% for critically damped and 10%-90% for overdamped. The output signal of a system is characterized also by fall time: both parameters depend on rise and fall times of input signal and on the characteristics of the system.
Rise time is an analog parameter of fundamental importance in high speed electronics, since it is a measure of the ability of a circuit to respond to fast input signals. Many efforts over the years have been made to reduce the rise times of generators, analog and digital circuits, measuring and data transmission equipment, focused on the research of faster electron devices and on techniques of reduction of stray circuit parameters (mainly capacitances and inductances). For applications outside the realm of high speed electronics, long (compared to the attainable state of the art) rise times are sometimes desirable: examples are the dimming of a light, where a longer rise-time results, amongst other things, in a longer life for the bulb, or digital signals apt to the control of analog ones, where a longer rise time means lower capacitive feedthrough, and thus lower coupling noise.
Simple examples of calculation of rise time
The aim of this section is the calculation of rise time of step response for some simple systems: all notations and assumptions required for the following analysis are listed here.
- is the rise time of the analyzed system, measured in seconds.
- is the low frequency cutoff (-3 dB point) of the analyzed system, measured in hertz.
- is high frequency cutoff (-3 dB point) of the analyzed system, measured in hertz.
- is the impulse response of the analyzed system in the time domain.
- is the frequency response of the analyzed system in the frequency domain.
- The bandwidth is defined as
- and since the low frequency cutoff is usually several decades lower than the high frequency cutoff ,
- All systems analyzed here have a frequency response which extends to 0 (low-pass systems), thus
- All systems analyzed are thought as electrical networks and all the signals are thought as voltages for the sake of simplicity: the input is a step function of volts.
Gaussian response system
A system is said to have a Gaussian response if it is characterized by the following frequency response
where is a constant, related to the high frequency cutoff by the following relation:
Applying directly the definition of step response
Solving for t's the two following equations by using known properties of the error function
the value is then known and since
One stage low pass RC network
For a simple one stage low pass RC network, the 10% to 90% rise time is proportional to the network time constant :
Solving for t's
We call t1 the time needed to go from 0% to 10% of the steady-state value, and t2 the one to 90%. Thus t1 is such that and t2 is such that . Solving the previous equation for these two values we find the analitical expression for t1 and t2:
We obtain t2 in the same way, resulting in
Subtracting from we obtain the rise time, whis is therefore proportional to the time constant:
Now, noting that
(see here for the proof of the previous equation) then
and since the high frequency cutoff is equal to the bandwidth
Rise time of cascaded blocks
Consider a system composed by cascaded non interacting blocks, each having a rise time and no overshoot in their step response: suppose also that the input signal of the first block has a rise time whose value is . Then its output signal has a rise time equal to
Factors affecting rise time
Rise time values in a resistive circuit are primarily due to stray capacitance and inductance in the circuit. Because every circuit has not only resistance, but also capacitance and inductance, a delay in voltage and/or current at the load is apparent until the steady state is reached. In a pure RC circuit, the output risetime (10% to 90%), as shown above, is approximately equal to .
Rise time in control applications
In control theory, for overdamped systems, rise time is commonly defined as the time for a waveform to go from 10% to 90% of its final value.
However, the proper calculation for rise time from 0 to 100% of an under-damped 2nd-order system is:
where ζ is the damping ratio and ω0 is the natural frequency of the network.
- Precisely, Levine (1996, p. 158) states: "The rise time is the time required for the response to rise from x% to y% of its final value. For overdamped second order systems, the 0% to 100% rise time is normally used, and for underdamped systems...the 10% to 90% rise time is commonly used". See also the textbook Nise 2008.
- This beautiful one-page paper does not contain any calculation. Henry Wallman simply sets up a table he calls dictionary paralleling concepts from electronics engineering and probability theory: the key of the process is the use of Laplace transform. Then he notes that, following the correspondence of concepts established by the dictionary, that the step response of a cascade of blocks corresponds to the central limit theorem and states that: "This has important practical consequences, among them the fact that if a network is free of overshoot its time-of-response inevitably increases rapidly upon cascading, namely as the square-root of the number of cascaded network".(Wallman 1950, p. 91)
- Levine, William S. (1996), The control handbook, Boca Raton, FL: CRC Press, p. 1548, ISBN 0-8493-8570-9.
- Nise, Norman S. (2008), Control Systems Engineering (Fifth ed.), John Wiley & Sons, pp. xvii+880, ISBN 978-0-471-79475-2
- United States Federal Standard 1037C: Glossary of Telecommunications Terms
- Valley, George E., Jr.; Wallman, Henry (1948), Vacuum Tube Amplifiers, MIT Radiation Laboratory Series 18, New York: McGraw-Hill., pp. xvii+743 Paragraph 2 of chapter 2 and paragraphs 1 to 7 of chapter 7 .
- Wallman, Henry (1950), "Transient response and the central limit theorem of probability", Proceedings of Symposia in Applied Mathematics (Providence: AMS.) 2: 91, MR 0034250, Zbl 0035.08102.