Rise time

Among rise time and fall time ( engl. rise time and fall time ) is understood in the measurement technology and digital technology, the time required for a change in level of a (ideally) rectangular signal real to its signal level between two defined intermediate values ​​( customary 10% and 90 %) to change.

The times are caused by the finite cut-off frequency of the involved switching elements and transmission lines.

Rise and fall times described in the digital technology and the switching transistors in the switching times of the characteristic in which the signal is no longer holding the old and the new are not defined logic level ("0 " or " 1") or switch state. See also slope.

In digital technology, in this case the worst case (worst case) are called guaranteed time mostly. They describe the amount of time ( for example, a computer processor ) requires a signal for safe switching between the two binary states. The specified for a component rise and fall times are often not measured values ​​but a secured through the design or the manufacturing process characteristic of the block or the considered logic family for certain parameters (operating voltage, temperature).

All digital technology is ultimately based on analog operating circuit elements which are optimized for the processing of digital signals. It should be noted that, while the assumption for digital signals "1" or "0" and "Power On" or "0", but is usually carried out in practice, with voltage and current levels which are not equal to zero. Furthermore, the behavior of the circuit is in state change mostly unbalanced, so rise and fall time are then different lengths.

For a logic signal in a circuit, it is necessary to define thresholds. For a logic "0 " in TTL, for example, an allowable range of 0 ... 0.4 V and set as logic " 1", an area of 2 ... 5 V. The actual switching threshold is the logic blocks ( approximately 1.4 V ) in the range between these two values ​​, which is also referred to as forbidden area. It is therefore in the range of rise and fall time, resulting in a very quick pass through the forbidden region, which is important and often specified with a minimum allowable rate of voltage rise for many logic devices.

Rise and fall - time to describe the time intervals in which this undefined "forbidden" state occurs during switching.

This is particularly important for edge triggered circuits, i.e., circuits which react to the change of the signal (for example, edge-triggered flip-flop ). Otherwise, it may cause a malfunction; see also race condition.

To obtain a security against disturbances, the allowed level also apply to the outputs of these blocks. For example, for a gate output in the classical TTL technology max. 0.4 V is " 0", and min. 2.4 V for " 1" guaranteed.

Very short rise and fall times of the signal also mean that in the spectrum of the signal very high frequency components are present, leading to the emission ( emission ) of electromagnetic waves. By these false signals other circuit parts can be influenced in their function.

To ensure electromagnetic compatibility, so the outputs of digital and driver circuits are designed so that the rise and fall times are only needed as short as absolutely. For this purpose, the slope is slew rate of the output driver limited.

Measurement

In metrology, and also for the characterization of analog and digital circuits, the values ​​of 10 % and 90 % of the switching threshold or setpoint signal can be defined to specify the hours most.

Also switching transistors and other power electronic components can be characterized thus.

Analog amplifier or amplifiers, for example, light emitting diodes, lasers or photodiodes and phototransistors are also characterized by the rise and fall times, which provide them as a response to a step function.

Here are the following relationships for a 1st order low pass or any comparable scheme, which is rise and fall time:

• About 2.2 times the time constant of
• Approximately 0.35 times the reciprocal of the 3-dB cutoff frequency

Both in digital circuits as well as amplifiers, sensors and actuators delay times are in addition (german delay time ) characteristic, the pass before the output voltage begins to change for an input signal jump. When switching stages these times hot engl. turn- on delay and turn- off delay. The total time delay when passing through a switching stage or another quadrupole thus results from the delay and about half the rise and fall time.

Many digital storage feature in its software via measurement functions for the rise and fall times and the delay times.

The above formulas are usually derived to first order filters. Interestingly, the error in higher-order systems is quite low, that is, one can, for example, even with higher order low-pass filters relatively accurately calculated from the 3-dB cutoff frequency of the rise time. The most commonly occurring overshoot it has relatively little influence. The pure time delay, however, is much more dependent on other effects, such as other poles and zeros of the transfer function or nonlinear memory effects.