# Find -3dB frequencies from rise time or decay time

Amplifiers often exhibit a low frequency and or high freqeuncy response similar to that of a single RC high pass or low pass network.

When driven by a square wave, the response hints the time constant of those elements.

This calculator allow specification of two voltages on the exponential waveform and the time between them, and calculates the corresponding -3dB frequency of the network.

 Inputs: Rise time / decay time rise decay V V1 V2 Time (V2-V1) (s) Results: -3dB Frequency (Hz)

Version:

V is the relevant peak of the waveform applied to the section, V1 is some point on the exponential part of the response; V2 is a later point on the same exponential curve,  and the Time (V2-V1) (s) is the time interval from V1 to V2. The examples below illustrate its working.

## Example 1

Example 1 is a measurement of rise time of a fast square wave. Classicaly, rise time is measured as the time from 10% to 90% of the exponential charge and that is what is done in thix example, but the calculator can use an arbitrary pair of points (though they should be selected on the steep part of the curve for best accuracy). Fig 1 shows a capture from an oscilloscope. The markers have been set to identify two points when are then used in finding the -3dB frequency. The button Example 1 on the calculator automatically enters this data.

### Example 2

Example 1 is a measurement of decay time of a fast square wave. In this example, it is important to note that after each rise, the voltage will decay exponentially towards 0V. Decay time is measured between two arbitrary points of the exponential discharge charge curve (though the points should be selected well apart on the curve for best accuracy). Fig 1 shows a capture from an oscilloscope. The markers have been set to identify two points when are then used in finding the -3dB frequency. The button Example 2 on the calculator automatically enters this data.

Notes:

1.  In both cases, V1 can be equal to V, it makes for a simpler set of measurements and does not compromise the calculated result.

The calculator does not do a lot of error checking, if you enter nonsense, it will probably produce nonsense.