# how to calculate raise time of 50 khz square wave frequency

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#### raman00084

##### Full Member level 6 rise time = 2.2RC
RISE TIME IS THE FASTEST INCOMING EDGE

To calculate the value of R and C, use the following steps:

Find the fastest edge of the incoming signal – or determine the fastest frequency of the incoming signal and assume an edge speed of 1/100th of the input period (a 1 kHz input frequency has an edge of 10 µs).
Select ‘R’. Usually this can be selected to a common value already in the system, such as 1 kΩ.
Use the equation in Figure 9 to determine the value of ‘C’.
In some cases, the input signal is a very slow moving signal (button press, switch closure, etc.), so the value of ‘C’ can be then changed to match a common value on the board, as long as the order of magnitude is maintained.
As shown in Figure 8, the values for R and C are 1 kΩ and 0.01 µF (assumed a maximum input frequency of 1 kHz). Figure 10 shows how this circuit works with the input switch circuit. Note how the overshoot edges are now gone compared to Figure 7. This is the effect of the capacitor.

KINDLY TEL ME WHAT IS RAISE TIME OF 50 KHZ FREQUENCY AND HOW TO CALCULATE IT

ALSO THE VALUE OF R AND C

THE ARTICLE IS IN THE LINK :http://www.digikey.com/en/articles/techzone/2012/apr/protecting-inputs-in-digital-electronics

REGARDS
KALYAN

#### paulfjujo hello

it is not possible to give you a correct answer , because to many parameters to involve..
RAISE time depend of internal impedance of the source (50KHz)
with is a part of the R value ....
Impedance of the input (C mainly capacitor value)
Band width of the receiver side..

• raman00084

### raman00084

points: 2

#### KlausST

##### Super Moderator
Staff member Hi,

just referring to the headline: an ideal square wave has zero risetime and zero falltime. In other words: dU/dt is ininfite.

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rise time = 2.2RC
here you bring two parameters int account: R and C. So i suggest you connect an RC combination to an ideal square wave output.
Risetime should be the time for a known voltage step. Maybe 10V/us. So without the knowledge of the square wave amplitude you can not calculate risetime. 2.2RC makes no sense in my eyes.

......
reading your post i always wondered what you are talking about. But the information is in the most bottom line....

now it´s clearing up.

Read wikipedia "RC time constant". there you find useful information. e.g. "2.2RC" is from 10% to 90% of voltage.

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square wave signal dU/dt is infinite (independent of square wave frequency). But after low pass filtering with an RC you get a calculable rise time.
Please read the wikipeda article and the digikey article again and ask a specific question, if any.

Klaus

• raman00084

points: 2