why high-pass filter response can be estimated this way

[Rainbow00]

Hi,

given a simple RC high pass filter and a step function at the input, i read a quick way to estimate the output at a certain time as below:

for example, assume step at 0ns. at 20ns the impedance of capacitor can be calculated using 25MHz.

i couldn't figure out why 25MHz frequency is used. can someone elaborate more on this?


thanks a lot
best regards
gd

 

[FvM]

Time-responce can be exactly calculated with exponential function, frequency response can be exactly calculated from frequency dependant impedances, no need to estimate or mix time and frequency domain in calculation.

 

[laktronics]

Hi,
If I assume that your specification is a step at every 20 nsec., starting from zero time, I guess, you get a squarewave of period 40 nanosec. which will have a fundamental frequency of 25 MHz. It is only a guess.
Regards,
Laktronics

 

[Rainbow00]

Hi,

thanks for your reply. i knew the time and frequency response can be calculated using exponential function and laplace transform respectively. however, calculating time response is more complicated as it involves exponential. don't you think the method stated in the first post is relatively quick and intuitive?

Laktronics,
no, the step input is 0 at 0ns and 1 thereafter. pumping this to a high pass filter would have a response of a peak and exponential decay. while in the first post the method can give the decaying voltage output at any instance after the step. i am wondering how it can be done this way?


thanks
best regards
gd