first order low pass filter

[Johnny_YU]

Hi,all

I have a problem with the first order low pass filter.

I kown how to calculate the H(s).

H(s) = 1/(sRC+1), the bode diagram is below.



In bode diagram , the slope is -20dB, how to calculate that slope?

Thank you

 

[shanmei]

H(s)=1/(SRC+1), when S is large, H(s)=1/SRC.

H1(s)=1/10RC, when s=10;
H2(s)=1/100RC, when s=100;

20log(H2(s)/H1(s))=20log(0.1)=-20dB

 

[CataM]

H(jω)=1/(1+jω·T) T=RC for simplification.

You are asking about the gain, so I will omit the phase.

|H(jω)|(dB)=-20·log[√1+(ω·T)² ] if ω·T>>>1 results in -20·log(ω·T)

Lets say -20·log(ω·T)=a [dB] => -20·log(10*ω·T)=-20+a [dB] i.e. every decade you subtract 20 dB.