# 3db frequency problem

Discussion in 'Homework Help' started by Millia Kang, Dec 18, 2014.

1. ### Millia Kang Thread Starter New Member

Dec 18, 2014
1
0
hello, I need to solve 3db frequency from transfer function in this picture....
buf... it's hard to me...plz help me

in the picture, the transfer functi on is
HLP(Z)=(1-a/2)*(z+1)/(z-a)

and the answer, 3db cutoff frequency is cos-1(2a/1+a^2)

I don't know how to prove this.
thank you

2. ### MrAl Distinguished Member

Jun 17, 2014
2,567
521
Hi,

Something doesnt look right. Is that supposed to be the inverse cosine function? If so that doesnt look right as it has too limited of a range.
Also, inside the parens, you have 2a/1 but you usually dont need the "1" when it is written like that, so is this written correctly?
Your image does not show up either.
Try to be more careful about writing out your question and make sure the transfer function is correct, and if you have a specified range for 'a' (like 0.1 to 0.2) or whatever.

Last edited: Dec 19, 2014