Filter Group delay 和 step response關係

s20192s20192

最近在看Filter相關書籍
/ a3 N2 r5 k# h# G* M$ N書中提到
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The time for the step response to reach the value 0.5 is an approximate measure of the average group delay
' b3 U/ @4 H, ~. o. A; i* ~and ringing in the step response indicates that the group delay varies strongly in the passband
3 T4 x$ f' [$ Z( i+ a5 I想請問為何從step response中就可以知道關於group delay
有什麼相關數學公式來驗證這些關係嗎
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2.
voltage mode and current mode
為何voltge mode較不適用在高頻阿
+ ^% F/ K- b9 |3 S1 Q# Z書中是說voltage mode 用 feedback
所以較不適用在高頻
但是用回授
不是頻寬增加!?

# j' f# w( m" t3 v, P  ~想請教版上各位大大這兩個問題

  |% {6 B" q6 K感謝

s20192s20192

再多補充一個問題
0 p, p, ?8 X1 |  a- R, t為何group delay (in passband)為何會隨頻寬減少而上升阿
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感謝