Filter Group delay 和 step response關係

s20192s20192

最近在看Filter相關書籍
7 E( p% f. K" x* Q1 L- @0 P' Y書中提到
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) k% D0 j, M; r$ @4 ?The time for the step response to reach the value 0.5 is an approximate measure of the average group delay
' e6 p$ s% v% ~0 n2 Pand ringing in the step response indicates that the group delay varies strongly in the passband
; r1 e0 `8 O: Z2 A' H% J& K想請問為何從step response中就可以知道關於group delay
) C4 v, D' G, M6 c# Y有什麼相關數學公式來驗證這些關係嗎
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1 @( S- Y/ s5 \/ uvoltage mode and current mode
為何voltge mode較不適用在高頻阿
2 m( b8 o3 a( ?. L/ o/ y書中是說voltage mode 用 feedback
! N3 S( G, `  c( I, ~9 r所以較不適用在高頻
' o  r- b' T& Z3 O( \9 e但是用回授
% @7 U! |& o9 M* I  W! y不是頻寬增加!?

想請教版上各位大大這兩個問題
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感謝

s20192s20192

再多補充一個問題
" T$ l" t" L8 l# y9 S為何group delay (in passband)為何會隨頻寬減少而上升阿

' W% C5 P- a0 v感謝