Filter Group delay 和 step response關係

s20192s20192

最近在看Filter相關書籍
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
5 f* k; p9 Z7 R5 E+ F% aand ringing in the step response indicates that the group delay varies strongly in the passband
想請問為何從step response中就可以知道關於group delay
6 t0 Q" V3 T/ G% Y有什麼相關數學公式來驗證這些關係嗎

2.
4 r1 }, |8 y3 @voltage mode and current mode
' t5 v) H: ]2 A' o" E: p  u4 n) `為何voltge mode較不適用在高頻阿
書中是說voltage mode 用 feedback
/ V+ b( d; _" G2 X9 v所以較不適用在高頻
但是用回授
4 |2 I0 s+ B2 a* o不是頻寬增加!?
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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
為何group delay (in passband)為何會隨頻寬減少而上升阿

  Y" R( @3 D) _  d0 h  J& h/ e( g感謝