[SI-LIST] 3dB or Knee Frequency

  • From: Anand.Kuriakose@xxxxxxxxxx
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 13 Jun 2002 10:16:45 +0630

Hi  All,

When we talk about high edge rate signals, we talk about two terms namely
1>  3 dB frequency, which is related to the edge rate as approximately 0.35/Tr
2>  Knee frequency (mentioned in Howard Johnson's famous book and many SI-list
mails) which is given as 0.5/Tr.

1>   What is Knee frequency? Does it relate to the highest harmonic present in
the signal with significant energy? How relevant it is in High speed board
design    as against 3dB frequency?

2>   To determine the decoupling caps, we have to determine the highest
significant frequency involved in the switching signals. Now the question is
should we      consider the 3dB frequency ( mentioned in High Speed Digital
Design by Stephen Hall) or the knee frequency ( in Howards Johnson's Black
MAgic)  as     the best approximation?  Let us consider Tr=1ns. Then we have 3dB
frequency = 350Mhz and Knee frequency = 500 Mhz.


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