From: Mike Saunders ([email protected])
Date: Fri May 26 2000 - 07:29:09 PDT
Howard Johnson's book uses a D flip flop clocked at several nominal rates
as an example of a typical digital signal. The output merely toggles, with
10%-90% rise/fall times which are about 1% of the overall period. Then, he
shows a plot of the spectral power density of the output, based on
frequency components relative to the sample clock. In this plot, the
spectral power density rolls off at a rate of about -20dB/decade when
plotting signal amplitude (y) vs. frequency (x, log scale). However, by
the knee frequency, the spectrum is already at 6.8dB(V) below the straight
-20dB/decade slope. After this knee frequency, the slope rolls off
exponentially, indicating that since most of the energy is contained below
the knee frequency, the behavior of a circuit at frequencies above this
frequency will hardly affect digital performance. He also asserts that the
behavior of a circuit AT the knee frequency determines how well it can
handle a steep edge. Dr. Johnson does qualify Fknee, saying it can be used
as a quick way to relate the time domain to the frequency domain.
Therefore, it can be used as a quick rule of thumb to determine whether or
not particular frequencies should be taken into account. He also states
that for EMI, frequencies well above Fknee ARE important and should not be
At 03:35 PM 5/25/2000 -0700, you wrote:
> I've seen the derivation for the BW = 0.35/tr(10% - 90%).
> I haven't seen the derivation for the BW = 0.5/tr
>Barry Ma wrote:
>> There is another way to figure out what is the highest frequency we
might need to take care of:
>> Howard Johnson call it Fknee = 0.5/Tr. Given Tr = 0.4 ns, We have Fknee
= 1.25 GHz.
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