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Specifying a LOW PASS FILTER
The Pass Band extends from Fo to Fc, Fo is the start frequency of the Pass Band,
usually 0, but not necessarily. Fc is the cut-off or maximum frequency of the
Pass Band. Ao is the flat ( or minimum) loss of the filter. This can very low, a
fraction of a dB or even 0 dB. Ap is the maximum loss in dB in the Pass Band.
The amplitude variation in the Pass Band is the difference between Ap & Ao, and
consists of roll-off & ripple. If there is a ripple constraint that is less than
Ap - Ao it should be stated.
As is the minimum permissible attenuation in the Stop Band with respect to Ao,
where the Stop Band begins at Fs and extends to Fm. The Stop Band is not
necessarily flat and equal as shown in the above drawing .It can have segments
that have different flat or slope attenuation requirements
The Transition Region is usually not defined unless constraints are required.
Return Loss in dB over the Pass Band is specified in terms of the minimum
required at the input and output. The higher the Return Loss the better the
match of the filter with the source and load, both usually 50 OHMS unbalanced.
If Group Delay is an issue then the maximum allowable relative parabolic, slope,
and ripple components must be stated.
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