Re: uBITX SSB filter response question #ubitx

ajparent1/kb1gmx
 

An ideal filter would have a flat to and the skirts would be vertical.
Typical commercial filters are specified as 3 db flatness in the pass band for low cost and higher
end might be 1db across the passband.

3db pass band flatness is satisfactory for all but critical applications.

The other specification is the ultimate stopband attenuation typically for a transmit filter
60db is adequate and for receiving anywhere from 80 to 140db is seen.  For really good
stop bands shielding and grounding of the cans is a must. Usually they have a
specification for the 6 to 60db points where a shape factor of 1.2 is very good and 1.4
is good and 2 is only ok.

If you want to you could replace the capacitors with a smaller value and then pad smaller
value caps while watching the shape.  You can also adjust the termination at both ends for
best shape and response.  This is an iterative approach in the empirical realm.  However
with patience the results can be extremely good.  I usually use 0803 size caps for this 
and from known sources SMT parts can be good to excellent.  Here is a trick for those 
doing high performance attempts. All capacitors have unintended series resistance
which lowers their Q.  One way to get around that is if the value is say 100PF use
two 50 pf in parallel as the series resistance is in parallel as well the effective
Q just doubled.  FYI you could use three 33pf with an effective Q of 3x.   It also
makes it easier to use smaller values to get larger ones.  Stacking SMT caps and
resistors at HF works well with no ill effects and allows one to get any value
desired even non standard.


Allison

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