On Wednesday, August 09, 2006 at 7:18 AM Wayne Burdick wrote:
A much better approach to the "Filter Shifing" problem would be to
actually *narrow* the first crystal filter, protecting all subsequent
stages.
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May I second Wayne's suggestion. In practice I have found that it is very
useful to be able to select roofing filters having different bandwidths, at
the moment down to 1.5 kHz, with good stopband attenuation. In spite of the
protection offered by *narrow* roofing filters, I believe that it is wise
to
have all the stages before the second IF filter(s) capable of handling
strong signals, i.e. IIP3s of +40dbm or better, to reduce the crud
generated by those unwanted signals that are very close in and get through
a
narrow bandwidth roofing filter. One price is high current draw which is
probably not acceptable if the rig is to be used portable. The phase noise
of the LOs must also be suitably low to avoid compromising dynamic range
etc. Lastly the filters themselves must also be capable of handling strong
signals.
73,
Geoff
GM4ESD
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