[Elecraft] IF Shift, PB Tuning, and why neither completely eliminates QRM

[Geoffrey Mackenzie-Kennedy]

 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