[Elecraft] CW/DATA modes: Why you'll want a narrow roofing filter

[wayne burdick]

Let's suppose the only crystal roofing filter you have installed is 2.8 
kHz wide. You put the rig into CW (or data) mode, then dial the DSP 
bandwidth down to 400 Hz. Everything will be fine -- unless an 
extremely strong signal appears that's *outside* the DSP's passband but 
still *inside* the crystal filter passband.

This can cause what's called "pumping," meaning activation of the AGC 
or desensing. Many transceivers with I.F. DSP don't have the ability to 
do anything about this, so they get "trashed" on receive. Such 
transceivers typically have only a 3 kHz (or wider) roofing filter, 
allowing strong signals to cause trouble.

The K3 has roofing filter bandwidths as low as 200 Hz, giving you a 
means to completely eliminating problems with nearby signals. If 
installed, these will be automatically selected as you narrow the DSP's 
bandwidth using the shift/width/hicut/locut controls. Or you can select 
them manually.

We'll add this to the FAQ.

73,
Wayne
N6KR



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