[Elecraft] K3 - Roofing filter for SWLing

[David Woolley (E.L)]

jpk5lad at cox.net wrote:
> I read, with interest, the discussion about the K3's inability to
+ copy (or perhaps a better way to say it is to reproduce the audio of)
+ shortwave broadcast stations well without a wide FM filter. I just

The problem isn't for shortwave broadcast stations in general, most of 
which are already restricted to the AM filter bandwidth, but for certain 
ones, particularly intra-USA ones, that use more than one 5kHz channel.

+ wondered if there is not, or could not be made, a provision to have no
+ roofing filter for someone who wants to listen to shortwave. I haven't

This has already been covered, more than once.  Without the filter, one 
will get image responses at 30kHz less twice the DSP IF filter 
bandwidth.  You will also, probably, get aliasing at other frequencies. 
  You may also get desensitisation and IMD, as the result of signals 
that not actually aliased into the DSP filer bandwidth.

The LC filter, that others have  proposed, may be enough to constrain to 
reject BC to BC interference, unless you are listening to something well 
out of area, but no filter at all is a non-starter.

I believe there is a certain amount of LC filtering built in, but that 
will not be designed to provide adequate image rejecting and elimination 
of other close in aliases.


-- 
David Woolley
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