K3 - Roofing filter for SWLing

[K5LAD]

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 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 done a lot of shortwave listening but is there actually even a need to have the selectivity benefits of a roofing filter for SWLing?  Perhaps there is.

I am still awaiting my K3 so I ask this as an anxious observer, but I do have the
extra bandpass filters as part of my order.  Since receivers for years did not have roofing filters, it's obvious that some receivers can operate without one.  This would just let the DSP provide the necessary selectivity.  The K3 may not have, or want, that ability.

I realize that asking for a super-deluxe receiver to be able to remove a roofing
filter is a bit like buying a Rolls Royce without the engine and replacing the engine with a riding lawnmower engine.

Just a curious question............... while I impatiently wait.

Jim - K5LAD

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[David Woolley (E.L)]

[hidden email] 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.


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David Woolley
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[David Woolley (E.L)]

David Woolley (E.L) wrote:

> This has already been covered, more than once.  Without the filter, one
> will get image responses at 30kHz less twice the DSP IF filter

Sorry, I meant twice the audio bandwidth.  For AM it is the IF bandwidth.

> bandwidth.  You will also, probably, get aliasing at other frequencies.


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David Woolley
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Elecraft products and more general topics related ham radio"
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