[Elecraft] K3 FM filter required for full AM receive width

[David Woolley (E.L)]

KM5Q wrote:

> I like using the K3 for shortwave listening. I would rather not be 
> required to buy the FM xtal filter to get the full bandwidth on AM. Will 
> this change with future revisions?

I don't understand the question; it seems to me to be a matter of physics.

A roofing filter is needed to suppress the image response at the final 
(DSP) IF.

True AM detection requires a bandwidth of twice the highest modulation 
frequency.  By "full AM" I presume you mean medium wave AM (9kHz 
channels in Europe, or shortwave AM that uses more than one channel, 
e.g. 5kHz maximum modulation frequency, so you need a 10kHz bandwidth.

The only filter available from Elecraft that has at least 10kHz 
bandwidth is the FM one, and you therefore need that if you want audio 
beyond about 2.5kHz on AM.  You need an additional filter for any true 
AM detection with a sensible bandwidth.  Not having a filter at all is 
not an option, becuause of the image response.



-- 
David Woolley
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