[Elecraft] LC Bandpass Filter for AM Reception

[Don Wilhelm]

Jack,

About the only way I can foresee to use LC filtering for a 13 to 15 kHz 
bandwidth would be to take the filter input and downconvert it to 
something like 455 kHz, run it through an LC filter and upconvert to the 
K3 IF.  Seems like for the size, effort and potential for problems makes 
the cost of the proper 13 kHz filter inexpensive.

For those who don't mind having the same signal appear at 2 frequencies 
30 kHz apart, an 8 MHz LC filter might be a solution - just don't 
complain about "strange happenings" while using it.

73,
Don W3FPR

Jack Smith wrote:
> When I looked at the feasibility of an LC bandpass IF filter for AM 
> reception a few months ago, I concluded that it was not feasible with 
> practical components.
>
> The K3's 2nd IF is 15 KHz, so in order to be effective at removing the 
> image response, the 8215 KHz centered bandpass filter must be, say, 
> 60-80 dB down at 30 KHz from center.
>
> In order to achieve 10-15 KHz bandwidth at 8215 KHz, and be 60-80 dB 
> down at 30 KHz from the filter center and to do this with LC components 
> with a reasonable insertion loss requires components with Q values in 
> the tens of thousands. (Typical crystals as employed in crystal filters 
> have motional Q values in the 100,000 range.)
>
> Other than using superconducting wire, I could not come up with a design 
> that meets all these requirements.
>
> Am I missing something?
>
> Jack K8ZOA
> www.cliftonlaboratories.com
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