[Elecraft] LC Bandpass Filter for AM Reception

[Jack Smith]

Don:

Yes, that would work although if one is going to that degree of effort, 
you might also fit a Collins mechanical filter as an option instead of 
455 KHz LC filters.

It would also require some careful work with the mixers to avoid 
degrading the K3's IP3 with two extra mixers, and also shielding to 
reduce the prospect of birdies from the oscillator.

I think it would also be challenging to fit it all into the space 
occupied by a standard K3 crystal filter.

Jack


Don Wilhelm wrote:
> 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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