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LC Bandpass Filter for AM Reception

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Jack Smith-6

LC Bandpass Filter for AM Reception

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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Don Wilhelm-4

Re: LC Bandpass Filter for AM Reception

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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KK7P

Re: LC Bandpass Filter for AM Reception

> 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.

The problem is much worse than that.

Why?

The K3 is a transceiver.

So even if *you* are willing to put up with the image, when you transmit
you'll be on two frequencies and others may be less willing to live with
the image than you are.

73,

Lyle KK7P

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Jack Smith-6

Re: LC Bandpass Filter for AM Reception

In reply to this post by Don Wilhelm-4

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
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>> ------------------------------------------------------------------------
>>
>>
>> No virus found in this incoming message.
>> Checked by AVG - www.avg.com
>> Version: 8.0.238 / Virus Database: 270.11.30/2026 - Release Date: 03/27/09 07:13:00
>>
>>
>>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
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> Please help support this email list: http://www.qsl.net/donate.html
>
>