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

[Stewart Baker]

Just to pass the time I used a wire to link across an unused 
filter position (with a series 0.1u DC blocking capacitor), 
adjusted the filter bandwidth to 8kHz, and listened to AM 
broadcast stations. Quite a respectable performance, much better 
than with a 2.7kHz filter.

Now I know that it is worth making even a fairly loose LC filter 
just for occasional AM reception.

73
Stewart G3RXQ
On Mon, 05 May 2008 20:25:59 +0100, David Woolley (E.L) wrote:
> Jerry Flanders wrote:
>> for SWL. Only downside (a minor one, IMO) is that you might get 
response
>> from images 30 KHz away. How much weaker is a signal when you 
tune to
>> its image 30 KHz up/down?
>>
> Less than 30kHz.  The whole point of the exercise is to use wide
> filters, so, assuming that the interference is from a carrier 
and
> appears in the demodulated audio, the image can be 25 (5kHz 
audio) to
> 20kHz (10kHz audio) away from the carrier (20 and 10kHz from the
> passband edge).  If the interference is from an audio component 
at the
> extreme of the bandwidth and the transmit is the same as the 
receive
> bandwidth, the carrier can be 20 to 10kHz away (although in the 
latter
> case would have other problems).  In practice, I would expect 
the
> carrier to get past a weak filter better than an extreme 
sideband frequency.
>
> Note this means that if the signals are trying to reproduce 
audio up to
> 10kHz, the adjacent channel will be an issue.