[Elecraft] New K3 SN#207 built but question on AM Filter

[Barry N1EU]

This is admittedly topic skew, but for non-critical occasional am listening,
it would be nice to have a much lower cost option than the $120 filters.  A
cheap ceramic or 2-xtal filter, about 15Khz bw would have been a nice
addition to the K3, maybe even as a st'd component. 

73,
Barry N1EU



Joe Subich, W4TV-3 wrote:
> 
> 
>> Even with the 10 kHz channel spacing used in the USA, AM broadcast
>> stations do not have 5 kHz audio bandwidth.  The FCC requires a guard
>> band between stations.  As I recall, rgulations require that the audio
>> start to drop off at about 4 kHz so that it can be down 20 dB 
>> or so by 6 kHz (the passband edge of the adjacent station).
> 
> I don't think that's right ... or wasn't the last time I was around 
> an AM station (I spent most of my career in TV).  I remember the AM 
> guys doing proof to 10 KHz. 
> 
> Admittedly, many of the directional stations could not maintain 10 KHz 
> through the phasors and the high end got trashed at night but the old 
> allocation systems generally kept first adjacent situations far enough 
> apart that 10 KHz could be obtained on groundwave during the daytime.  
> 
> "In the day" most receivers would start to roll off somewhere around 
> 6 KHz and the better ones had a 10 KHz notch for nighttime conditions. 
> 
> Given the DSP demodulation in the K3, it's a shame that there isn't 
> an "offset" option to do "vestigial sideband" demodulation (offset the 
> AM filter to the upper sideband or lower sideband) and demodulate 
> carrier and one sideband for better fidelity.  This would work quite 
> well if the carrier were placed at the -6dB point on the composite 
> filter passband since it would keep the proper ratio between carrier 
> and sideband.  Alternatively, the carrier could be moved to 1 KHz 
> from the -6 dB point and the DSP could equalize out the 6 dB boost 
> in audio below 1 KHz from the "opposite" sideband. 
> 
> 73, 
> 
>    ... Joe, W4TV 
>  
> 
> 
> 
>> -----Original Message-----
>> From: elecraft-bounces at mailman.qth.net 
>> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Alan Bloom
>> Sent: Sunday, January 13, 2008 10:48 PM
>> To: David Woolley
>> Cc: elecraft at mailman.qth.net
>> Subject: Re: [Elecraft] New K3 SN#207 built but question on AM Filter
>> 
>> 
>> On Sun, 2008-01-13 at 02:49, David Woolley wrote:
>> > Paul Webb wrote:
>> > 
>> ...
>> > The basic reason has already been explained, however, there 
>> are very few 
>> > cases were exceeding 3kHz audio bandwidth is useful.  HF broadcast 
>> > stations use 5kHz channelling, which would only allow them 
>> 2.5kHz with 
>> > brick wall filtering at both transmit and receive ends, 
>> although they 
>> > probably do expect to suffer significant adjacent channel 
>> interference.
>> > 
>> > MF broadcast stations use 9kHz channelling in Europe and 
>> 10kHz in the 
>> > USA, but I suspect that adjacent channel interference is less 
>> > acceptable.  As they were designed to be received with LC 
>> IF filters, 
>> > with poor shape factors, I suspect they don't even make use 
>> of the full 
>> > channel, and if they did, they would probably be required to have 
>> > filters which put the adjacent channel into the filter stop band.
>> 
>> Even with the 10 kHz channel spacing used in the USA, AM broadcast
>> stations do not have 5 kHz audio bandwidth.  The FCC requires a guard
>> band between stations.  As I recall, rgulations require that the audio
>> start to drop off at about 4 kHz so that it can be down 20 dB 
>> or so by 6
>> kHz (the passband edge of the adjacent station).
>> 
>> So there's not much point in the receiver audio being wider 
>> than 4 kHz.
>> 
>> Al N1AL
>> 
>> 
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> 
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