[Elecraft] Wide Filters (was:New K3 SN#207 built but question on
AM Filter)
Joe Subich, W4TV
lists at microham-usa.com
Mon Jan 14 10:17:51 EST 2008
> 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.
I don't know if off the shelf ceramic filters are available at
the right frequency. The issue with two or three crystal filters
would be image rejection (the image is only 30 KHz away). Based
on data for the INRAD 6-pole 7 MHz "front end" filter, the 8-pole
filter (KFL3B-FM) should have sufficient rejection for transmit
use.
I suspect someone could build their own wide filter if/when parts
are available. The key would be to make sure it could not be used
for transmit!
73,
... Joe, W4TV
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Barry N1EU
> Sent: Monday, January 14, 2008 7:37 AM
> To: elecraft at mailman.qth.net
> Subject: RE: [Elecraft] New K3 SN#207 built but question on AM Filter
>
>
>
> 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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