[Elecraft] Wide Filters (was:New K3 SN#207 built but question on AM Filter)

Mon Jan 14 10:39:37 EST 2008

I think part of the problem is not only finding an economical AM filter,
but ALSO being able to PREVENT users from trying to TRANSMIT THROUGH IT!

Tom   N0SS

At 09:17 01/14/2008, Joe Subich, W4TV wrote:

> > 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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> > --
> > View this message in context:
> > http://www.nabble.com/New-K3-SN-207-built-but-question-on-AM-F
>ilter-tp14780362p14800931.html
>Sent from the Elecraft mailing list archive at Nabble.com.
>
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