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New K3 SN#207 built but question on AM Filter

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AC7AC

RE: AM bandwidth, the rest of the story :=)

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Kevin Cozens-2

Re: AM bandwidth, the rest of the story :=)

Ron D'Eau Claire wrote:
> The "clear channel flamethrowers" running
> 500,000 watts are relatively few by comparison.

500kW? I assume that must be a typo. AFAIK, the limit on the AM BCB is 50kW in
Canada and the US.

--
Cheers!

Kevin.

http://www.ve3syb.ca/ |"What are we going to do today, Borg?"
Owner of Elecraft K2 #2172 |"Same thing we always do, Pinkutus:
| Try to assimilate the world!"
#include <disclaimer/favourite> | -Pinkutus & the Borg
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Phil Kane-2

RE: AM bandwidth, the rest of the story :=)

In reply to this post by Alan Bloom

On Tue, 15 Jan 2008 09:51:38 -0800, Ron D'Eau Claire wrote:

> Many AM stations today limit their audio bandwidth to 5 or 6 KHz,
> maximum, and even more consumer AM receivers sold today limit the
> audio bandwidth to 5 kHz or less!

> Some of the reasons for the limits at the transmitter have to do
> with going to digital audio. I haven't worked in the broadcast
> industry since digital AM started to appear so I don't know the
> overall issues, but I see it mentioned regularly in the
> broadcasting press.

Yes, it has to do with what is known as the NRSC Mask which was
adopted in preparation for AM digital transmissions to confine
the analog signal within a 10 kHz linearized channel. A lot of
older transmitters and antenna array tuning units ("phasers")
had to be retuned or replaced in order to meet the NRSC linearity
requirements.

> A few articles I've seen in the broadcast press even note that
> since almost 1/2 of the American public under 50 years of age is
> now functionally deaf above about 6 or 7 KHz, thanks to abusing
> their ears with loud music as children, they are completely
> unaware of the change in audio response anyway.

How very true.

--
73 de K2ASP - Phil Kane
Elecraft K2/100 s/n 5402

Philip M. Kane P.E. / Esq
VP - General Counsel & Engineering Manager
CSI Telecommunications Inc. - Consulting Engineers
San Francisco, CA - Beaverton, OR

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AC7AC

RE: AM bandwidth, the rest of the story :=)

In reply to this post by Kevin Cozens-2

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Alan Bloom

RE: AM bandwidth, the rest of the story :=)

In reply to this post by Joe Subich, W4TV-3

Joe,

Somehow they have to limit their occupied bandwidth to less than 10
kHz. If they don't there will be severe adjacent-channel interference.
That means at most 5 kHz audio bandwidth, assuming a brick-wall filter.

Using a real-world filter, it has to be substantially less than 5 kHz.
You can argue whether the right number is 4 kHz or 4.5 kHz, but for sure
it isn't 10 kHz.

73,

Al N1AL

On Tue, 2008-01-15 at 07:22, Joe Subich, W4TV wrote:

> > OK< I looked it up. According to Title 47, part 73.44 of the FCC
> > regulations, <http://www.fcc.gov/mb/audio/bickel/amfmrule.html#AM>
> > the modulation of an AM broadcast station must be down 25 dB at
> > 10.2 kHz from the carrier.
>
> The information at 73.44(b) only applies to out of band emissions.
> You will note that the section applies additional limitations at
> 20 KHz from carrier, 30 KHz from carrier, etc.
>
> > Assuming a 3-pole low-pass filter (e.g. a pi-network), the filter
> > attenuation is 18 dB per octave, which implies a cutoff frequency
> > of no more than 3.9 kHz. The -3 dB bandwidth would be a little
> > higher than that.
>
> Any AM station that used a simply pi-network filter would have
> real problems since they would sound "muddy" an lack punch. I
> think you will find that they are using filters much more complex
> than a simple pi-network - usually with a sharp cut-off ("brick
> wall") response to maintain response out to 10 KHz. For example,
> the audio response specification for the Harris AM transmitters
> is: +0.2/-0.8 dB, 20 Hz to 10 kHz. Ref. 1 kHz at 95% modulation.
> See www.broadcast.harris.com/radio/transmission/analog.asp and
> look at the various specification sheets for the Analog AM
> transmitters.
>
> 73,
>
> ... Joe, W4TV
>
>
>
> > -----Original Message-----
> > From: Alan Bloom [mailto:[hidden email]]
> > Sent: Tuesday, January 15, 2008 1:50 AM
> > To: Joe Subich, W4TV
> > Cc: [hidden email]
> > Subject: AM bandwidth, the rest of the story :=)
> >
> >
> > OK< I looked it up. According to Title 47, part 73.44 of the FCC
> > regulations, <http://www.fcc.gov/mb/audio/bickel/amfmrule.html#AM> the
> > modulation of an AM broadcast station must be down 25 dB at 10.2 kHz
> > from the carrier. Assuming a 3-pole low-pass filter (e.g. a
> > pi-network), the filter attenuation is 18 dB per octave,
> > which implies a
> > cutoff frequency of no more than 3.9 kHz. The -3 dB
> > bandwidth would be
> > a little higher than that.
> >
> > That's about what I remember from my broadcasting days many,
> > many years
> > ago. If you think about it, a double-sideband AM signal can't have a
> > bandwidth greater than 1/2 the channel spacing without
> > interfering with
> > adjacent channels. And it has to be somewhat less than that given
> > real-world filters. So there is not much point in having a receiver
> > with much more than 4 kHz or so audio response (8 kHz or so RF
> > bandwidth).
> >
> > > I remember the AM guys doing proof to 10 KHz.
> >
> > Right, in order to confirm that the modulation is down 25 dB at 10.2
> > kHz.
> >
> > Al N1AL
> >
> >
> > On Sun, 2008-01-13 at 20:40, Joe Subich, W4TV 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
> > >
> >
> >
> >

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David Cutter

Re: RE: AM bandwidth, the rest of the story :=)

I haven't read anyone refer to the antenna bandwidth, perhaps I missed it.
Small, local, transmitters would have "small" antennas with much lower
bandwidth than a big transmitter and therefore some additional self-limiting
of transmitted bandwidth.

Is the transmitted bandwidth part of the measurements required for broadcast
transmitters? It would make a lot more sense to me than what is fed to the
antenna.

David
G3UNA

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Phil Kane-2

Re: RE: AM bandwidth, the rest of the story :=)

On Tue, 15 Jan 2008 23:02:06 -0000, David Cutter wrote:

> Is the transmitted bandwidth part of the measurements required
> for broadcast transmitters? It would make a lot more sense to me
> than what is fed to the antenna.

Yes. A "proof of performance" to NRSC standards requires
measurement of the transmitted signal including the antenna.

--
73 de K2ASP - Phil Kane
Elecraft K2/100 s/n 5402

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Joe Subich, W4TV-3

RE: RE: AM bandwidth, the rest of the story :=)

In reply to this post by David Cutter

> I haven't read anyone refer to the antenna bandwidth, perhaps
> I missed it.

There are standards - generally requiring the antenna to be
reasonably well behaved +/- 10 KHz or more - is order to
support both AM stereo (if used) and "HD (digital) radio"

> Small, local, transmitters would have "small" antennas with
> much lower bandwidth than a big transmitter and therefore some
> additional self-limiting of transmitted bandwidth.

Not necessarily. Antenna height is driven to a large degree
by the requirement to maintain specified field strength and
minimize high angle radiation that causes skywave interference
within the coverage area.

> Is the transmitted bandwidth part of the measurements
> required for broadcast transmitters?

Not specifically for audio response measurements.

73,

... Joe, W4TV

> -----Original Message-----
> From: David Cutter [mailto:[hidden email]]
> Sent: Tuesday, January 15, 2008 6:02 PM
> To: [hidden email]; Joe Subich, W4TV
> Cc: [hidden email]
> Subject: Re: [Elecraft] RE: AM bandwidth, the rest of the story :=)
>
>
> I haven't read anyone refer to the antenna bandwidth, perhaps
> I missed it.
> Small, local, transmitters would have "small" antennas with
> much lower
> bandwidth than a big transmitter and therefore some
> additional self-limiting
> of transmitted bandwidth.
>
> Is the transmitted bandwidth part of the measurements
> required for broadcast
> transmitters? It would make a lot more sense to me than what
> is fed to the
> antenna.
>
>
> David
> G3UNA
>
>
>

Joe Subich, W4TV-3

RE: AM bandwidth, the rest of the story :=)

In reply to this post by Alan Bloom

> Somehow they have to limit their occupied bandwidth to less
> than 10 kHz. If they don't there will be severe adjacent-channel
> interference.

No they don't. Allocation practices do not place adjacent channel
stations in the same service area. The rules only impose limits
at +/- 10 KHz, not +/- 5 KHz. I am sure the transmitters contain
brickwall audio filters that allow flat response to 10 KHz while
still meeting the standard at 10.5 KHz.

73,

... Joe, W4TV

> -----Original Message-----
> From: Alan Bloom [mailto:[hidden email]]
> Sent: Tuesday, January 15, 2008 5:40 PM
> To: Joe Subich, W4TV
> Cc: [hidden email]
> Subject: RE: AM bandwidth, the rest of the story :=)
>
>
> Joe,
>
> Somehow they have to limit their occupied bandwidth to less than 10
> kHz. If they don't there will be severe adjacent-channel
> interference.
> That means at most 5 kHz audio bandwidth, assuming a
> brick-wall filter.
>
> Using a real-world filter, it has to be substantially less
> than 5 kHz.
> You can argue whether the right number is 4 kHz or 4.5 kHz,
> but for sure
> it isn't 10 kHz.
>
> 73,
>
> Al N1AL
>
>
> On Tue, 2008-01-15 at 07:22, Joe Subich, W4TV wrote:
> > > OK< I looked it up. According to Title 47, part 73.44 of the FCC
> > > regulations,
> <http://www.fcc.gov/mb/audio/bickel/amfmrule.html#AM>
> > > the modulation of an AM broadcast station must be down 25 dB at
> > > 10.2 kHz from the carrier.
> >
> > The information at 73.44(b) only applies to out of band emissions.
> > You will note that the section applies additional limitations at
> > 20 KHz from carrier, 30 KHz from carrier, etc.
> >
> > > Assuming a 3-pole low-pass filter (e.g. a pi-network), the filter
> > > attenuation is 18 dB per octave, which implies a cutoff frequency
> > > of no more than 3.9 kHz. The -3 dB bandwidth would be a little
> > > higher than that.
> >
> > Any AM station that used a simply pi-network filter would have
> > real problems since they would sound "muddy" an lack punch. I
> > think you will find that they are using filters much more complex
> > than a simple pi-network - usually with a sharp cut-off ("brick
> > wall") response to maintain response out to 10 KHz. For example,
> > the audio response specification for the Harris AM transmitters
> > is: +0.2/-0.8 dB, 20 Hz to 10 kHz. Ref. 1 kHz at 95% modulation.
> > See www.broadcast.harris.com/radio/transmission/analog.asp and
> > look at the various specification sheets for the Analog AM
> > transmitters.
> >
> > 73,
> >
> > ... Joe, W4TV
> >
> >
> >
> > > -----Original Message-----
> > > From: Alan Bloom [mailto:[hidden email]]
> > > Sent: Tuesday, January 15, 2008 1:50 AM
> > > To: Joe Subich, W4TV
> > > Cc: [hidden email]
> > > Subject: AM bandwidth, the rest of the story :=)
> > >
> > >
> > > OK< I looked it up. According to Title 47, part 73.44 of the FCC
> > > regulations,
> <http://www.fcc.gov/mb/audio/bickel/amfmrule.html#AM> the
> > > modulation of an AM broadcast station must be down 25 dB
> at 10.2 kHz
> > > from the carrier. Assuming a 3-pole low-pass filter (e.g. a
> > > pi-network), the filter attenuation is 18 dB per octave,
> > > which implies a
> > > cutoff frequency of no more than 3.9 kHz. The -3 dB
> > > bandwidth would be
> > > a little higher than that.
> > >
> > > That's about what I remember from my broadcasting days many,
> > > many years
> > > ago. If you think about it, a double-sideband AM signal
> can't have a
> > > bandwidth greater than 1/2 the channel spacing without
> > > interfering with
> > > adjacent channels. And it has to be somewhat less than that given
> > > real-world filters. So there is not much point in having
> a receiver
> > > with much more than 4 kHz or so audio response (8 kHz or so RF
> > > bandwidth).
> > >
> > > > I remember the AM guys doing proof to 10 KHz.
> > >
> > > Right, in order to confirm that the modulation is down 25
> dB at 10.2
> > > kHz.
> > >
> > > Al N1AL
> > >
> > >
> > > On Sun, 2008-01-13 at 20:40, Joe Subich, W4TV 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
> > > >
> > >
> > >
> > >
>
>
>

N2EY

Re: AM bandwidth, the rest of the story :=)

In reply to this post by Alan Bloom

In a message dated 1/15/08 3:18:00 PM Eastern Standard Time, [hidden email]
writes:

> 500kW? I assume that must be a typo. AFAIK, the limit on the AM BCB is 50kW
> in
> Canada and the US.
>

There was one 500 kW AM BC station in the USA, however. IIRC, it was WLW, and
it ran that power level in the 1930s. Special experimental permit or some
such. WW2 caused reduction to 50 kW - I don't know if WLW was allowed to increase
power after the war.

Google "WLW" for history, pictures, etc.

73 de Jim, N2EY

**************
Start the year off right. Easy ways to stay in
shape.

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w7aqk

Re: AM bandwidth, the rest of the story :=)

In reply to this post by Kevin Cozens-2

Well, maybe he did mean 50 KW, but then again there are (were) stations like
good ole' XERF, Del Rio, Texas! That station (and a bunch of others from
south of the border, ran a heck of a lot more than 50 KW.

I remember also that several other stations in the NW ran 50 KW, like KEX in
Portland, and maybe another Portland stations (KWJJ ?). Albuquerque had
KOB, which shared 770 khz with only one other station in NYC--that's changed
now. KGO in S.F. was 50 KW also I think. There used to be lots of them.

Dave W7AQK

----- Original Message -----
From: "Kevin Cozens" <[hidden email]>
To: "K2" <[hidden email]>
Sent: Tuesday, January 15, 2008 1:17 PM
Subject: Re: [Elecraft] AM bandwidth, the rest of the story :=)

> Ron D'Eau Claire wrote:
>> The "clear channel flamethrowers" running
>> 500,000 watts are relatively few by comparison.
>
> 500kW? I assume that must be a typo. AFAIK, the limit on the AM BCB is
> 50kW in Canada and the US.
>
> --
> Cheers!
>
> Kevin.
>
> http://www.ve3syb.ca/ |"What are we going to do today, Borg?"
> Owner of Elecraft K2 #2172 |"Same thing we always do, Pinkutus:
> | Try to assimilate the world!"
> #include <disclaimer/favourite> | -Pinkutus & the Borg
> _______________________________________________
> Elecraft mailing list
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> Elecraft web page: http://www.elecraft.com
>

Ken Kopp

OT - Antenna bandwidth

In reply to this post by David Cutter

Two items ...

The now-QRT 20 kHz and the present 60 kHz transmitters
at WWVL / WWVB had / have such high antenna Q that a
close-overhead thunder storm cloud can detune the antennas
enough to cause a TX overload / shut-down.

I've read that VLF BC stations limit audio bandwidth to avoid
exceeding their antenna's bandwidth.

Ken Kopp - K0PP

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Augie "Gus" Hansen

Re: AM bandwidth, the rest of the story :=)

In reply to this post by w7aqk

David Yarnes wrote:
> Well, maybe he did mean 50 KW, but then again there are (were)
> stations like good ole' XERF, Del Rio, Texas! That station (and a
> bunch of others from south of the border, ran a heck of a lot more
> than 50 KW.

I used to listen to XERF, XEG, and several other "south of the border"
megawatt stations from my childhood home in Rhode Island. In winter I
could hear them almost any time except for a few hours around noon.

> ... There used to be lots of them.

There still are many. AC6V has compiled a list of stations that run 50KW
at night (as well as during the day) to help those seeking "heard all
states" status. Check the following URLs for details:

http://www.ac6v.com/clearam.htm#50
http://www.ac6v.com/clearam.htm#USA

Happy BCB DXing,

Gus Hansen
KB0YH
Denver, CO

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Alan Bloom

RE: AM bandwidth, the rest of the story :=)

In reply to this post by Joe Subich, W4TV-3

> Allocation practices do not place adjacent channel
> stations in the same service area. The rules only impose limits
> at +/- 10 KHz, not +/- 5 KHz.

That makes sense. If the rules only specify -25 dB at +/-10.2 kHz, that
still allows a lot of QRM at the adjacent channel edges at +/-5 kHz.
They must be assuming no adjacent channel.

> I am sure the transmitters contain
> brickwall audio filters that allow flat response to 10 KHz while
> still meeting the standard at 10.5 KHz.

It's actually 10.2 kHz, not 10.5. I don't think I'm going too far out
on a limb to state that it's absolutely impossible to build an analog
filter that is flat to 10 kHz but down 25 dB at 10.2 kHz. It might be
possible to do that with a digital filter, but remember DSP wasn't even
thought of when those regulations were written.

What would it take to build a coil-capacitor filter that is flat out to,
let's say, 9 kHz and down 25 dB at 10.2 kHz? That works out to about
138 dB per octave, which implies something like a 23-pole filter. Some
of those poles would have extremely high loaded Q so the inductors would
have to be huge. But the main problem is that a filter that sharp would
have a horrible peak in the group delay that would degrade the audio
quality you are trying to achieve.

A more-reasonable analog low-pass filter is something like the so-called
"half wave" pi-network. It has a loaded Q of 1 and reasonably-flat
group delay. For such a filter, the bandwidth has to be less than 4 kHz
or so to achieve -25 dB at 10.2 kHz.

I think any analog filter design with a reasonable number of poles,
decent group delay, and 25 dB attenuation at 10.2 kHz will end up with a
bandwidth of less than 5 or 6 kHz.

Al N1AL

On Tue, 2008-01-15 at 15:51, Joe Subich, W4TV wrote:

> > Somehow they have to limit their occupied bandwidth to less
> > than 10 kHz. If they don't there will be severe adjacent-channel
> > interference.
>
> No they don't. Allocation practices do not place adjacent channel
> stations in the same service area. The rules only impose limits
> at +/- 10 KHz, not +/- 5 KHz. I am sure the transmitters contain
> brickwall audio filters that allow flat response to 10 KHz while
> still meeting the standard at 10.5 KHz.
>
> 73,
>
> ... Joe, W4TV
>

Joe Subich, W4TV-3

RE: AM bandwidth, the rest of the story :=)

> > I am sure the transmitters contain
> > brickwall audio filters that allow flat response to 10 KHz while
> > still meeting the standard at 10.5 KHz.
>
> It's actually 10.2 kHz, not 10.5. I don't think I'm going too far out
> on a limb to state that it's absolutely impossible to build an analog
> filter that is flat to 10 kHz but down 25 dB at 10.2 kHz. It might be
> possible to do that with a digital filter, but remember DSP
> wasn't even thought of when those regulations were written.

There s no doubt the modern transmitters don't use analog filters.
They're all using digital filtering of the audio and digital modulation
methods. However, even 30 years ago before many of the digital methods
transmitters were making 8 KHz or more of audio bandwidth while still
meeting the limits at 10.2 KHz.

73,

... Joe, W4TV

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