Posted by
Alan Bloom on
Jan 16, 2008; 6:20am
URL: http://elecraft.85.s1.nabble.com/New-K3-SN-207-built-but-question-on-AM-Filter-tp458592p458600.html
> 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
>
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