K3 FM filter required for full AM receive width

[KM5Q]

Wayne,

In the new firmware announcement you say:

> AM RECEIVE WIDTH RANGE INCREASED
> ...
> NORM selects an IF bandwidth of 6.0 kHz in this mode. WIDTH settings  
> above 6.0 kHz will not be useful unless you have an FM-bandwidth  
> crystal filter installed (at FL1).

I like using the K3 for shortwave listening. I would rather not be  
required to buy the FM xtal filter to get the full bandwidth on AM.  
Will this change with future revisions?

Windy KM5Q
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[Julian, G4ILO]

Windy Dankoff KM5Q wrote

I like using the K3 for shortwave listening. I would rather not be  
required to buy the FM xtal filter to get the full bandwidth on AM.  
Will this change with future revisions?

How can you have an IF bandwidth wider than 6KHz without using a filter that's wider than 6KHz?

[David Woolley (E.L)]

KM5Q wrote:

> I like using the K3 for shortwave listening. I would rather not be
> required to buy the FM xtal filter to get the full bandwidth on AM. Will
> this change with future revisions?

I don't understand the question; it seems to me to be a matter of physics.

A roofing filter is needed to suppress the image response at the final
(DSP) IF.

True AM detection requires a bandwidth of twice the highest modulation
frequency.  By "full AM" I presume you mean medium wave AM (9kHz
channels in Europe, or shortwave AM that uses more than one channel,
e.g. 5kHz maximum modulation frequency, so you need a 10kHz bandwidth.

The only filter available from Elecraft that has at least 10kHz
bandwidth is the FM one, and you therefore need that if you want audio
beyond about 2.5kHz on AM.  You need an additional filter for any true
AM detection with a sensible bandwidth.  Not having a filter at all is
not an option, becuause of the image response.



--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
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[KM5Q]

My widest filter is 2.8. I am allowed to widen the DSP to 4. So why  
not in AM, for BC receiving, be allowed wide enough for high fidelity,  
without a roofing (or xmit) filter? (AM can be widened to "10", but it  
sounds narrow and muffled)

I get much better bandwidth (fidelity) now using SSB to receive AMl,  
but it makes noisy band-browsing because of all the heterodyne noise.

Windy KM5Q  K3 764

> How can you have an IF bandwidth wider than 6KHz without using a  
> filter
> that's wider than 6KHz?
>
> -----
> Julian, G4ILO  K3 s/n: 222 K2 s/n: 392
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[Stewart Baker]

Why not an LC filter for AM reception instead of an expensive
crystal roofing filter ?

73
Stewart G3RXQ
On Mon, 05 May 2008 08:29:29 -0600, KM5Q wrote:
> My widest filter is 2.8. I am allowed to widen the DSP to 4. So
why
> not in AM, for BC receiving, be allowed wide enough for high
fidelity,
> without a roofing (or xmit) filter? (AM can be widened to "10",
but it
> sounds narrow and muffled)
>
> I get much better bandwidth (fidelity) now using SSB to receive
AMl,
> but it makes noisy band-browsing because of all the heterodyne
noise.
>
> Windy KM5Q  K3 764
>
>> How can you have an IF bandwidth wider than 6KHz without using
a

>> filter
>> that's wider than 6KHz?
>>
>> -----
>> Julian, G4ILO  K3 s/n: 222 K2 s/n: 392
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[Dave Martin-2]

On 5/5/08, Stewart Baker <[hidden email]> wrote:
> Why not an LC filter for AM reception instead of an expensive
>  crystal roofing filter ?
>
That's what I did.  I cut a scrap of PC board to the size of a filter
and used three toroids with tuning and coupling caps.  It worked just
fine in position one, set up as 13 KHz.  The DSP hasn't suffered at
all.  Maybe with more antenna gain one might see some overload from
nearby signals, but not with my modest wire.  I sure wouldn't transmit
with it, though.  It's hugely wide, offering no attenuation of
unwanted mixer products.

Dave  W5DHM
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[Stewart Baker]

Blast ! you got there before me Dave.
I have been modeling some suitable LC filters, but as you say they
are a bit wide for transmit. However looking back in my log shows
that I haven't had an AM QSO for 10's of years, so it's unlikely I
will have one now. :-)

73
Stewart G3RXQ
On Mon, 5 May 2008 11:01:13 -0400, Dave Martin wrote:
> On 5/5/08, Stewart Baker <[hidden email]> wrote:
>> Why not an LC filter for AM reception instead of an expensive
>> crystal roofing filter ?
>>
> That's what I did.  I cut a scrap of PC board to the size of a
filter
> and used three toroids with tuning and coupling caps.  It worked
just
> fine in position one, set up as 13 KHz.  The DSP hasn't suffered
at
> all.  Maybe with more antenna gain one might see some overload
from
> nearby signals, but not with my modest wire.  I sure wouldn't
transmit

> with it, though.  It's hugely wide, offering no attenuation of
> unwanted mixer products.
>
> Dave  W5DHM
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[KK7P]

> ...  I sure wouldn't transmit
> with it, though.  It's hugely wide, offering no attenuation of
> unwanted mixer products.

If the filter is too wide, then poor 15 kHz image rejection, aliasing
and other unwanted things might happen in Rx.  The DSP depends on
bandwidth limiting prior to digitizing, and the K3 system design places
that burden on the crystal roofing filter.

That said, congratulations on your ingenuity to do this!

73,

Lyle KK7P

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[Jerry Flanders]

Sounds good for SWL. Only downside (a minor one, IMO) is that you
might get response from images 30 KHz away. How much weaker is a
signal when you tune to its image 30 KHz up/down?

Jerry W4UK

At 11:01 AM 5/5/2008, Dave Martin wrote:

>On 5/5/08, Stewart Baker <[hidden email]> wrote:
> > Why not an LC filter for AM reception instead of an expensive
> >  crystal roofing filter ?
> >
>That's what I did.  I cut a scrap of PC board to the size of a filter
>and used three toroids with tuning and coupling caps.  It worked just
>fine in position one, set up as 13 KHz.  The DSP hasn't suffered at
>all.  Maybe with more antenna gain one might see some overload from
>nearby signals, but not with my modest wire.  I sure wouldn't transmit
>with it, though.  It's hugely wide, offering no attenuation of
>unwanted mixer products.
>
>Dave  W5DHM
>_______________________________________________

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[Dave Martin-2]

> Blast ! you got there before me Dave.
>  I have been modeling some suitable LC filters, but as you say they
>  are a bit wide for transmit. However looking back in my log shows
>  that I haven't had an AM QSO for 10's of years, so it's unlikely I
>  will have one now. :-)
>
>  73
>  Stewart G3RXQ
>
I had plenty of time while waiting for #605 to arrive, so I played
around with designs on Elsie.  I had a filter tested and rough tuned
when the radio came.  I don't think I'd want to transmit on AM in our
neighborhood anyway.  I'd surely come in loud and clear on some device
in one of the nearby houses.  At least no one can identify you on SSB
or CW.  Hi.  AM's better for a countryside QTH.

And thanks, Lyle.  I was a bit concerned, but I can rock the width
back and forth at the point where it changes roofing filters and never
tell any difference.  But I seldom see a signal much over S9.  Maybe
it'd be worse with stronger signals.

Dave  W5DHM
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[Dave Martin-2]

On 5/5/08, Jerry Flanders <[hidden email]> wrote:
> Sounds good for SWL. Only downside (a minor one, IMO) is that you might get
> response from images 30 KHz away. How much weaker is a signal when you tune
> to its image 30 KHz up/down?
>
>  Jerry W4UK
>
I can't hear anything, Jerry, even from our local jukebox station at
40 over.  But I was thinking that the two pole filter following the IF
amp might be enough to minimize any problem.  I had some fun building
the filter, and it's saved me some money until I decide if I want to
order something else.

Dave  W5DHM
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[Julian, G4ILO]

Stewart G3RXQ wrote

Why not an LC filter for AM reception instead of an expensive
crystal roofing filter ?

I was going to suggest that, then I thought of the points Lyle made and guessed that it wouldn't work. But if it works well enough for casual listening and people are willing to accept the limitations then perhaps there is scope for someone to have this made up as a kit.

As for opening up the DSP to 4KHz when you only have a 2.7KHz filter, I didn't realize you could do this, since I have both the AM and FM filters. I can't really see the point. As the DSP filters are sharper than the 2.7KHz crystal filter I guess it lets you hear a bit more bandwidth, but it's a bit misleading. The AM filter is quite sharp already so I doubt if allowing the DSP to be wider would gain you much.

[David Woolley (E.L)]

Jerry Flanders wrote:
> for SWL. Only downside (a minor one, IMO) is that you might get response
> from images 30 KHz away. How much weaker is a signal when you tune to
> its image 30 KHz up/down?

Less than 30kHz.  The whole point of the exercise is to use wide
filters, so, assuming that the interference is from a carrier and
appears in the demodulated audio, the image can be 25 (5kHz audio) to
20kHz (10kHz audio) away from the carrier (20 and 10kHz from the
passband edge).  If the interference is from an audio component at the
extreme of the bandwidth and the transmit is the same as the receive
bandwidth, the carrier can be 20 to 10kHz away (although in the latter
case would have other problems).  In practice, I would expect the
carrier to get past a weak filter better than an extreme sideband frequency.

Note this means that if the signals are trying to reproduce audio up to
10kHz, the adjacent channel will be an issue.




--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
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[Stewart Baker]

Just to pass the time I used a wire to link across an unused
filter position (with a series 0.1u DC blocking capacitor),
adjusted the filter bandwidth to 8kHz, and listened to AM
broadcast stations. Quite a respectable performance, much better
than with a 2.7kHz filter.

Now I know that it is worth making even a fairly loose LC filter
just for occasional AM reception.

73
Stewart G3RXQ
On Mon, 05 May 2008 20:25:59 +0100, David Woolley (E.L) wrote:
> Jerry Flanders wrote:
>> for SWL. Only downside (a minor one, IMO) is that you might get
response
>> from images 30 KHz away. How much weaker is a signal when you
tune to
>> its image 30 KHz up/down?
>>
> Less than 30kHz.  The whole point of the exercise is to use wide
> filters, so, assuming that the interference is from a carrier
and
> appears in the demodulated audio, the image can be 25 (5kHz
audio) to
> 20kHz (10kHz audio) away from the carrier (20 and 10kHz from the
> passband edge).  If the interference is from an audio component
at the
> extreme of the bandwidth and the transmit is the same as the
receive
> bandwidth, the carrier can be 20 to 10kHz away (although in the
latter
> case would have other problems).  In practice, I would expect
the
> carrier to get past a weak filter better than an extreme
sideband frequency.
>
> Note this means that if the signals are trying to reproduce
audio up to
> 10kHz, the adjacent channel will be an issue.


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[Vic K2VCO]

Stewart Baker wrote:

> Now I know that it is worth making even a fairly loose LC filter
> just for occasional AM reception.

Keep in mind that you can copy AM stations quite well in SSB mode by
zero-beating the carrier and choosing the better sideband (some SWBC
stations seem to transmit only one sideband plus carrier).

I'm not suggesting that this provides high-fidelity reception, but it is
surprisingly good, especially for speech.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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[Bill VanAlstyne W5WVO]

Vic K2VCO wrote:

> Stewart Baker wrote:
>
>> Now I know that it is worth making even a fairly loose LC filter
>> just for occasional AM reception.
>
> Keep in mind that you can copy AM stations quite well in SSB mode by
> zero-beating the carrier and choosing the better sideband (some SWBC
> stations seem to transmit only one sideband plus carrier).
>
> I'm not suggesting that this provides high-fidelity reception, but it
> is surprisingly good, especially for speech.

I'm becoming increasingly amused by this back-and-forth about AM short-wave
broadcast reception and what is "right" in terms of receiver performance. As
an SWL for nearly 50 years, let me put this in perspective by asserting that
there are two kinds of short-wave listeners:

There is the guy (if a ham, he is probably a DXer) who wants to dig out that
weak, garbled signal from some obscure African republic that is hiding 5 kHz
from (and about 60 dB below) 40-over-S9 Radio Nederland.

And then there is the guy (if a ham, he is probably a rag-chewer) who wants to
put on his smoking jacket and slippers, sit back in his favorite armchair with
a pipe and a snifter of brandy, and actually LISTEN to Radio Nederland.

I have great affinity for both these approaches to SWLing! No need to argue
about it. :-)  They do require two entirely different and fundamentally
incompatible approaches to receiver design and performance. If you can embody
both approaches in one receiver by providing adequate configuration options,
so much the better. That's one of the things I'm really looking forward to in
the K3 once the AM options are completely fleshed out.

Bill W5WVO

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