Roofing filters and DSP bandwidth tuning

[NJMike]

If I understand it correctly, as you turn the bandwidth knob
counterclockwise, the filter narrows.  At some point, it hits the next
roofing filter and then....narrows more?  Just trying to understand so I can
get the right filters.  Rig A (below) is what Elecraft recommends for my
operating profile.

What would be the difference between the following three examples, as far as
how narrow the bandwidth could actually be adjusted?

Rig A with 2.8, 2.1, 400, 250, 200 filters
Rig B with 2.8, 2.1, 400, 250 filters
Rig C with 2.8, 2.1, 400, 200 filters



--
Sent from: http://elecraft.365791.n2.nabble.com/
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

These are roofing filters, they do not effect how narrow the bandwidth  
can be adjusted.  FWIW, I have both the 400 and 250 8 pole filters. If  
money is an issue, I believe that there is very little bandwidth  
difference between the two.

John KK9A


NJ Mike wrote:

If I understand it correctly, as you turn the bandwidth knob
counterclockwise, the filter narrows.  At some point, it hits the next
roofing filter and then....narrows more?  Just trying to understand so I can
get the right filters.  Rig A (below) is what Elecraft recommends for my
operating profile.

What would be the difference between the following three examples, as far as
how narrow the bandwidth could actually be adjusted?

Rig A with 2.8, 2.1, 400, 250, 200 filters
Rig B with 2.8, 2.1, 400, 250 filters
Rig C with 2.8, 2.1, 400, 200 filters

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[k6dgw]

NJ Mike:  If I understand your question, the answer is "Yes, as you
narrow the DSP BW with the 'WIDTH" knob, the roofing filters will switch
such that the narrowest filter that still includes the DSP BW will be
selected".

To your second question, the ultimate BW is set by the DSP, and on my
K3, that's 50 Hz.  It doesn't really matter what roofing filter is
selected.  The xtal filters are in the 1st IF [8 MHz] and their primary
[maybe only] purpose is to suppress strong signals outside the DSP BW
that would activate the AGC and affect the level of the desired signal
inside the DSP BW, even though you can't hear them.  It follows that
you'd probably like the selection of roofing filters to follow the DSP
BW as closely as possible.  You won't see any difference between your
Rigs A, B, and C.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

> NJ Mike wrote:
>
> If I understand it correctly, as you turn the bandwidth knob
> counterclockwise, the filter narrows.  At some point, it hits the next
> roofing filter and then....narrows more?  Just trying to understand so
> I can
> get the right filters.  Rig A (below) is what Elecraft recommends for my
> operating profile.
>
> What would be the difference between the following three examples, as
> far as
> how narrow the bandwidth could actually be adjusted?
>
> Rig A with 2.8, 2.1, 400, 250, 200 filters
> Rig B with 2.8, 2.1, 400, 250 filters
> Rig C with 2.8, 2.1, 400, 200 filters

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[Jim Brown-10]

On 1/27/2021 12:14 PM, [hidden email] wrote:
> FWIW, I have both the 400 and 250 8 pole filters. If money is an issue,
> I believe that there is very little bandwidth difference between the two.

I have both as well in all my radios. I can clearly hear the difference
when switching between them. I mostly use the 250 Hz filter for CW and
the 500 Hz filter for RTTY.

73, Jim K9YC
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

I have 8 pole 400 Hz filters in my K3s's, not 500 Hz as well as 250 Hz.
According to W0YK's presentation linked below the 400 filter has a bandwidth
of 435 and the 250 filter has a bandwidth of 370. This data is for 8 pole
crystal filters made by Inrad, I am not sure what filters Elecraft currently
sells.

http://www.nccc.cc/archived_meetings/pdf/K3%20Filters,%20Jan%202009.pdf 

John KK9A


Jim Brown K9YC wrote:

On 1/27/2021 12:14 PM, john at kk9a.com wrote:
> FWIW, I have both the 400 and 250 8 pole filters. If money is an issue,
> I believe that there is very little bandwidth difference between the two.

I have both as well in all my radios. I can clearly hear the difference
when switching between them. I mostly use the 250 Hz filter for CW and
the 500 Hz filter for RTTY.

73, Jim K9YC

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[David Woolley (E.L)]

On 27/01/2021 20:35, Fred Jensen wrote:
>    It follows that you'd probably like the selection of roofing filters
> to follow the DSP BW as closely as possible.

Having the roofing filter too close to the DSP filter is not necessarily
a good thing, as the roofing filters are likely to have worse passband
ripples and will have non-linear phase responses, which can compromise
digital modes.  At least some of the DSP filters are finite impulse
response, meaning they are also linear phase, which means that pulses
will not get smeared out.

--
David Woolley

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[k6dgw]

True.  Statement was focused on preventing AGC action from signals
outside the DSP BW and to do that, you'd like the roofing filters to
include the DSP BW but not much more.  The "effective" BW of the xtal
filters is also something larger than the 2.5 kHz or 0.5 kHz or 0.25 kHz
in the name ... more poles = steeper skirts. Phase response is fairly
irrelevant on CW and almost so on SSB.  If you're operating digital
modes, phase response [and passband ripple] becomes important if the
desired signal BW fills the filter BW.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 1/27/2021 3:06 PM, David Woolley wrote:

> On 27/01/2021 20:35, Fred Jensen wrote:
>>    It follows that you'd probably like the selection of roofing
>> filters to follow the DSP BW as closely as possible.
>
> Having the roofing filter too close to the DSP filter is not
> necessarily a good thing, as the roofing filters are likely to have
> worse passband ripples and will have non-linear phase responses, which
> can compromise digital modes.  At least some of the DSP filters are
> finite impulse response, meaning they are also linear phase, which
> means that pulses will not get smeared out.
>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[Jim Brown-10]

On 1/27/2021 2:48 PM, [hidden email] wrote:
> I have both as well in all my radios. I can clearly hear the difference
> when switching between them. I mostly use the 250 Hz filter for CW and
> the 500 Hz filter for RTTY.

Sorry, I mis-spoke. I have the 8-pole 250 Hz and 400 Hz filters, but I
set the DSP IF for 500 Hz for RTTY. I'm well aware how similarly they
measure -- W0YK and I did that independently in 2008-9 when we first
installed them.

73, Jim
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[Jim Brown-10]

On 1/27/2021 3:06 PM, David Woolley wrote:
> Having the roofing filter too close to the DSP filter is not necessarily
> a good thing, as the roofing filters are likely to have worse passband
> ripples and will have non-linear phase responses, which can compromise
> digital modes.  At least some of the DSP filters are finite impulse
> response, meaning they are also linear phase, which means that pulses
> will not get smeared out.

K1JT strongly urges 3kHz or greater IF bandwidths for his modes, and for
exactly that reason. And it's why top RTTY contesters have abandoned the
K3's dual-peak filter in favor of 500 Hz IF bandwidth. My professional
life in audio system design taught me that speech intelligibility is
degraded by time/phase distortion and suspected the result would be the
same with RTTY, but I was derided when I started preaching that to RTTY
guys. Several years later, author of the 2Tone RTTY software G3YYD said
the same thing, and folks started believing it. It's also why I find
that the 2.1 kHz 8-pole provides better speech intelligibility than the
1.8 kHz filter.

Years ago, I tried using narrow SSB realignments of the K2's CW crystal
filters in contests. I had carefully tweaked them per the build
instructions, noting that their amplitude response looked like the
profile of a mountain range. I wasn't surprised that those settings made
signals much harder to copy. The SSB TX filter sounded fine on RX.

When I switched from the 2.7 kHz 5-pole filters to the 2.8 kHz filters
for TX in one of my K3s I noticed considerably less incidental AM on
RTTY, and immediately converted the other two. I didn't do that in the
2nd RX, because I only use it for weak signal CW work on the lower bands.

73, Jim K9YC


______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]