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Roofing Filters

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AnnLatz

Roofing Filters

Can someone explain the meaning of "roofing filters". I have searched everywhere and can't find a good explaination. Lisa recommended the FAQ secftion of Elecraft and there they just mention them. Maybe I should order an updated Ham bible from ARRL.

Thanks, Alan KA9UCP
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Ken Wagner K3IU

Re: Roofing Filters

You can check here...

http://www.elecraft.com/K3/Roofing_Filters.htm

73, Ken K3IU

AnnLatz wrote:

> Can someone explain the meaning of "roofing filters". I have searched everywhere and can't find a good explaination. Lisa recommended the FAQ secftion of Elecraft and there they just mention them. Maybe I should order an updated Ham bible from ARRL.
>
> Thanks, Alan KA9UCP
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>

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Bill W4ZV

Re: Roofing Filters

In reply to this post by AnnLatz

AnnLatz wrote

Can someone explain the meaning of "roofing filters". I have searched everywhere and can't find a good explanation. Lisa recommended the FAQ section of Elecraft and there they just mention them. Maybe I should order an updated Ham bible from ARRL.

Thanks, Alan KA9UCP

See below, written by W2VJN of Inrad:

http://www.qth.com/inrad/roofing-filters.pdf

73, Bill W4ZV

AC7AC

RE: Roofing Filters

In reply to this post by AnnLatz

CONTENTS DELETED

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Dave Van Wallaghen

RE: Roofing Filters

First, thank you Ron for putting that in a very comprehensive manner that
even I could understand ;-) As usual, you've turned on the "light bulb".

Now, when I purchased my K3 and debated on my purchase of the roofing
filters, I spoke with a few hams who said that the roofing filters were
primarily for blocking out close in and powerful signals and that if I
didn't have that problem where I live, that the standard DSP filter with the
standard 2.7khz filter would probably suffice. And that is what I did.

But I wonder (always dangerous), if operating on some of the noisier bands,
would there be a benefit to using a narrower filter to cut down on the
amount of energy hitting the DSP in order to help with digging out weak
signals? Or is that not a consideration with this particular implementation
of DSP in the K3?

As is, it has been fun playing around with digging out weak signals and
enhancing them. I need to get a better antenna to rig switching setup so I
can make a/b comparisons with my K2.

Thanks in advance,
Dave W8FGU

>
> Can someone explain the meaning of "roofing filters". I have searched
> everywhere and can't find a good explaination. Lisa recommended the FAQ
> secftion of Elecraft and there they just mention them. Maybe I should
> order
> an updated Ham bible from ARRL.
>
> --------------------------------------------
>
> The K3 receiver, like many current superhetrodyne receivers, has more than
> one high-performance filter in the system.
>
> I don't know how long you've been tinkering with superhets, but a few
> decades we ago we just called the "roofing filter" the first I.F. filter.
>
> You probably know that a superheterodyne receiver takes the selected
> incoming signal, no matter where it is in the tuning range of the
> receiver,
> and converts it to one fixed frequency before it is demodulated. That
> fixed
> frequency is called the intermediate frequency (I.F.).
>
> >From a design standpoint, it's a good idea to put all the selectivity as
> early in the receiver chain as possible so the first I.F. filter has been
> the most important in the system. Indeed, we normally tried to do all the
> significant filtering there. That's who the K2 is designed.
>
> Nowadays higher performance amplifiers and mixers allow more signal
> processing before it's necessary to use a filter to strip off all but the
> exact signal we want to hear.
>
> We use the first I.F. filter to reject signals that are completely outside
> the range of interest, then use a second filter further along in the
> signal
> path to finish the filtering to set the final bandwidth to just what we
> want.
>
> The first I.F. filter, therefore, defines how far off each side of the
> center frequency we can "hear". That is, it sets the limit or "roof" on
> the
> bandpass. (I think of it as the upper and lower "sides" to the bandpass,
> but
> no one checked with me before choosing the name "roofing filter".)
>
> The K3's second filter is provided by digital signal processing (DSP) and
> allows controlling the bandpass and center frequency to set it anywhere
> within the range of the first I.F. (roofing) filter bandpass.
>
> Even though mixers and amplifiers are much better today than a few years
> ago, it's still a good idea to reject all unwanted signals as early in the
> signal path as possible. That's why the K3 offers up to five "roofing
> filters" of various bandwidths to fit the sort of signal you're receiving
> from very wide FM signals down to a very narrow CW/PSK signal. Within the
> bandpass of the roofing filter, the DSP filter provides additional
> flexibility to set the exact upper and lower frequency limits, provide a
> notch filter to remove a specific heterodyne, etc.
>
> Ron AC7AC

KK7P

Re: Roofing Filters

> But I wonder (always dangerous), if operating on some of the noisier bands,
> would there be a benefit to using a narrower filter to cut down on the
> amount of energy hitting the DSP in order to help with digging out weak
> signals? Or is that not a consideration with this particular implementation
> of DSP in the K3?

In general, the DSP can handle signals and noise below about S9+40 or so
without undesired effects. Signals between about S9+40 and S9+60 will
start to activate the hardware AGC (to protect the analog to digital
converter from overload), so if there are strong signals inside the
roofing filter passband but outside the DSP IF filter passband, they can
make their presence known by activating (or "pumping") the AGC.

So, it isn't the weak signals or noise that are of concern, but the
presence of adjacent signals that are (a) stronger than the one you are
trying to copy and (b) above the hardware AGC threshold.

Contests -- and therefore weekends -- come to mind :-)

73,

Lyle KK7P

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Dave Van Wallaghen

RE: Roofing Filters

Hey Lyle,

Thanks for the reply. Man, if anyone knows about how the K? DSP works...

So if I read this right (bear with me, I'm learning), in keeping strong
signals out of its passband, it will keep the hardware AGC from activating
to eliminate "pumping". And that makes perfect sense. Keeping the high
energy out from interfering with the signal you want.

But, in the same scenario, if I had a 2.7khz filter and a 200hz filter with
no strong signals, just band noise (or maybe static crash type QRN), the DSP
filter would act the same and my desired signal would come through the same
way regardless of which roofing filter I used. In other words, if I had a
weak signal just above the noise floor in the above scenario, it would not
make a difference which roofing filter I chose, the signal would sound the
same.

Obviously, I would narrow the DSP filter to chop off some of the noise. And
that is where, in my above scenario, the elimination of extra energy from
noise would be eliminated. Reduction in bandwidth in the roofing filter
would have no redeeming effect.

And if you would permit me one more scenario. In the case of strong static
crash type QRN, in my case, either the noise blanker or noise reduction
would be able to handle the really heavy stuff. I know the hardware noise
blanker is set in front of the roofing filters and well ahead of the DSP,
therefore protecting it. I guess that statement is more of a question than a
statement.

Am I getting this clear?

73,
Dave W8FGU

>
> > But I wonder (always dangerous), if operating on some of the noisier
> bands,
> > would there be a benefit to using a narrower filter to cut down on the
> > amount of energy hitting the DSP in order to help with digging out weak
> > signals? Or is that not a consideration with this particular
> implementation
> > of DSP in the K3?
>
> In general, the DSP can handle signals and noise below about S9+40 or so
> without undesired effects. Signals between about S9+40 and S9+60 will
> start to activate the hardware AGC (to protect the analog to digital
> converter from overload), so if there are strong signals inside the
> roofing filter passband but outside the DSP IF filter passband, they can
> make their presence known by activating (or "pumping") the AGC.
>
> So, it isn't the weak signals or noise that are of concern, but the
> presence of adjacent signals that are (a) stronger than the one you are
> trying to copy and (b) above the hardware AGC threshold.
>
> Contests -- and therefore weekends -- come to mind :-)
>
> 73,
>
> Lyle KK7P

AC7AC

RE: Roofing Filters

In reply to this post by Dave Van Wallaghen

CONTENTS DELETED

The author has deleted this message.

KK7P

Re: Roofing Filters

In reply to this post by Dave Van Wallaghen

Hello Dave!

> So if I read this right (bear with me, I'm learning), in keeping strong
> signals out of its passband, it will keep the hardware AGC from activating
> to eliminate "pumping"...

Correct.

> But, in the same scenario, if I had a 2.7khz filter and a 200hz filter with
> no strong signals, just band noise (or maybe static crash type QRN), the DSP
> filter would act the same and my desired signal would come through the same
> way regardless of which roofing filter I used...

Also correct. The key is if signals stronger than the desired signal
are in the roofing filter passband, and are also strong enough to
activate the hardware AGC.

> And if you would permit me one more scenario. In the case of strong static
> crash type QRN, in my case, either the noise blanker or noise reduction
> would be able to handle the really heavy stuff. I know the hardware noise
> blanker is set in front of the roofing filters and well ahead of the DSP,
> therefore protecting it. I guess that statement is more of a question than a
> statement.

We have pulse-suppressing AGC in addition to DSP and IF blankers. Won't
kill all the noise types, but makes a very effective arsenal against
noise. Narrow roofing filters can help in some cases here, too.

In the end, you may not often need narrow roofing filters in your
particular station's interference environment, but when you need 'em,
you need 'em :-)

73,

Lyle KK7P

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Dave Van Wallaghen

RE: Roofing Filters

Thanks again Lyle, I believe I now have the proper prospective (looking at
the block diagram of the K3 helped out too). So far in my limited operation
of K3 #338, anytime I've tried to dig out a weaker CW signal next to a
stronger one, the DSP filter has worked just fine. And I found when it was a
little too close I was able to shift the filter away from the offending
signal enough to hear the desired one very comfortably. Of course, these
were in non contest and non-aggressive situations.

I saw a post the other day about the notch filter. I have used it a couple
of times in the old novice portion of 40m (where I hang out with the QRS
crowd). When some of the BC stations start up, I was able to notch out that
carrier just fine and turn an irritating listening situation into something
very comfortable.

Very nice work. Thanks again Lyle (hope I didn't bore too many on the list).

73,
Dave
W8FGU

>
> Hello Dave!
>
> > So if I read this right (bear with me, I'm learning), in keeping strong
> > signals out of its passband, it will keep the hardware AGC from
> activating
> > to eliminate "pumping"...
>
> Correct.
>
> > But, in the same scenario, if I had a 2.7khz filter and a 200hz filter
> with
> > no strong signals, just band noise (or maybe static crash type QRN), the
> DSP
> > filter would act the same and my desired signal would come through the
> same
> > way regardless of which roofing filter I used...
>
> Also correct. The key is if signals stronger than the desired signal
> are in the roofing filter passband, and are also strong enough to
> activate the hardware AGC.
>
> > And if you would permit me one more scenario. In the case of strong
> static
> > crash type QRN, in my case, either the noise blanker or noise reduction
> > would be able to handle the really heavy stuff. I know the hardware
> noise
> > blanker is set in front of the roofing filters and well ahead of the
> DSP,
> > therefore protecting it. I guess that statement is more of a question
> than a
> > statement.
>
> We have pulse-suppressing AGC in addition to DSP and IF blankers. Won't
> kill all the noise types, but makes a very effective arsenal against
> noise. Narrow roofing filters can help in some cases here, too.
>
> In the end, you may not often need narrow roofing filters in your
> particular station's interference environment, but when you need 'em,
> you need 'em :-)
>
> 73,
>
> Lyle KK7P

Dave Van Wallaghen

RE: Roofing Filters

In reply to this post by AC7AC

Thanks Ron, that was a good read and reiterated what you and Lyle stated
here. I read that before, but I think now that I've been playing around with
#338, it is just sinking in now.

Thanks again for the help.

73,
Dave W8FGU

> -----Original Message-----
> From: [hidden email] [mailto:elecraft-
> [hidden email]] On Behalf Of Ron D'Eau Claire
> Sent: Wednesday, February 20, 2008 7:35 PM
> To: [hidden email]
> Subject: RE: [Elecraft] Roofing Filters
>
> I see that Lyle (Mr. DSP) stepped in with the answer I couldn't have given
> you anyway <G>. That's the beauty of this reflector: lots of expertise and
> resources. I seldom "answer" a question without learning something myself!
>
> Someone else dropped me a note reminding me that Wayne discusses roofing
> filters on the Elecraft web site and, in particular how he addressed the
> design issues of employing them in the K3. That FB write up is here:
>
> http://elecraft.com/K3/Roofing_Filters.htm
>
> Ron AC7AC
>
>
>
> -----Original Message-----
> From: Dave Van Wallaghen [mailto:[hidden email]]
> Sent: Wednesday, February 20, 2008 2:21 PM
> To: 'Ron D'Eau Claire'; [hidden email]
> Subject: RE: [Elecraft] Roofing Filters
>
>
> First, thank you Ron for putting that in a very comprehensive manner that
> even I could understand ;-) As usual, you've turned on the "light bulb".
>
> Now, when I purchased my K3 and debated on my purchase of the roofing
> filters, I spoke with a few hams who said that the roofing filters were
> primarily for blocking out close in and powerful signals and that if I
> didn't have that problem where I live, that the standard DSP filter with
> the
> standard 2.7khz filter would probably suffice. And that is what I did.
>
> But I wonder (always dangerous), if operating on some of the noisier
> bands,
> would there be a benefit to using a narrower filter to cut down on the
> amount of energy hitting the DSP in order to help with digging out weak
> signals? Or is that not a consideration with this particular
> implementation
> of DSP in the K3?
>
> As is, it has been fun playing around with digging out weak signals and
> enhancing them. I need to get a better antenna to rig switching setup so I
> can make a/b comparisons with my K2.
>
> Thanks in advance,
> Dave W8FGU
>
> >
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

KK7P

Re: Roofing Filters

In reply to this post by Dave Van Wallaghen

> ...Thanks again Lyle (hope I didn't bore too many on the list).

Thank you for your interest in all this. Self-education is one of the
pillars of our avocation, and it is always a pleasure to try to help in
this process when I am able.

73,

Lyle KK7P

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Brett Howard

RE: Roofing Filters

In reply to this post by Bill W4ZV

So in reading the inrad description of roofing filters at:

http://www.qth.com/inrad/roofing-filters.pdf

It seems that they all but tell you to buy the elecraft 5-pole filters when
talking about 500Hz and narrower bandwidths. Does this hold true in the
case of the K3 in that the higer pole count results in a lower "roof" I
guess you could call it and then end up needing more gain in the next stage
therefore ending up with less dynamic range?

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Bill W4ZV
Sent: Wednesday, February 20, 2008 12:30 PM
To: [hidden email]
Subject: Re: [Elecraft] Roofing Filters

AnnLatz wrote:
>
> Can someone explain the meaning of "roofing filters". I have searched
> everywhere and can't find a good explanation. Lisa recommended the FAQ
> section of Elecraft and there they just mention them. Maybe I should order
> an updated Ham bible from ARRL.
>
> Thanks, Alan KA9UCP
>
>

See below, written by W2VJN of Inrad:

http://www.qth.com/inrad/roofing-filters.pdf

73, Bill W4ZV

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temilinrogers

Re: Roofing Filters

In reply to this post by KK7P

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