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Roofing Filters - An Empirical Test

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Edward A. Dauer

Roofing Filters - An Empirical Test

I would be interested in knowing if anyone has done an empirical test and
observed the difference directly. Specifically, since the sub receiver is
identical to the main receiver in every way, if someone has a K3 with, for
example, a 400 Hz filter in the main but only a wider set in the sub, and
then set the DSP bandwidth on both receivers to 200 Hz or so, what
differences they actually noticed. I have no doubt the theory and the
engineering are sound - just curious what the difference sounds like in
the field . . .

Ted, KN1CBR

>On May 12, 2014, at 9:33 AM, Jerome Sodus <[hidden email]> wrote:
>
>>Hello Bill,
>>The term "roofing-filter" made sense back in the 1980's when I designed
>>roofing-filters at 70 MHz.
>>Bandwidths would be in tens of KHz.
>>The purpose then was to protect downstream circuitry by rejecting very
>>strong out-of-band signals that could cause overload; selectivity was not
>>the purpose.
>>Selectivity was done further downstream.
>>So the term has become corrupted over the years.
>>73 Jerry KM3K
>>-----Original Message-----
>>From: Elecraft [mailto:[hidden email]] On Behalf Of
>>Bill
>>Turner
>>Sent: Monday, May 12, 2014 1:07 AM
>>To: [hidden email]
>>Subject: Re: [Elecraft] [K3] Roofing filters are misunderstood
>>ORIGINAL MESSAGE: (may be snipped)
>>On 5/11/2014 7:25 PM, Fred Jensen wrote:
>>>I too think roofing filters are really not well understood.
>>REPLY:
>>A large part of the misunderstanding is due to the name. Whoever chose
>>the name "roofing" did a great disservice. A better name would simply be
>>it's function: 1st I.F. filter.
>>That's what it is and that's what it does.
>>I have always thought that "roofing" was a marketing ploy to imbue it
>>with some kind of magical powers.
>>73, Bill W6WRT
>>
>>
>
>
>------------------------------

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Scott Manthe-2

Re: Roofing Filters - An Empirical Test

The January 2009 QST has a table list the IMD numbers with various
filters. Quite informative. There's is no easy way to cut/paste the data
into an email, unfortunately.

73,
Scott, N9AA

On 5/12/14 3:54 PM, Dauer, Edward wrote:

> I would be interested in knowing if anyone has done an empirical test and
> observed the difference directly. Specifically, since the sub receiver is
> identical to the main receiver in every way, if someone has a K3 with, for
> example, a 400 Hz filter in the main but only a wider set in the sub, and
> then set the DSP bandwidth on both receivers to 200 Hz or so, what
> differences they actually noticed. I have no doubt the theory and the
> engineering are sound - just curious what the difference sounds like in
> the field . . .
>
> Ted, KN1CBR
>
>
>

wayne burdick

Re: Roofing Filters - An Empirical Test

Administrator

In reply to this post by Edward A. Dauer

It's quite simple. If you have a very strong interfering signal that falls inside the SSB filter passband (2.7 or 2.8 kHz), but outside an installed narrow filter, the hardware AGC will not get "pumped".

Lacking such a filter, a strong enough signal (typically S9+20 or higher) *will* activate hardware AGC, which can be annoying in mild cases and debilitating if it's really strong, has key clicks, etc.

This is why the K3 has slots for narrow filters. On a very busy band with lots of strong signals, it's the difference between a usable radio or not.

73,
Wayne
N6KR

On May 12, 2014, at 12:54 PM, "Dauer, Edward" <[hidden email]> wrote:

wayne burdick

Re: Roofing Filters - An Empirical Test

Administrator

I should have pointed out that this effect is most noticeable with CW or narrow data modes, where there's a large difference in bandwidth between "wide" and "narrow" filters. In SSB mode, a somewhat narrower filter (say 1.8 kHz) will provide additional margin on an active band with strong signals, but some of the advantage is lost due to IMD products inherent in the received voice signals.

Wayne

On May 12, 2014, at 1:19 PM, Wayne Burdick <[hidden email]> wrote:

> It's quite simple. If you have a very strong interfering signal that falls inside the SSB filter passband (2.7 or 2.8 kHz), but outside an installed narrow filter, the hardware AGC will not get "pumped".
>
> Lacking such a filter, a strong enough signal (typically S9+20 or higher) *will* activate hardware AGC, which can be annoying in mild cases and debilitating if it's really strong, has key clicks, etc.
>
> This is why the K3 has slots for narrow filters. On a very busy band with lots of strong signals, it's the difference between a usable radio or not.
>
> 73,
> Wayne
> N6KR
>
> On May 12, 2014, at 12:54 PM, "Dauer, Edward" <[hidden email]> wrote:
>
>> I would be interested in knowing if anyone has done an empirical test and
>> observed the difference directly. Specifically, since the sub receiver is
>> identical to the main receiver in every way, if someone has a K3 with, for
>> example, a 400 Hz filter in the main but only a wider set in the sub, and
>> then set the DSP bandwidth on both receivers to 200 Hz or so, what
>> differences they actually noticed. I have no doubt the theory and the
>> engineering are sound - just curious what the difference sounds like in
>> the field . . .
>>
>> Ted, KN1CBR
>>
>
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Vic Rosenthal

Re: Roofing Filters - An Empirical Test

In reply to this post by Edward A. Dauer

My K3 was originally set up this way, 400 Hz in the main and 2.8 kHz in
the sub.

When working DX pileups with the main on the DX and the sub on the
pileup, I definitely noticed the 'pumping' effect on the sub only. It
was sometimes annoying when the DX was working a weak caller and the
usual loud idiots were continuing to call.

On 5/12/2014 12:54 PM, Dauer, Edward wrote:

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/

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Rick Tavan N6XI

Re: Roofing Filters - An Empirical Test

You can mitigate that problem somewhat by listening to the DX on the wide
sub and the pileup with the narrow main rx. If the split is (roughly)
larger than the filter bandwidth difference, then loud, continuous callers
in the pile don't bother the DX signal, only your ability to locate the
weak caller in the strong pile. That being said, I put a narrow filter in
both receivers.

The unconventional technique of listening to the DX on the sub and the
pileup on the main has another benefit - you don't have to use SPLIT mode
which degrades keying at high speed and disables QRQ mode. It took me a
while to get used to it after decades of doing it the other way, but I
think the benefits outweigh the change in my cage.

73,

/Rick N6XI

On Mon, May 12, 2014 at 2:49 PM, Vic Rosenthal K2VCO <[hidden email]>wrote:

> My K3 was originally set up this way, 400 Hz in the main and 2.8 kHz in
> the sub.
>
> When working DX pileups with the main on the DX and the sub on the pileup,
> I definitely noticed the 'pumping' effect on the sub only. It was sometimes
> annoying when the DX was working a weak caller and the usual loud idiots
> were continuing to call.
>
>
> On 5/12/2014 12:54 PM, Dauer, Edward wrote:
>
>> I would be interested in knowing if anyone has done an empirical test and
>> observed the difference directly. Specifically, since the sub receiver is
>> identical to the main receiver in every way, if someone has a K3 with, for
>> example, a 400 Hz filter in the main but only a wider set in the sub, and
>> then set the DSP bandwidth on both receivers to 200 Hz or so, what
>> differences they actually noticed. I have no doubt the theory and the
>> engineering are sound - just curious what the difference sounds like in
>> the field . . .
>>
>> Ted, KN1CBR
>>
>
> --
> 73,
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco/
>
>
> ______________________________________________________________
> 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]
>

--
Rick Tavan N6XI
Truckee, CA
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Vic Rosenthal

Re: Roofing Filters - An Empirical Test

Actually, I switched to this method too, mainly because of QRQ mode. And
I got a 400 Hz filter for the main rx.

On 5/15/2014 8:27 AM, Rick Tavan N6XI wrote:

> You can mitigate that problem somewhat by listening to the DX on the
> wide sub and the pileup with the narrow main rx. If the split is
> (roughly) larger than the filter bandwidth difference, then loud,
> continuous callers in the pile don't bother the DX signal, only your
> ability to locate the weak caller in the strong pile. That being said, I
> put a narrow filter in both receivers.
>
> The unconventional technique of listening to the DX on the sub and the
> pileup on the main has another benefit - you don't have to use SPLIT
> mode which degrades keying at high speed and disables QRQ mode. It took
> me a while to get used to it after decades of doing it the other way,
> but I think the benefits outweigh the change in my cage.
>
> 73,
>
> /Rick N6XI
>
>
> On Mon, May 12, 2014 at 2:49 PM, Vic Rosenthal K2VCO
> <[hidden email] <mailto:[hidden email]>> wrote:
>
> My K3 was originally set up this way, 400 Hz in the main and 2.8 kHz
> in the sub.
>
> When working DX pileups with the main on the DX and the sub on the
> pileup, I definitely noticed the 'pumping' effect on the sub only.
> It was sometimes annoying when the DX was working a weak caller and
> the usual loud idiots were continuing to call.