[INRAD] Filter Board

[Tommy-W4BQF]

Yep! And a good way to look at it Bob. A roofing filter limits the amount of
bandwidth the remaining circuitry has to handle.

On my Omni 6 Plus, it originally had a 20kc wide filter before the first IF,
so the IF would have to handle all signals in that wide bandwidth, up to the
IF filters themselves. Included, of course, in that 20kc is atmospheric
noise (static, QRN, lightning crashes, etc.). With a 600 hz filter BEFORE
the first IF stage, now any signal outside of a 600 hz bandwidth, is reduced
and the IF stage(s) now have to handle the atmospheric noise contained in
the 600 hz bandwidth.

For example, on 80m with no roofing filter, my Omni 6 Plus may read +20 dB
of noise plus weak signals. With the 600 hz roofing filter, the noise drops
to S-5 - S-7, BUT the weak signal your trying to hear does not drop in
strength at all. A big improvement for both your radio and your ears!!

Tom - W4BQF


----- Original Message ----- 
From: "N5IET Bob" <rtnmi at sbcglobal.net>
To: <list at inrad.net>
Sent: Tuesday, October 25, 2005 10:29 PM
Subject: Re: [INRAD] Filter Board


> So the answer to the original question "what is a roofing
> filter" is that it is a filter that puts a roof or limit
> on what the rest of the IF's have to handle is that
> correct.
>
> They limit the work for the rest of the radio. By placing
> a sort of top (roof) on the first incoming signals.
>
>  73 fer nw,
> Bob N5IET
>
> 10X# 37210, FP#-1141, SMIRK#-5177
> http://www.n5iet.com/
> Code may be taking a back seat for now,
> but the pioneering spirit that put the code
> there in the first place is out front of it all.
>
>
> > -----Original Message-----
> > From: List-bounces at inrad.net
> [mailto:List-bounces at inrad.net]
> > On Behalf Of Tommy-W4BQF
> > Sent: Tuesday, October 25, 2005 8:33 PM
> > To: list at inrad.net
> > Subject: Re: [INRAD] Filter Board
> >
> >
> > I think Steve is close to correct, however a roofing
> filter
> > normally is installed before the first IF, in some rigs,
>
> > right after the first mixed. And yes, because most rigs
> have
> > the ability to receive FM (why?), the 'normal' filter is
>
> > something like 20 kc wide. With that your first IF has
> to
> > handle signals and noise in a 20kc wide spectrum.
> Install a
> > 600 hz roofing filter, and your IF stages have to handle
> a
> > bandwidth of only 600hz, meaning both strong signals
> close to
> > your QRG plus the noise, are both greatly reduced.
> >
> > In the Ten Tec Omni 6 Plus rigs, the improvement is
> astounding!
> >
> > Tom - W4BQF
> >
> > ----- Original Message ----- 
> > From: "Dick Powers" <powersharp at juno.com>
> > To: <steve.root at culligan4water.com>
> > Cc: <list at inrad.net>
> > Sent: Tuesday, October 25, 2005 9:19 PM
> > Subject: Re: [INRAD] Filter Board
> >
> >
> > > So it's kind of like a funnel right in the front end
> to
> > eliminate the
> > > unwanted stuff right off the bat I guess, huh? Thanks
> Steve (Boy I
> > > haven't seen the term "kc" used in awhile.) Dick
> WB9PWQ 73
> > >
> > > On Tue, 25 Oct 2005 20:37:46 -0400
> <steve.root at culligan4water.com>
> > > writes:
> > > > A "Roofing filter" is a crytal filter that is used
> in the
> > first IF.
> > > > My understanding of the problem is that the decision
> to include
> > > > general coverage reception dictated a first IF
> frequency
> > in the VHF
> > > > range, like 70 Mc.  Until fairly recently it was
> > difficult to make a
> > > > narrow filter that operated at these frequencies.
> The first IF
> > > > stage was pretty broad, on the order of 12 Kc wide.
> > > >
> > > > I have a lot of time in with both the FT-1000MP and
> the FT- 1000D
> > > > before and after adding the roofing filters.  The
> differences are
> > > > immediately apparent.  The MP in particular was much
> improved.
> > > > During periods of heavy activity like a CW contest I
>
> > usually ran the
> > > > MP with the pre-amp off and 6 or 12 db of
> attenuation in line to
> > > > keep the receiver from "crunching".  After adding
> the
> > roofing filter
> > > > the attenuation wasn't necessary as stong signals
> outside the
> > > > passband were eliminated by the filter.  In a word,
> it's
> > > > great!
> > > >
> > > > 73 Steve K0SR
> > > >
> > > >
> > >
> > >
> > > Dick
> > >
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