[INRAD] Filter Board

[Tommy-W4BQF]

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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