[ICOM] Fwd: Re: Hope to see a roofing filter for the IC-775DSP

[Steve N4LQ]

Jan:
Isn't the first IF of the Orion at 45mhz and the roofing filter is 9 mhz? So
the "narrow roofing filter" is not really ahead of that first IF if I am
thinking straight. I believe the 9 mhz "roofing" filter is the same, exact
filter used in the Omni VI, Omni V, Paragon, Corsair and others so it's
really nothing new when viewed in that light. I think you'll find that
TenTec put the roofing filter a few floors down from the roof.
Hey....That 238 tuner will make a great roofing filter!
73
Steve N4LQ
----- Original Message ----- 
From: "Jan C. Robbins" <swanman at cfu.net>
To: "ICOM Reflector" <icom at mailman.qth.net>
Sent: Saturday, September 11, 2004 9:36 PM
Subject: Re: [ICOM] Fwd: Re: Hope to see a roofing filter for the IC-775DSP


> The front ends of most radios these days--that is, ahead of the first
> IF--are wide open, typically 13 to 20khz minimum.  If everything in that
> passband gets to and into the first IF, it grabs the AGC, and makes weak
> signals uncopyable (it also blows by the IF filters, increases phase
> noise, etc., but we don't need to get into all that).
>
> One of the BEST ways to improve ANY receiver is to place a filter AHEAD
> of the first IF that is no wider than is absolutely necessary to copy
> the signal YOU want to copy--250hz for CW or RTTY, for  example, or
> 1.8khz for SSB (in the days before DSP, many receivers did exactly that;
> it is a fact that DSP has produced a deteriation in receiver design).
> What a roofing filters do, as Ten-Tec learned a long time ago (and Orion
> does better than anything now on the market), is exactly that: shut down
> the "window" by which signals get into the reciver in the first place.
> That means that almost nothing you DON'T want into the reciver ever GETS
> into the reciver, so the entire RF strip and everything else after that
> narrow opening does its job correctly.
>
> You might want to check on Rob Sherwood's analysis of how to evaluate
> receivers and what matters most in them.  Rob knows more about reciever
> performance than almost anyone alive, and has been quite generous in
> sharing what he knows with the rest of us.  I've listed the Sherwood
> Engineering website below.
>
> What you'll find is that Rob not only rates receivers by how narrow the
> front end is, but also how it gets there.  The BEST way to get there is
> a narrow, tracking preamplifier--as in an R390A.  Almost nonexistent in
> radios today (no, the Icom 7800 doesn't match up). But the Orion is the
> best second best by far.  He also makes it clear that ANY
> radio--including esp. the messy Icom Pros--that rely entirely on IF DSP
> AFTER the front end cannot possibly perform at the level of radios that
> don't.  One of the reasons why NO "pro-type" front end radio ends up
> even above the median in receiver performance.
>
> Here's the Sherwood Engineering website; sorry this has been so
long-winded.
>
> > http://www.sherweng.com/
>
>
>
> Durwydd MacTara wrote:
>
> > What is a "roofing filter" and why are they desireable?
> >
> >
> >
> >  "Carpe` Diem!"
> >             Durwydd MacTara
> >
> >
> > ----
> > Your Moderator: Dick Flanagan K7VC, icom-owner at mailman.qth.net
> > Icom Users Net: Sundays, 1700Z, 14.316 MHz
> > Icom FAQ: http://www.qsl.net/icom/
> >
> >
>
> -- 
> "There is no end to what you can accomplish
> if you don't care who gets the credit."   Anon.
>
>
> ----
> Your Moderator: Dick Flanagan K7VC, icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> Icom FAQ: http://www.qsl.net/icom/
>