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

[D C Macdonald]

The sharp pre-selector of the older Drake receivers was one
of their best features.  In the days that Loran A was still in
operation, that pre-selector was able to cut off (or greatly
reduce) the racket in adjacent 25 kHz segments so that you
could hear signals in  YOUR  25 kHz segment.

I fail to see, however, how this would do much good in a
situration where interfering signals were as close as 2 kHz.

Mac, K2GKK/5



----Original Message Follows----
From: "Jan C. Robbins" <swanman at cfu.net>
Reply-To: ICOM Reflector <icom at mailman.qth.net>
To: ICOM Reflector <icom at mailman.qth.net>
Subject: Re: [ICOM] Fwd: Re: Hope to see a roofing filter for the IC-775DSP
Date: Sat, 11 Sep 2004 20:36:41 -0500

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