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

[Jan C. Robbins]

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
> 
> 
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if you don't care who gets the credit."   Anon.