In order to contain strong signals and spurs, as well as perform their
fundamental job of image-rejection, roofing filters must be installed as
near the front-end of the receiver as possible. The preferable location is
immediately following the first mixer in a superhet.
Most IF filters are located far enough down the chain for the previous
stages to be affected by the signals true roofing filters are aimed at
keeping out.
I guess the bottom line is that conventional IF filters are used for signal
selection and noise-bandwidth reduction. Because of their usual location
within the signal chain, they may or more generally may not be very
effective as "roofing filters."
A singular counterexample is the Elecraft K2 in which the IF filter follows
the post-mixer amplifier directly and serves as both the roofing filter and
the IF signal-selection filter. The post-mixer amplifier is a special
high-current design with a very large dynamic range. It follows a ring-diode
mixer with a similarly large dynamic range. No AGC is applied anywhere in
the front-end, only to the IF amplifier following the IF filter.
The ORION, the K2 and the Icom 7800 have several design features in common.
Usually though, simply using a narrow IF filter does not have much effect
upon the front-end performance of a receiver in the sense of increasing its
TOI or IMDDR.
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:w5yr@att.net>
----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: <tentec@contesting.com>
Sent: Monday, June 16, 2003 4:37 PM
Subject: Re: [TenTec] Omni6+ roofing filter
> Fascinating -- I learn something new every day. I wonder why the Omni's
> IMD performance isn't even better, then? According to Inrad's table of
> available filters, there are only two IFs where filters can be installed,
> at ~9 and ~6.3 MHz. Right?
>
> 73, Pete N4ZR
>
> PS tried to send this directly to you, but it bounced back with a note
> about Adelphia being banned by qvssoftware for spam(?)
>
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