[R-390] Why use a Roofing Filter?

[Perry Sandeen via R-390]

List,

Roy Morgan wrote: I suspect that there are some of
us who:
 
- are not all that sure what a roofing filter is
 
- wonder why the R-390A needs to be “improved” by
adding one
 
- think that maybe the conditions under which we
use our radios at our places, does or does not warrant the improvement.
 
So, a short description of what the thing is,
where it goes in the radio, and why it might be an improvement would be
welcome.
 
Great questions, Roy.
 
Here is the information Dallas Lankford gave about
his mod:
 
“There are two reasons to put a roofing filter in
anR-390A:
 
(1) to improve the close spaced 3rd order intercepts,
and
 
(2) to provide a more appropriate wide AM
bandwidth. The close spaced 3rd order 
intercepts for 2, 4, 8, and 16 kHz BW’s without a
roofing filter is about -20 dBm, which can 
definitely cause 3rd order intermod in high RF
environments. 
 
With a 6 kHz BW roofing filter placed at the input
of the IF deck, the close spaced 3rd order intercepts are improved to better
than 0 dBm. The 8 and 16 kHz BW’s are too wide for general AM listening in most
cases, so a 6 kHz roofing filter kills two birds with one stone. 
 
You could put a roofing filter in the RF deck,
immediately after the 3rd mixer, and the performance 
would be the same. But removing and reinstalling
an RF deck is an order of magnitude more 
difficult than removing and reinstalling an IF
deck. 
 
Since the signal path from the RF deck to the IF deck
is via two mini-coax cables and two quick release mini BNC connectors, you
could implement the roofing filter external to the IF deck, provided you had
appropriate connectors. But it seems simpler to put it in the IF deck. The
general idea behind the mod is quite simple. Below are “Before” and “After”
schematics for the mod that I do. [Snip]
 
One can find the rest of the article’s details, with
pictures and schematics, in the Y2KR3 manual Chapter 11 page 135.
 
Hopefully that will answer all questions.
 
Regards,
 
Perrier