[R-390] Why use a Roofing Filter?

[Charles Steinmetz]

Dennis wrote:

>Charles, I noticed in Dallas' article that he puts his filter just 
>before the first IF...a stage before the mechanical filters.  Would 
>a filter there offer more improvement?  Your note suggested that the 
>roofing mod that Dallas describes simply sticks a 6 kc filter where 
>the rest of the mech filters are now.

Whether it goes on the grid side or the plate side of V501 doesn't 
really matter.  (By "just before the existing filters," the grid side 
was what I meant -- it is the first practical point before the 
existing filters.)  The 390A's IMD characteristic is basically all 
generated way before the 3rd mixer, so nothing you do at 455 kHz is 
going to help much (except with the close-in IMD that is simply a 
reflection of the IF bandwidth).  So there is no practical difference 
between putting another filter on the grid side of V501 or just using 
it to replace one of the mechanical filters.

>I've been toying with ways to output the IF of my 390A for further 
>processing by SDR software.  It seems to me that something without 
>any of the mechanical filter influence would be good...let as much 
>of the filtering be done in software as possible, and with as wide 
>as practicable slice of spectrum as possible.  So, maybe a nice wide 
>ceramic (?) filter ahead of the 1st if, or even in place of the 16kc 
>mech filter (which in my receiver is dead).

The 390A already has a perfectly serviceable IF output, and you 
should let the radio provide at least a roofing filter for the SDR 
(BW wider than you are going to use on the SDR).  So just select 
whatever BW you want with the 390A BW switch and feed the existing IF 
output to the SDR.  That's the one thing a 390A 16 kHz filter is good 
for.  You aren't going to get a wider slice of spectrum (than you get 
with a 16 kHz filter) out of the 390A in any case, because the BW of 
the IF cans is just barely wider than the 16 kHz filter.  (A 16 kHz 
filter is still be better than a jumper wire, however, because it 
suppresses the stop band more than the IF cans do.)  So, fix your 16 
kHz filter, find one that works, or just use the 8 kHz filter when 
you feed the SDR.

Best regards,

Charles