[Elecraft] Roofing Filter Usage

[Ron D'Eau Claire]

Good point Wes, but clearly Wayne used it in that context answering
someone's query who called the first I.F. filter by that name. He's picked
up that name (in parenthesis) in the latest Owner's manual as well in the
interest of communicating with people who think "roofing" simply means the
first I.F. filter. 

 

But you can't order a "roofing filter" for the K3 from the Elecraft page
under that name :-) 

 

The term "Roofing" isn't wrong in the way the K3 uses filters, but it's
misleading to those who have worked with receiver architectures that
up-convert to a low-VHF frequency, pass the signal through a "roofing
filter" whose purpose is to block some mixing products. Adequate filters for
taking final selectivity simply aren't available for the high first I.F.
those receivers use so the final selectivity must be taken in subsequent
stages at a lower second or third I.F. It was in these receivers that the
term "roofing" first appeared to designate that wide, first I.F. filter.  

 

While that architecture has some advantages in a receiver that covers a very
wide bit of the RF spectrum, it also has significant disadvantages that
Wayne chose to avoid in the K3.

 

So the K3's first I.F. filter is at a pretty typical low I.F. for these days
--  just above 8 MHz. At that frequency filters capable of isolating the
desired signal can be made, and that's why Elecraft offers a selection of
crystal filters with bandwidths to suit the different modes and preferences.


 

In the K3, the DSP filter in the second I.F. is usually a secondary
function. The main selectivity to eliminate unwanted signals is taken at the
first I.F. crystal filter. 

 

Ron AC7AC

 

 

 

 


Unfortunately, they do.  Personally I dislike the term.

For example:

A "A narrow roofing filter -- compatible with the communications bandwidth 
required -- will protect "downstream" stages. The K3's 
shift/width/hicut/locut controls automatically select and properly 
position the optimal roofing filter. You'll see the FL1-FL5 icons 
selected as you rotate these controls." 

http://elecraft.365791.n2.nabble.com/Re-roofing-filters-when-to-use-them-td4
46665.html#a446666

--- On Thu, 10/28/10, Ron D'Eau Claire <ron at cobi.biz> wrote:

 

That's why the Elecraft doesn't call the first I.F. crystal filters "roofing
filters". Unlike some receivers, the first I.F. crystal filters in the K3
are there for selectivity to isolate the desired signal. That is further
enhanced in the second conversion with the variable DSP filter, but the
crystal first I.F. filters are a key part of the selectivity equation.