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 <[email protected]> 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. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
