Thanks for the clarification Geoff. The fellow with the original question asked what why he was still hearing QRM even when using the 1.8 kHz crystal filter (presumably listening to SSB) and David responded, "The purpose of the roofing filter is...not to provide selectivity."
I was respectfully disagreeing with David when speaking of the K3. In the K3 the first I.F. filter (what some call the "roofing filter") provides the main selectivity. That's why a variety of bandwidths are provided for various modes and why so much attention was paid to the design of those filters to provide excellent stop band attenuation. The DSP filter at the 2nd I.F. provides additional filtering but it's the first I.F. filter in the K3 that is responsible for providing the bulk of the selectivity. Personally, I don't use the term "roofing filter" for such a filter. Rather when I see "roofing filter" I think of a filter such as David described that cleans up unwanted mixer products so that the final selectivity can be taken later in the signal path. That's how I've always seen "roofing filter" used in the commercial equipment I've worked with. IIRC the term roofing filter didn't come up with respect to the K3 until after it was introduced and Hams were trying to compare its architecture with Ham rigs that up-convert to VHF for the first I.F. Wayne answered many questions here on the reflector and finally wrote the paper on the web site explaining it further. He has added "roofing filter" in parenthesis in places in the Owner's manual to help those Hams understand the K3 Crystal Filter selected is the first I.F. filter. However, it's clear that applying the name to the K3 skews the definition of roofing filter from what Dave and I have always known in the past. Ron AC7AC -----Original Message----- With respect Ron, as far as I know the term "Roofing Filter" was in use many years before up-conversion superhet receivers with a fist IF at VHF started to appear in the market. It was at least fifty years ago when I first ran across the term, which was used to describe the first IF filter used in a voice/data multichannel Independent Sideband down conversion superhet receiver manufactured by our company at the time. In this particular case the bandwidth of this Roofing Filter was wide enough to allow both the upper and lower sidebands of the incoming signal to pass, and further downstream in the IF two filters described as USB and LSB IF filters were used to separate the signal's sidebands for further processing. It could be argued that the input bandpass filters of a receiver act as Roofing Filters, and that all filters which follow should be described as IF filters, Audio filters or whatever. However the applicable "rules" of terminology as I have understood them since those ancient times restricts the use of the term Roofing Filter to the first IF Filter, but the term should only be used if a second and narrower *IF* filter follows the first ( which would include DSP filters, but only if working at IF not audio). The use of relatively wide bandwidth Roofing Filters at VHF in up-conversion receivers is actually not such a problem that it might appear to be, because the use of a very narrow filter behind the mixer can be counter productive in terms of close in dynamic range, especially if the filter is followed by a well designed decent IF. The same is true of down-conversion. LO phase noise is a problem when up-converting to VHF. 73, Geoff GM4ESD ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
