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 On Friday, October 29, 2010, at 03:04 +0100, Ron D'Eau Claire wrote: > The term "roofing filter" came into use when receiver designers started > up-converting to a first I.F. in the VHF range. There are no practical > narrow filters at those frequencies but it was necessary to filter out > mixing products far from the desired frequency, so very wide (several kHz > wide) filters were used to define the general range to be further > processed. ______________________________________________________________ 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
