B.M. Sosin, "A breakthrough in h.f. receiver design", Point-to-Point Communication, January 1970, pp 4-14 Citat papirside 11: "...In the second conversion, the frequency is changed down to 30MHz and a narrow band 'roofing' crystal filter is used. (The term 'roofing filter' is applied to a filter of narrow bandwidth, at an early stage in the receiver, to give protection against adjacent frequency interference)...After passing the 'roofing' filter more amplification is possible because high interference has been eliminated..."
elecraft.com: K3S Crystal 'Roofing' Filters. What "Roofing Filter" means to Elecraft. Wayne Burdick, N6KR Citat: "...A "Roofing filter" is simply a filter in the radio's first IF through which all signals must pass before they will be "seen" by later receiver stages. The narrower this filter is, the less exposure later stages will have. Thus a "narrow" roofing filter is desirable -- but "narrow" is relative, as I'll explain. ...A roofing filter protects later stages, including amplifiers, mixers, narrower filters, and DSP subsystems, just as the roof on your house keeps rain out of all of the rooms. But a roofing filter can be equally at home at a low first IF, if that is how the radio is designed. It still provides the same protective function...So why don't they offer much narrower roofing filters that can be switched in for CW and data modes, or at times when adjacent-channel SSB QRM is very high? It's because they can't make filters any narrower at such a high IF...", backup
elecraft.com: K3S Crystal 'Roofing' Filters. What "Roofing Filter" means to Elecraft. Wayne Burdick, N6KR Citat: "...A "Roofing filter" is simply a filter in the radio's first IF through which all signals must pass before they will be "seen" by later receiver stages. The narrower this filter is, the less exposure later stages will have. Thus a "narrow" roofing filter is desirable -- but "narrow" is relative, as I'll explain. ...A roofing filter protects later stages, including amplifiers, mixers, narrower filters, and DSP subsystems, just as the roof on your house keeps rain out of all of the rooms. But a roofing filter can be equally at home at a low first IF, if that is how the radio is designed. It still provides the same protective function...So why don't they offer much narrower roofing filters that can be switched in for CW and data modes, or at times when adjacent-channel SSB QRM is very high? It's because they can't make filters any narrower at such a high IF...", backup
