Editing Roofing filter

A '''roofing filter''' is a type of filter used in a [[High frequency|HF]] [[Receiver (radio)|radio receiver]] that limits the [[passband]] in the early stages of the [[radio receiver|receiver]] electronics. It blocks strong signals outside the receive channel which can overload following [[amplifier]] and [[frequency mixer|mixer]] stages.

== Purpose ==
The '''roofing filter''' is usually found after the first receiver [[frequency mixer|mixer]] (which normally contains an amplifier) to limit the first [[intermediate frequency]] (IF) stage's [[passband]]. It prevents overloading later [[amplifier]] stages, which would cause [[Amplifier figures of merit#linearity|nonlinearity]] ("distortion") or [[Clipping (signal processing)|clipping]] ("buzz") even if the overload occurred on frequencies whose signal is not heard directly.

Roofing filters are usually [[crystal filter|crystal]] or [[Ceramic resonator#Ceramic filters|ceramic]] filter types, with a passband for general purpose [[shortwave radio]] reception of about 6–20&nbsp;[[Hertz|kHz]] (for [[amplitude modulation|AM]]–[[frequency modulation#narrowband_FM_anchor|NFM]]). The receiver's [[Bandwidth (signal processing)|bandwidth]] is ''not'' determined by the roofing filter passband, but instead by a follow-on [[crystal filter]], [[mechanical filter]], or [[digital signal processing|DSP]] filter, all of which allow a much tighter filtering curve than a typical roofing filter.

For more demanding uses like listening to weak [[Continuous wave|CW]] or [[sideband|SSB]] signals, a roofing filter is required that gives a smaller passband appropriate to the [[modulation|mode]] of the received signal. It is often used at a high first [[intermediate frequency|IF]] stage above 40&nbsp;[[Megahertz|MHz]], with passband widths of 250&nbsp;Hz, 500&nbsp;Hz (for CW), or 1.8&nbsp;kHz (for SSB). These narrow filters require that the receiver uses a first IF well below [[very high frequency|VHF]] range, perhaps&nbsp;9 or 11&nbsp;MHz.<ref>{{cite journal |last1=Coy |first1=R.J. |last2=Smith |first2=C.N. |last3=Smith |first3=P.R. |year=1992 |title=HF-band radio receiver design based on digital signal processing |journal=[[Electronics & Communication Engineering Journal]] |volume=4 |issue=2 |pages=83 |doi=10.1049/ecej:19920016 |doi-broken-date=7 December 2024 |lang=en |url=https://digital-library.theiet.org/content/journals/10.1049/ecej_19920016|url-access=subscription }}</ref>

==See also==
* [[Bandpass filter]] – category that includes roofing filters
* [[Preselector]] – an external device that serves a similar function

==References==
{{reflist|25em}}

{{DEFAULTSORT:Roofing Filter}}
[[Category:Radio electronics]]
[[Category:Radio technology]]
[[Category:Receiver (radio)]]
[[Category:Wireless tuning and filtering]]