Roofing filter
Encyclopedia
A roofing filter is a type of filter used in a HF
receiver (shortwave
radio
). It is usually found after the first receiver mixer. The goal of a roofing filter is to reduce the passband
of the 1st intermediate frequency (IF) to about 6-20 kHz
, so that overloading and distortions by the following amplifier
stages and mixers are reduced. The receiver's Bandwidth (signal processing) is not determined by the roofing filter but by a following crystal filter
, mechanical filter or DSP
filter. These allow much better filtering curve than a roofing filter, which often uses a high 1st IF of higher than 40 MHz. Because of this, roofing filters are usually crystal filter types.
While a 6–20 kHz roofing filter is acceptable for general purpose HF radio reception, demanding uses like listening to weak CW
or SSB
signals requires the use of roofing filters that are a much smaller width appropriate to the reception mode in use. 250 Hz, 500 Hz, or 1.8 kHz would be acceptable values. These also require that the receiver uses a low first IF below VHF
range, perhaps 9 or 11 MHz.
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
receiver (shortwave
Shortwave
Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
). It is usually found after the first receiver mixer. The goal of a roofing filter is to reduce the passband
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
of the 1st intermediate frequency (IF) to about 6-20 kHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, so that overloading and distortions by the following amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
stages and mixers are reduced. The receiver's Bandwidth (signal processing) is not determined by the roofing filter but by a following crystal filter
Crystal filter
A crystal filter is a special form of quartz crystal used in electronics systems, in particular communications devices. It provides a very precisely defined centre frequency and very steep bandpass characteristics, that is a very high Q factor—far higher than can be obtained with conventional...
, mechanical filter or DSP
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
filter. These allow much better filtering curve than a roofing filter, which often uses a high 1st IF of higher than 40 MHz. Because of this, roofing filters are usually crystal filter types.
While a 6–20 kHz roofing filter is acceptable for general purpose HF radio reception, demanding uses like listening to weak CW
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
or SSB
Sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing power as a result of the modulation process. The sidebands consist of all the Fourier components of the modulated signal except the carrier...
signals requires the use of roofing filters that are a much smaller width appropriate to the reception mode in use. 250 Hz, 500 Hz, or 1.8 kHz would be acceptable values. These also require that the receiver uses a low first IF below VHF
Very low frequency
225px|thumb|right|A VLF receiving antenna at [[Palmer Station]], Antarctica, operated by Stanford UniversityVery low frequency or VLF refers to radio frequencies in the range of 3 kHz to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals...
range, perhaps 9 or 11 MHz.