Tuesday, 15 February 2011

Poor Man's MWLW Wideband Noise Reduction


Many radio signals in the medium and long wave bands (MW/LW) but also shortwave (SW) are infested by noise of wide variety and of such levels that weak stations are virtually obliterated. The worst noise is the wideband variety which stretches across several hundred kilohertz across the band. However the very fact that the noise is wideband in nature allows it to be suppressed using a little known and inexpensive method of using a second receiver tuned just beside the frequency you want to listen to. The principle is illustrated in the drawing. The second receiver effectively isolates the noise by adding it in anti-phase to the wanted signal.
As shown the loudspeaker outputs of the two radios are connected in phase while the loudspeaker in the second radio is disconnected or removed. With the second receiver tuned a little higher or lower than the main one, the loudspeaker in the main receiver is fed with the difference between the two signals. Using the volume adjustment on the second radio, a setting can be found that should result in considerable reduction of wideband noise. If an external antenna is used, it may be connected in parallel to the receiver inputs. Ferrite rod antennas need to turned in the same direction. Best results are obtained when two identical radios are used.

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