Compared to FM radio receivers, amplitude modulation (AM) radio receivers are very much susceptible to atmospheric noise interference. This is because of the fact that atmospheric noise is amplitude modulated. Most of the man made electromagnetic interference also is amplitude modulated. The undesirable noise interference in AM radio receivers and the instability along with noise patterns in the picture on a TV receiver connected to an antenna during thunder storms is a common experience. Similar problems encountered because of nearby electrical switching circuits (as for example, automobile ignition systems) also are common.
Even though I have not done any research in this field, I think the modulation envelope of the amplitude modulated noise signal may be more or less the same over a wide range of frequencies. If that is the case, it will be possible to extract this noise signal and phase shift it through the required angle so that it has a phase shift of 180 degrees with the noise signal appearing along with the received useful signal. After amplifying this phase shifted noise signal to the required extent so as to have the same amplitude as that of the noise signal appearing along with the useful signal, they are to be superimposed so that the net noise signal is canceled. The separate noise signal may be received from the lower or upper end of the transmission band. For the medium wave band, the required noise signal can be extracted using a separate tuned circuit at a frequency around 500 kHz or 1600 kHz. The phase shifting and the controlled amplification of the separate noise signal are to be done after demodulation. The superimposition of the signals also is evidently to be done after demodulation.
With modern operational amplifiers the above tasks may be simple.
So, Attention Electronics Engineers!
