However, higher channels can also suffer, depending on the strength of the received pager signal. This narrow notch filter protects adjacent signals. If the cause of the harmonic interference can not be located so that proper filtering can be negotiated, then the harmonic interference must be removed by a process known as Phase Cancellation which effectively suppresses the harmonic signal without degradation to the desired signal. In all the cases above the interference was caused by signal overload. Although the interfering frequency differed considerably from the disturbed frequency, the strength of the received signal overwhelmed the receiving apparatus.
Harmonic interference is the reception of a signal whose frequency is within the desired channel.
None of the above cures is applicable to harmonic interference. Suppressing the offending harmonic with a notch filter or other type of filter, at the point of reception, is not possible without unacceptable degradation of the desired signal which is very near to or identical with the interfering harmonic frequency. Least cost suppression of harmonic interference consists of installing an appropriate filter at the transmitter causing the interference. Harmonics are multiples of the transmitter frequency. Therefore, the appropriate filter type is a lowpass filter capable of withstanding the transmit power level.