In a conventional modulator, demodulator, phase comparator or the like, when multiplying an input electrical signal, i.e. a multiplicand signal, by another electrical signal, i.e. a multiplier signal, if the multiplier signal includes high harmonic components therein, the harmonic components will be simultaneously multiplied by unwanted signal components or noise components which might be included in the multiplicand signal so that undesirable spurious signal components will occur in the output product signal obtained by the multiplication.
Although the above described problem should be theoretically resolved by the use of a sinusoidal signal as the multiplier signal, in practice distortion components occur in the output product signal and thus undesirable spurous signal components are produced because it is extremely difficult to obtain a sine wave having negligibly small distortion. Furthermore, the difficulty with the use of a sine wave as a multiplier signal is that the sine wave multiplier signal does not allow switching techniques to be employed for a multiplication has to be done linearly.
On the other hand, in various conventional demodulators or the like, in which a switching technique is utilized, a square wave signal having the same frequency as a multiplier signal, by which an input signal is intended to be multiplied, and a duty cycle of 50 percent, has been used as a multiplier or switching signal.
However, as is well known, since a square wave signal having a duty cycle of 50 percent includes a third harmonic component having a level one third the level of the fundamental frequency although no even harmonics are included therein, if unwanted signals or noise components having a frequency three times the fundamental frequency is included in the input electrical signal, and are multiplied by the multiplier signal in the form of a square wave signal of 50 percent duty cycle, the signal to noise ratio of the resultant product signal will be deteriorated suffering from the beat interference components included in the demodulated signal obtained as the result of the multiplication.
For instance, if a decoding or switching signal used for stereo multiplex decoding includes harmonic distortion components, unwanted signals or noise components, which might be included in a composite stereo signal from an FM detector, will be multiplied by the harmonic distortion components so that interference signals or noises having an audio frequency may occur in the demodulated signals. Assuming that the stereo multiplex decoding signal having a fundamental frequency of 38 KHz includes a third harmonic component (114 KHz), and further the composite stereo signal, which is the objective of the stereo multiplex demodulation, includes an unwanted signal having a frequency in the vicinity of 114 KHz, an audible interference signal will occur in the demodulated signals as the result of the multiplication of the unwanted signal in the vicinity of 114 KHz by the third harmonic component included in the decoding signal.