Is the High-Frequency Signal Necessary for the Resonance in the Delayed System?

In a delayed system excited by low-frequency and high-frequency signals,the necessity of the high-frequency signal on the resonance is discussed.By adjusting the delay time,the resonance occurs in a wide scope of frequencies,including the primary,subharmonic and superharmonic frequencies.Only for very few cases does the high-frequency signal have a positive effect on the resonance.It is the traditional vibrational resonance phenomenon.In most situations,the high-frequency excitation is unnecessary for the resonance.An appropriate delay,rather than the high-frequency signal,is the key factor in improving the weak low-frequency signal.