Q378 : Differences Between Modulated and Unmodulated Response in FFTSES

Question
What are the differences between the unmodulated and modulated response in FFTSES?

The unmodulated response curves represent the response of the system to a unit energization at all frequencies. This is an intrinsic characteristic of the system being studied. Once you obtain the unmodulated response of a system, you can use it to compute essentially all electromagnetic quantities for that system. Physically, the unmodulated response of a system at a given frequency is the peak value of the field (or, in your case, potential) that would result when the system is energized with a perfect sine wave with a unit amplitude at that frequency.

The modulated response of a system represents the actual response of the system to a specific energization signal. It combines the unmodulated response described above and the spectrum of the input signal. It is therefore meaningful only for the specific input signal that you used. The modulated response at any frequency is the product of the input signal for that frequency and the unmodulated response for that frequency. The modulated response is linked directly to the time-domain response of the system through a Fourier transformation.

For a specific frequency to contribute significantly to the time-domain response, both the unmodulated response of the system and the strength of the input signal for that frequency should be sizable. If the system reacts strongly to an energization at that frequency (i.e., the unmodulated response is large) but the input signal is very weak, the overall response can be quite small. Likewise, if the system responds very weakly at that frequency, the overall response may well be quite small even if the energization signal is very strong at that frequency. Only the product of the two is meaningful.

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• Created on 05/29/2002