The broadband carrier signals 1828 may be generated in a variety of ways, as described in detail below with respect to FIG. 18C. In one embodiment, a desired spectrum of frequencies (e.g., 10 Hz to 300 Hz) is defined. The frequency components are transformed from the frequency domain representation of a flat power spectrum to the time domain by taking the inverse Fourier transform. In another embodiment, a noise process is simulated by aggregating a large number of sinusoids (e.g., 10-1000 sinusoids). A continuum of frequencies is approximated by adding successive frequencies with a small step size, e.g., 1-10 Hz. In another embodiment, a smaller number of sinusoids (e.g., 3-10) are aggregated that span the frequency range of responsiveness of the skin. In another embodiment, the broadband carrier signals 1828 may be constructed in the time domain by taking each time sample of the broadband carrier signals 1828 as being a realization of a random number, e.g., a Gaussian distributed random variable that is generated independently of the values that preceded it. Such a method would result in a white Gaussian noise process. In another embodiment, periodic, low-frequency, non-sinusoidal signals may be used to generate the broadband carrier signals 1828, since such signals have frequency spectra that are comprised of a fundamental frequency and harmonic frequencies.
