Detection of subthreshold oscillations in a sinusoid-crossing sampling

The detection threshold B of a sinusoid-crossing (SC) detector is improved using a new dithering technique. In a real SC detector, B is always greater than zero because the crossings could be located only with finite accuracy. Dithering is employed to determine the frequency f_s and the amplitude A_s of the subthreshold oscillation s(t)=A_s cos(2πf_st), where A_s<B. The data representation of an analog input signal of bandlimit W, consists of locations {t₁,t₂,…,t_2M}={t_i} where the signal intersects with the reference sinusoid r(t)=A cos(2πf_rt). A crossing exists within each interval Δ=1/2 f_r=T/2M, where T is the sampling period. If W/2<~f_r, and the signal amplitude is less than A for all t values within T, then SC sampling satisfies the Nyquist sampling theorem. The unknown f_s value is determined from the power spectrum of the crossing locations of [s(t)+n_σ(t)], where n_σ(t) is the noise of variance σ². The A_s value is approximated from the signal-to-noise ratio (R) vs σ plot. The performance of the technique is studied from the R plots for different A_s, f_s, and T values.