In this paper multiple feedback(FB) active filter configurations for DM EMI noise in the input side of AC/DC converters has been analyzed using developed models and then their performance was compared over single FB active filters using measurements. First, a single FB active filter circuit model is developed in the open loop and then based on DM EMI noise model of AC/DC converters, a closed loop model is developed. Then, based on the single FB model, closed loop models for series and parallel FB active filter topologies are developed. Single FB, series FB and parallel FB topologies have been implemented in hardware. Loop gains and insertion gains for the topologies are firstly derived and then measured using network analyzer. Comparing the topologies, it was found that series FB topology can give much higher attenuation. Finally, spectral domain measurements in the conducted EMI frequency range were done to verify the frequency domain performance of series FB topology.

In this paper, a noise robust formant frequency estimation scheme is developed based on a spectral model matching algorithm. Considering the vocal tract as an autoregressive system, a spectral model of repeated autocorrelation function (RACF) of band-limited speech signal is proposed. It is shown that because of the repeated autocorrelation operation on band-limited signal, the proposed model can exhibit prominent formant characteristics. First from given noisy speech observations, an adaptive band selection criterion is developed. Next, on each resulting band-limited noisy speech signal, a repeated autocorrelation operation is carried out, which not only reduces the effect of noise but also strengthens the dominant poles corresponding to the formant frequencies. Finally, spectrum of the RACF is computed and instead of direct spectral peak picking, a model fitting scheme is introduced to find out model parameters which lead to formant estimation. The proposed algorithm has been tested on natural vowels as well as some naturally spoken sentences in the presence of different environmental noises. It is found that the proposed scheme provides better formant estimation accuracy in comparison to some of the existing methods at low levels of signal-to-noise ratio.