The spectrotemporal receptive field (STRF) provides a versatile and integrated (spectral and temporal) functional characterization of single cells in primary auditory cortex (AI). We explore in this paper the origin and relationship between several different ways of measuring and analyzing the STRF. Specifically, we demonstrate that STRFs measured using a spectrotemporally diverse array of broadband stimuli --- such as dynamic ripples, spectrotemporally white noise (STWN), and temporally orthogonal ripple combinations (TORCs) --- are very similar, confirming earlier findings that the STRF is a robust linear descriptor of the cell. We also present a new deterministic analysis framework that employs the Fourier series to describe the spectrotemporal modulation frequency content of the stimuli and responses. Additional insights into the STRF measurements, including the nature and interpretation of measurement errors, is presented using the Fourier transform, coupled to singular-value decomposition (SVD), and variability analyses including bootstrap. The results promote the utility of the STRF as a core functional descriptor of neurons in AI.