An Analog Baseband Spectrum Sensing Circuit Employing Voltage Follower-Based Multiorder Channel Selection Filters

An analog baseband (ABB) spectrum sensing circuit is proposed for fast spectrum sensing. Contrast to the conventional RF scanner where it takes a relatively long period to detect channel and interference condition by sequentially controlling the center frequency of the filter, the proposed ABB circuit can scan it at a time using different bandwidths of low-pass filters with an almost equal out-of-band attenuation slope to sense the powers and frequencies of channel and interference. The ABB circuit is composed of two chains of five cascaded real and complex low-pass filters. The cut-off frequencies of five cascaded low-pass filters are set to be 10, 8, 6, 4 and 2 MHz respectively, regarding the channel bandwidth of 2 MHz. The orders of the five filters are optimized to be <inline-formula> <tex-math notation="LaTeX">$4^{\mathrm{ th}}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$4^{\mathrm{ th}}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$3^{\mathrm{ rd}}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$2^{\mathrm{ nd}}$ </tex-math></inline-formula>, and <inline-formula> <tex-math notation="LaTeX">$2^{\mathrm{ nd}}$ </tex-math></inline-formula> order respectively to make the rejected out-of-band power same for each filter for accurate detection. The filter core is designed on the basis of a flipped voltage follower and a super source follower for a low power dissipation and a high linearity, and the low power technique for implementing high-order filter is newly proposed in the design of filter chain. The measured frequency response at each output of the filter chain was almost the same to the simulation result. The measurement results show a noise figure (NF) of 19.2 dB, 22.1 dB, 24.8 dB, 25.1 dB, and 27 dB for each filters, and an in-band input-referred third-order intercept point (IIP3) of 9.2 dBm, 7.2 dBm, and 15.7 dBm at the output of <inline-formula> <tex-math notation="LaTeX">$1^{\mathrm{ st}}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$3^{\mathrm{ rd}}$ </tex-math></inline-formula>, and last stages. The total power consumption of the filter chain is 10.4 mW from a 1.2 V supply voltage.