A Programmable Active Polyphase Filter Based on a High-Linearity, Low-Noise Voltage Subtractor for High-Speed Wideband Spectrum Sensing Receivers

A tunable active polyphase filter (PPF) based on a high linearity, low noise voltage subtractor is proposed for wideband and fast spectrum sensing receivers. The proposed active PPF is designed on the basis of the voltage subtractor, which is configured as a source follower (SF) stage stacked on a common source amplifier. Because of the linear voltage-to-voltage transfer function of the voltage subtractor by post linearization process and voltage-series feedback operation, it maintains high linearity while providing a certain amount of voltage gain with low noise. Consequently, it relaxes the gain burden of the preceding low noise amplifier (LNA) stage in comparison with the traditional passive RC-based PPF. In addition, the proposed PPF is a highly desirable structure for achieving frequency tunability because it includes only grounded tunable capacitors, and as a result, the proposed three-stage PPF achieves a wide image rejection frequency range from −2 to −30 MHz by varying the notch frequency using a grounded switched capacitor array. The measurement results show a conversion gain greater than +14.8 dB, an image rejection ratio (IRR) exceeding 30 dB, a noise figure (NF) below 19 dB, an in-band (IB) output-referred third-order intercept point (OIP3) greater than +14.5 dBm, and an out-of-band (OOB) OIP3 greater than +26.5 dBm across the operating frequency range. The total power consumption of the receiver, excluding the measurement buffer, was approximately 16.8 mW from a 1.2 V supply voltage.