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A crystal-less 500kHz voltage and temperature independent CMOS Gm-C low-pass-filter with automatic frequency tuning

Authors Shea, Man Kit
Issue Date 2012
Summary Nowadays, wireless receivers for baseband communication, such as GSM, Bluetooth CDMA mainly utilize zero-IF structure between the down-conversion mixer and A/D converter. Therefore, requirements of baseband filter become more and more important. There are various kinds of on-chip filter design, such as passive RC filter, switch capacitor filter, active RC filter and Gm-C filter. Among four kinds of filter architecture, Gm-C filter can operate in wide range of frequencies. Accounting the area concerns, Gm-C filter can be tunable easily to meet both frequency requirement and area requirement. Therefore, for baseband receiver application, Gm-C filter is an excellent choice. Most of the existing frequency compensated Gm-C filter needs to have a crystal oscillator. In order to minimize the cost and area of wireless receiver, it is preferable to remove the crystal oscillator in some of the baseband communication systems. In this thesis, a noval FVC type automatic frequency control is proposed to implement frequency compensated Gm-C filter without using any crystal oscillator. The main concerns of linearity, cutoff frequency variation and DC offset minimization of CMOS Gm-C filter are studied and optimized in this thesis. Three 500kHz low-pass-filters are fabricated in AMS 0.35μm CMOS process and characterized. All filters are 4th-order low-pass-filter constructed by 2 cascade stages of biquads. The cutoff frequency is around 500kHz for baseband applications. The purpose of first filter design acts as a reference to compare with the newly proposed idea. Based on simplicity, only linearity, common mode rejection and cutoff frequency variation are investigated and studied. This design is based on a PMOS input regulated cascode configuration for improvement in Gm stabilization. The measured THD is 0.055%, cutoff frequency variation across 0.5V-2.5V input common mode voltage is 200%, and cutoff frequency variation across supply voltage 3-3.6V and temperature from -40°C to 85°C is 45%. The purpose of second design is to resolve the problem of first design with poor input common-mode rejection and large cutoff frequency variation. A newly proposed OTA design and automatic frequency tuning method are proposed. The OTA design is modified with an additional rail-to-rail input stage and current summing output stage to reject input common-mode voltage variation. Also, an automatic frequency tuning method, crystal-less automatic frequency control (AFC) is designed to stabilize the cutoff frequency variation. The measured THD is 0.032%, cutoff frequency variation across 0.5V-2.5V input common-mode voltage is reduced to 20%, and cutoff frequency variation across supply voltage and temperature is reduced to 4.7%. However, the DC offset voltage in some samples was as large to 100mV. The chip areas of combining the first and second design are 2mm x 1.9mm. The final design is to resolve the offset voltage problem and optimization of the second design. A newly proposed offset cancellation technique for Gm-C filter is proposed. Based on the chopping technique, it helps minimize the output DC offset voltage to less than 5mV. The measured THD is 0.0565%, cutoff frequency variation across supply voltage and temperature is slightly reduced to 4.5%. The chip area is 1.3mm x 1mm.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2012
Language English
Format Thesis
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