Low-if Filter In The Rf System Design And Research

As the wireless communication system develops, the fully integrated RF system on chip has become a hot research filed. This thesis deals with low-IF filters in RF system. The methods of active filter synthesis and auto-tuning as well as some structures of transconductance amplifiers are introduced first in this paper. Two filters used in Bluetooth system are presented: one is the low-IF bandpass filter. The other is low-IF complex filter.Because of the variance of the process and aging, the integrated filter must employ auto-tuning circuit to adjust its center frequency. The methods of auto-tuning, including PLL-based tuning and single integrator-based tuning are analyzed, which are used in controlling frequency response of the filters designed.The oscillating condition of such kind of VCO, which constitutes of the transconductance amplifier and capacitors, is studied in the PLL-based tuning circuit design. The influence of parasitic parameters on VCO oscillating frequency is derived. The conclusion shows that the accuracy of auto-tuning circuit can be improved by increasing the output resistance of the OTA. Meanwhile, a novel simple transconductor is invented to limit the oscillating amplitude of VCO. On the bases of 0.35um TSMC digital process, a bandpass filter using this kind of auto-tuning circuit is fabricated. The control voltage is stable, which is the output of the PLL-based tuning circuit. The frequency of the filter is correct. Thus, it can be concluded that the novel transconductor works well, and the oscillating amplitude is limited in the range we wanted.When single-integrator-based tuning is designed, models are built to study the circuit. It reveals that the loop may be oscillating when the loop filter is a single stage. A zero and a pole are added to the loop filter, which becomes a two stage filter. Thus the loop is stabilized. Simulation proves that this analysis result is right. Also, a rail-to-rail fully differential buffer is used to drive large differential signals in tuning circuit. A complex filter adopts this tuning circuit, which is proved by filter's testing result. Designed on 0.35um charter analog process, the complex filter has been designed and the testing results are presented .Finally, two filters cited above and their tuning circuits are compared. The future work about filters are presented.