A Research And Design Of Multi-passband Complex Filter

With the rapid development of the wireless communication technology, the wireless products have made a great influence on the human's life. Among the wireless communication products, the RF transceiver, as the interface of the communication chip, affects the performance of the transceiver signal and then determines the quality of the communication products. In the structure of the RF transceiver, the Low-IF receiver, which has the advantages of high integration density and avoiding the DC offset, has been widely used. However, the signals in the Low-IF receiver have an image signal, which will interference the useful signal and restrict the property of the Low-IF receiver. Complex filter can reject the image signal effectively, thus the complex filter with high integration, low power and high linearity has become the hot research topic.Firstly, this paper introduced the function and the development of the complex filter. It can be found that Most of the exiting complex filters have one or two pasbands, resulting in that the complex filters' applications have been limited in a large degree. In response to the problem, this paper shows a 4 passbands complex filter with a 3-order Butterworth structure after analyzing the basic structure and function. Because the multi-passband complex filter has more poles, this paper shows a newly-proposed capacitor array, which has a miller compensation structure inside. Under the function of the array, the filter's stability has been improved.Then, in order to improve the quality of the filter, many auxiliary circuits have been finished. This paper shows an auto frequency calibration circuit, made of a RC oscillator circuit and a digital module. Because of the operation of the oscillator and the digital module, the frequency deviations caused by the craftwork and temperature have been calibrated successfully. Besides 2 groups of DCOC circuits has been designed to eliminated the DC offset, 2-order PGA circuits has been proposed to guarantee the output signal's amplitude and the buffer circuits are used to balance the difference of the common voltage.At last, the whole schematic and layout has been completed based on the SMIC 130 nm and the simulation has been shown as well. The simulation result shows that the center frequencies of the 4 passband modes are 100 kHz, 200 kHz, 30 kHz and 500 kHz and the corresponding bandwidths are 50 kHz, 100 kHz, 150 kHz and 250 kHz. The gain of the whole circuits stretches from 0dB to 55 dB and can be adjusted by the step of 0.875 dB. When the gain of the circuits reaches the maximum value, the circuits' noise factor is less than 30 dB and linearity more than-40 dBm. The error of the frequency calibration is less than 3% and the total current is less than 1uA.