The Design Of Tunable Bandwidth Low-pass Filter For3G/4G Mobile Communication Transmitter

During the upgrading of mobile communication systems, communicationterminals should be compatible to achieve the long-term coexistence among differentcommunication protocol. This dissertation presents a variable-bandwidth filter formobile transmitter, which is compatible with both TD-SCDMA and TD-LTEprotocols. It succeeds in reusing the chip and increasing the integration of the system.The main contents are expressed as follows:(1)Analyze the physical channel requirements of TD-SCDMA and TD-LTE protocols;obtain the index of the multimode transmitter compatible for both protocols.(2)Introduce the basic analog filter theories, the design methods and the actual designrules of analog filters. Defined the technical index of the filter; five kinds ofbandwidths:1.6MHz,3MHz,5MHz,10MHz,15MHz, and the double cut-offfrequency attenuation exceed to50dB.(3)Design a folded-cascode two stage operational amplifier for the active-RC filter,analyzes that its GBW is the most important parameter and the phase margin can berelaxed. The simulation results show that the GBW reached800MHz, and the gain is80dB, the phase margin is40°. After analyzing the effect of the finite GBW on filter, acompensation structure is added to compensate the effect.(4)The layout design and the taping out of the filter use the UMC0.18μm CMOSprocess, chip area is1059μm×867μm. Design the test solution and make the PCB testboard for chip test, the test results show that the filter can be tuned of five kinds ofbandwidths, double cut-off frequency attenuation reach above50dB, the IIP3is-4.4dBm, meet the requirements of the design.(5)Analyze and summarize the results; make some recommendations to follow-up.