使用正交相位調變器及高整合度功率放大器之2.4 GHz 互補式金氧半射頻發射器的設計

Abstract

本篇論文主旨在設計一個可應用於2.4GHz ISM 頻帶的射頻發射器。主要包括調變器、壓控震盪器、功率放大器及內部偏壓電路。基頻輸入訊號為10MHz, 發射信號大於4dBm，發射頻譜符合Bluetooth規格的規範。 整個設計使用差動式電路以增強抗干擾能力。論文中使用一直接昇頻式的正交相位調變器。兩個相同的震盪器透過差動反相器耦合，用以產生精準的正交相位信號。10MHz的基頻輸入信號經由調變器與2.4GHz的震盪器輸出信號混波後，直接昇頻至2.4GHz ISM 頻帶。調變器的輸出再經由一AB類的差動功率放大器放大，以加強發射功率。一個經由設計的內部偏壓電路可以補償實際製造過程中元件尺寸的偏差。發射器的特性已由Hspice模擬驗證。 以點25微米1P5M n-well製程互補式金氧半技術設計並製造的發射器電路已經完成量測。在量測過程中發現，差動功率放大器間的不匹配將會增強震盪器信號漏溢、鏡相信號、交互調變信號。這些信號都是輸出端不希望看到的。為了得到較乾淨的輸出頻譜，功率放大器的偏壓點和電源電壓被調整在低準位。然而輸出信號強度也被限制住了。由量測結果可知，壓控震盪器的可調頻率為2.09GHz到 2.718GHz。當輸出信號被調整至2.2568GHz時，有最低的震盪器信號漏溢，其值為-34.33dBc。在此量測條件下，鏡相信號為-32.83dBc，二次交互調變信號為-33.5dBc，三次交互調變信號為-36.5dBc，輸出信號強度為-25dBm。當輸出信號被調至其他頻率，或是輸出信號強度被調大時，震盪器信號漏溢和交互調變信號會跟著變大。當發射器的電源電壓為2.5v，功率放大器DC偏壓增加至1.4v時，發射器輸出功率可達5dBm。但此時發射頻譜很髒且消耗功率極大。因為在實際製作晶片上的螺旋電感的品質因數小於模擬時所用的值，及電感電容負載有一些飄移，實際量到的發射器輸出功率小於模擬時的結果。這些由量測中發現的問題亟需解決，以在將來實現一個實用的射頻發射器。

In this thesis, the design of CMOS RF transmitter operated at 2.4GHz ISM band is presented. The circuits included a quadrature modulator, quadrature VCO, power amplifier, and self-bias circuits. The frequency of baseband inputs is 10MHz, and the output power is above 4dBm. The transmit band should meet the specifications of Bluetooth. Differential circuits are used in this design to reinforce the ability of interference rejection. A direct-conversion quadrature modulator is applied in this thesis. Two identical LC tank oscillators coupled by differential inverters generate accurate quadrature LO signals. The 10MHz baseband signals and 2.4GHz LO signals are mixed by the modulator. The modulator output is input to a class AB differential power amplifier for strengthening the transmit power. A self-bias circuit is designed here to compensate the technique process variations. The performance of the transmitter have been proven by the Hspice simulation. The transmitter, which is designed and fabricated by 0.25μm 1P5M n-well CMOS technology, has been measured. During the experiment, it has been observed that the mismatch between differential power amplifiers will enhance the strength of undesired output signals such as LO leakage, image signal, and intermodulation signals. For getting clear output spectrum, the power supply and DC bias of PA must keep low. However the output power will be limited. From the measurement results, the tuning range of the quadrature VCO is 628Mhz from 2.09GHz to 2.718GHz. The minimum LO leakage is –34.33dBc when the desired output signal is set at 2.2568GHz. At this measurement condition, the image ratio is 32.83dBc, the second-order intermodulation is –33.5dBc, and the third-order intermodulation is –36.5dBc. Moreover, the output power is -25dBm.When the desired output signal is set to other frequency or the output power is set to be higher, the LO leakage and intermodulation signals will be larger. If the power supply of the transmitter is 2.5v and the DC bias of the power amplifiers is increased to 1.4v, the output power could be 5dBm. However the output spectrum is noisy and the power dissipation is huge. Due to the smaller quality factor of the real spiral inductors and the variation of the LC load, the measured output power is lower than simulation results. These problems observed in the measurement results should be solved in the future to realize a practical RF transmitter.