二戰期間，為了能在戰爭中有效監測敵方部隊位置，及遠距離辨別敵我部隊的需求，雷達就這樣誕生了；二戰結束後，隨著各個科學領域的進步，雷達技術不斷發展，相較於其他感測機制如紅外線、超音波、雷射等，雷達能在大範圍感測之下保持性能穩定，較不受其他環境因素的影響；如今雷達早已克服了諸多限制而普及於交通、安全、醫療等民間應用上，在這個科技發達的時代，其重要性可說是與日俱增。

在一個雷達系統接收端中，天線所接收到的射頻訊號，會先透過低雜訊放大器將其放大到後端電路可辨識處理的程度，再經由前端電路混頻器降頻後，送交給類比基頻電路做訊號放大處理，並利用高階濾波器濾除頻帶外雜訊，最後將類比資料送交給類比數位轉換器，取得我們要的數位訊號；簡言之，類比基頻電路在接收端後端電路所扮演的腳色，就是提供所需之增益及濾波。

本論文提出一個類比基頻電路的設計，其組成包含了電壓緩衝器、可變增益放大器、四階巴特沃茲低通濾波器、二階等化器以及另一個電壓緩衝器，有著0至100dB的可變增益範圍，其截止頻率為20MHz，並在通帶內有著良好的相位響應特性，此類比基頻電路使用台積電65奈米的CMOS製程來設計，其操作電壓為1.2伏特。

During the period of World War II, radar has been created for searching the position of enemy forces effectively or distinguishing the troops from a long distance. After the Second World War, radar has been highly developed because of the progress of Scientific and technological in different fields. Compared with other sensing mechanisms, like Infrared, Ultrasound or Laser, radar is relatively unaffected by other
Environmental factors and more stable in a wide range of sensing. Today radar has overcome many limitations and become many daily applications in our life. The importance of radar is steadily on the increase.

In the beginning of receiver in a radar system, a RF signal is received by an antenna. And it will be gained by the low noise amplifier (LNA) that the back-end circuit can deal with. After passing the down-conversion Mixers of the front-end circuit, it will be brought to the baseband circuit for amplifying the signal. And the noise will be filtered by a higher-stage low pass filter. In the last, the analog information will be referred to an ADC for the new digital information that we expect. In short, the baseband circuit is to amplify the signal and eliminate the noise outside the cutoff frequency.

This thesis presents an analog baseband circuit design. It includes an unit gain buffer, a programmable gain amplifier, a fourth-order Butterworth low pass filter, a second-order equalizer and another unit gain buffer. The circuit has zero to hundred dB programmable gain. The cutoff frequency is 20MHz. It also has a nice phase response in passband frequency. The baseband circuit is implemented with TSMC 65nm process, operating at 1.2 volt.

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