This paper presents a wideband analog baseband (ABB) chain with precise gain step and tunable cutoff frequency in a 1.2V 40nm CMOS process. The chip has a core area of 0.21mm². A variable gain amplifier (VGA) realized in super source follower topology is designed to implement precise gain step. Besides, a 6^th-order lowpass filter (LPF) realized in Gm-C topology is designed to implement high and tunable cutoff frequency. The design achieves a maximum gain of 43.3dB and a dynamic gain range of 20.5dB with 0.5dB gain step. Meanwhile, the cutoff frequency can be tuned from 115MHz to 325MHz under 8 different bandwidth modes. The OP1dB is 5.5dBm and input referred noise (IRN) is 4.45 ~ 7.07nV/√Hz in band. The proposed analog baseband chain consumes 27 ~ 50mA in different gain modes and bandwidth modes under supply of 1.2V.