This paper presents a step attenuator with 0.22 dB flat attenuation states and low phase variation between 1.5-13.5 GHz. This work consists of 7 switched Π/T-type attenuation blocks. Isolated NMOS(iNMOS) devices were preferred instead of conventional NMOS devices to utilize switching to improve RF performance. A parallel capacitor was inserted in each Π-type attenuation bits to achieve flat attenuation states. The measured attenuator can cover 28 dB with 0.22 dB states. The RMS amplitude error of this chip is less than 0.22 dB between 2-9.5 GHz. The RMS amplitude error @13.5 GHz is 0.27 dB. The IP1dB of the attenuator is 12 dBm at 7.5 GHz. The IL of the attenuator was measured less than 15.8 dB.