The Design Of Automatic Gain Control Circuit With Digitally Assisted DC Offset Cancellation

Wireless transceiver is an important part in wireless communication and the design of automatic gain control circuit is the key of wireless transceiver. In this thesis, a digital automatic gain control circuit with digital assisted DC offset cancellation is studied.In this thesis the basic method eliminating the DC offset digitally is using controlled current DACs. Injecting current through two branches of the differential amplifier to calibrate the DC offset. For a multi stage programmable gain amplifier, the DC offset calibration should be applied to each stage. A method that calibrating from the front stage to the back stage is adopted. This would improve the calibration accuracy. For a single stage amplifier, DC offset calibration is performed by the improved bit by bit calibration algorithm. The direction of the injection current is decided according to the polarity of the original DC offset. And then the current is adjusted bit by bit to calibrate the DC offset. In this thesis, the automatic gain control is based on FSK modulation and is realized by digital detection and digital control. The structure of the programmable gain amplifer in this thesis is composed of the fixed gain amplifiers and the variable gain amplifier. Wherein the fixed gain amplifiers based on source degeneration resistanceare and gm-boosting structure is for coarse gain adjustment and the variable gain amplifier based on resistor feedback op amp is for fine gain adjustment. The digital detection is performed by digital RSSI and the digital gain control is achieved by a state machine for coarse gain adjustment and a look-up table for fine gain adjustment.The proposed AGC circuit is designed and simulated in SMIC 0.18 ?m CMOS process and is tested. The supply voltage is 1.8V, the gain range is 0-68dB, the noise figure is 36.09dB, the IIP3 is 20.685dBm, the bandwidth is 5MHz. The time of automatic gain control and digital DC offset cancellation is within 80 ?s, the DC output voltage is within 15mV for different gain.