Digitally Assisted Radio-Frequency Integrated Circuits

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Stewart, David
Microwave , RF , Integrated Circuits , Digital Assist , Electrical Engineering
In this thesis, three radio frequency integrated circuits (RFICs) were digitally assisted for varying signal power, frequency or both. Performance paramters were 'optimized' in the sense of obtaining the best performance improvement possible through the methods used. The digital assist method used a lookup table (LUT) of optimal bias points measured through extensive sweeps and linear interpolation to determine the optimal bias point of the chip between measured points. A Gilbert Cell was fabricated in 0.13 μm CMOS. Transistor gate bias voltages were swept with input power to find the optimal bias voltages for intermodulation distortion (IMD) performance. A power detector was on-chip for the digital assist. Linear interpolation was used to optimize biases for any input power between initially measured points. Whereas distortion generally increases with input power, the digitally assisted device reduced the distortion of large signals. The IIP3 was 2.83 dBm from -3.11 dBm, and the P1dB was -3.33 dBm from -12.06 dBm. The RF bandwidth was measured as 1 to 12 GHz and DC power consumption varied from 2.06 to 3.27 mW. A noise cancelling low noise amplifier (LNA) was designed and fabricated in 0.13 μm CMOS. A feedback capacitor was used to boost the gain of the input transistor for lower DC power and better S11. An on-chip frequency detector was implemented for the digital assist. The gain rolled off gradually from 1 GHz until the 3 dB cutoff at 7 GHz. The average noise figure was 4.1 dB and less than 5 dB across the band. With digital assist, the gain curve was flattened at about 15 dB with to a broader 8 GHz bandwidth without negatively affecting the noise figure. The S11 was below -12 dB. A power amplifier (PA) previously designed in 0.8 μm Gallium Nitride (GaN) used third order IMD cancellation by derivative superposition effective at a fixed frequency and output power. Digital assist eliminated the extreme sensitivity to variations. Digital assist and bilinear interpolation maintained a 10 dB improvement in OIP3 over the entire 1 to 6 GHz band for varying output power between 21 and 24 dBm.
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