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A new third-order transconductance (gm3) cancellation technique is proposed and applied to a conventional radio frequency (RF) mixer for improving circuit linearity. The bulk-to- source voltage is applied to adjust the peak value position of gms. The cancellation of gm3 is utilized by a negative peak gm3 transistor combined in parallel with a positive peak gm3 transistor. For a single device, the measured adjacent channel power ratio (ACPR) and third-order intermodulation (IMD3) distortion are both improved over 15 dB. A Gilbert-cell mixer in commercial 0.18-mum CMOS process was designed using the proposed method to further evaluate the linearity. The compensated gm3 device is placed in the input RF gm-stage and then reducing the principle nonlinearity source of the mixer. From the experiment results, the ACPR and IMD3 of the mixer are improved about 10 and 15 dB, respectively.
Date of Publication: May 2008