Real-time calibration of gain and timing errors in two-channel time-interleaved A/D converters for SDR applications: Page 5 of 6

June 25, 2014 //By Djamel Haddadi, Integrated Device Technology, Inc.
The explosion of mobile data is driving new receiver architectures in communication infrastructure in order to provide higher capacity and more flexibility.
Figure 4 shows the power spectrums of the data before and after calibration. The LTE carrier image, at -80 dBFS before calibration, has been reduced by about 30 dB to -110 dBFS level after calibration. The calibration signal and its image have been completely cancelled by the extraction and cancellation algorithm. This performance has been achieved within about 200 µs convergence time.

Figure 4: Power spectrums before (TOP) and after calibration (Bottom) with 300 MHz LTE carrier. Click image to enlarge.

The calibration signal was kept unchanged and the LTE carrier center frequency swept from 50 MHz to 400 MHz in order to assess the frequency behaviour. The resulting image rejection, as shown in Figure 5, shows that at least 30 dB dynamic range improvement is maintained across the two first Nyquist zones. As expected, the image rejection decreases with frequency limited by the contribution of the bandwidth error which is not corrected.

Figure 5: Image rejection versus the LTE carrier center frequency with fixed
calibration signal. Click image to enlarge.

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