Real-time calibration of gain and timing errors in two-channel time-interleaved A/D converters for SDR applications: Page 2 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.
Proposed calibration method

In practice the Nyquist bandwidth of an ADC is never fully used, and a fraction of it is usually dedicated to the roll-off of the anti-aliasing filter. This free band is exploited to inject a constrained calibration signal. A sine-wave is selected for calibration as it is easy to generate with high spectral purity on which two main constraints are imposed:

  1. The amplitude is kept small enough to avoid any impact on the dynamic range while providing enough estimation accuracy. Experiments show that -40 dBFS to -35 dBFS level range provides the best tradeoff for a 14-bit ADC.
  2. The frequency is limited to the following discrete values in order to reduce the complexity of the digital signal processing algorithms:

        
............................................................ (Equation 1)

Where Fs is the TIADC sampling frequency, P, K are unsigned integers and S=+-1 depending on the location of the calibration signal with relation to the edge of the Nyquist zone (see Figure 1). This signal can be easily generated on-chip with a fractional-N PLL using the clock of the ADC as a reference signal. By choosing K high enough, the harmonics of the calibration signal will alias outside the useful band which relaxes their filtering requirements. The swing adjustment can be achieved with a programmable attenuator placed at the output of the PLL.

Figure 1: Frequency plan showing the location of the calibration signal. Click image to enlarge.

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