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AD9269BCPZ-65 Datasheet(PDF) 29 Page - Analog Devices |
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AD9269BCPZ-65 Datasheet(HTML) 29 Page - Analog Devices |
29 / 41 page AD9269 Data Sheet Rev. A | Page 28 of 40 DC AND QUADRATURE ERROR CORRECTION (QEC) In direct conversion or other quadrature systems, mismatches between the real (I) and imaginary (Q) signal paths cause fre- quencies in the positive spectrum to image into the negative spectrum, and vice versa. From an RF point of view, this is equivalent to information above the local oscillator (LO) frequency interfering with information below the LO frequency, and vice versa. These mismatches may occur from gain and/or phase mismatches in the analog quadrature demodulator or in any other mismatches between the I and Q signal chains. In a single- carrier zero-IF system where the carrier has been placed symmetrically around dc, this causes self-distortion of the carrier as the two sidebands fold onto one another and degrade the error vector magnitude (EVM) of the signal. In a multicarrier communication system, this can be even more problematic because carriers of widely different power levels can interfere with one another. For example, a large carrier centered at +f1 can have an image appear at −f1 that may be much larger than the desired carrier at −f1. The integrated quadrature error correction (QEC) algorithm of the AD9269 attempts to measure and correct the amplitude and phase imbalances of the I and Q signal paths to achieve higher levels of image suppression than is achievable by analog means alone. These errors can be corrected in an adapted manner, in which the I and Q gain and quadrature phase mismatches are constantly estimated and corrected, allowing for constant tracking of slow changes in mismatches that are due to supply and temperature. The quadrature errors are corrected in a frequency independent manner on the AD9269; therefore, systems with significant mismatch in the baseband I and Q signal chains may have reduced image suppression. The AD9269 QEC still corrects the systematic imbalances. The convergence time of the QEC algorithm is dependent on the statistics of the input signal. For large signals and large imbalance errors, this convergence time is typically less than 2 million samples of the AD9269 data rate. LO Leakage (DC) Correction In a direct conversion receiver subsystem, LO to RF leakage of the quadrature modulator shows up as dc offsets at baseband. These offsets are added to dc offsets in the baseband signal paths, and both contribute to a carrier at dc. In a zero-IF receiver, this dc energy can cause problems because it appears in band of a desired channel. As part of the QEC function, the dc offset is suppressed by applying a low frequency notch filter to form a null around dc. In applications where constant tracking of the dc offsets and quadrature errors are not needed, the algorithms can be independently frozen to save power. When frozen, the image and LO leakage (dc) correction are still performed, but changes are no longer tracked. Bits[5:3] in Register 0x110 disable the respective correction when frozen. The default configuration on the AD9269 has the QEC and dc correction blocks disabled, and Bits[2:0] in Register 0x110 must be pulled high to enable the correction blocks. The quadrature gain, quadrature phase, and dc correction algorithms can also be disabled independently for system debugging or to save power by pulling Bits[2:0] low in Register 0x110. When the QEC is enabled and a correction value has been calculated, the value remains active as long as any of the QEC functions (DC, gain, or phase correction) are being used. QEC and DC Correction Range Table 13 gives the minimum and maximum correction ranges of the algorithms. If the mismatches are greater than these ranges, an imperfect correction results. Table 13. QEC and DC Correction Range Parameter Minimum Maximum Gain −1.1 dB +1.0 dB Phase −1.79 degrees +1.79 degrees DC −6 % +6% 0 –15 –30 –45 –60 –75 –135 –120 –105 –90 FREQUENCY (MHz) 4 3 2 5 6 IMAGE DC OFFSET Figure 57. QEC Mode Off 0 –15 –30 –45 –60 –75 –135 –120 –105 –90 FREQUENCY (MHz) 4 3 2 5 6 IMAGE DC OFFSET Figure 58. QEC Mode On |
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