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AD8349 Arkusz danych(PDF) 4 Page - Analog Devices |
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AD8349 Arkusz danych(HTML) 4 Page - Analog Devices |
4 / 8 page AN-1039 Application Note Rev. 0 | Page 4 of 8 Table 1. IQ Modulator Selection Table Showing Uncompensated Gain and Phase Imbalance PART NUMBER FREQUENCY (MHz) IQ 3dB BANDWIDTH (MHz) CARRIER SUPPRESS (dBm) SIDEBAND SUPPRESSION (dBc) GAIN IMBALANCE (dB) PHASE IMBALANCE (°) NOISE FLOOR (dBm/Hz) P1dB (dBm) OUTPUT IP3 (dBm) SUPPLY VOLTAGE (V) SUPPLY CURRENT (mA) SPECS @ (MHz) AD8345 AD8346 AD8349 ADL5370 ADL5371 ADL5372 ADL5373 ADL5374 ADL5375 ADL5385 ADL5386 ADL5590 ADL5591 140 TO 1000 800 TO 2500 700 TO 2700 300 TO 1000 500 TO 1500 1500 TO 2500 2300 TO 3000 3000 TO 4000 400 TO 6000 50 TO 2200 50 TO 2200 869 TO 960 1805 TO 1990 80 70 160 500 500 500 500 500 750 700 700 250 250 –42 –42 –45 –50 –50 –45 –32 –33 –46 –46 –38 –44 –39 –42 –36 –35 –41 –55 –45 –57 –50 –52 –50 –46 –50 –48 0.200 0.200 0.100 0.030 0.100 0.090 0.010 0.015 –0.050 –0.030 0.050 0.050 0.050 0.50 1.00 1.90 0.76 –0.03 0.21 0.10 0.25 –0.29 –0.39 –0.50 0.20 0.30 –155.0 –147.0 –155.0 –160.0 –158.6 –158.0 –157.0 –160.0 –160.0 –159.0 –160.0 –157.0 –157.0 2.5 –3.0 7.6 11.0 14.4 14.2 13.8 12.0 9.4 11.0 11.1 16.0 16.0 N/A N/A 21.0 24.0 27.0 27.0 26.0 22.8 27.0 26.0 25.0 29.0 30.0 2.70 TO 5.50 2.70 TO 5.50 4.75 TO 5.50 4.75 TO 5.25 4.75 TO 5.25 4.75 TO 5.25 4.75 TO 5.25 4.75 TO 5.25 4.75 TO 5.25 4.75 TO 5.50 4.75 TO 5.50 4.75 TO 5.50 4.75 TO 5.50 65 45 135 205 175 165 174 173 200 215 230 170 170 800 1900 900 450 900 1900 2500 3500 900 350 350 940 1960 CORRECTING MODULATOR IMPERFECTIONS Note that in Figure 1, in addition to the direct conversion signal chain, an optional loop-back or transmit observation receiver has also been incorporated into the radio. The primary function of this receiver is to analyze the adjacent channel power ratio (ACPR) of the transmitter that is primarily caused by distortion in the high power amplifier (HPA). By continually observing the ACPR of the transmitter, digital predistortion of the baseband signal can be employed to partially correct HPA nonlinearities while allowing the HPA to operate closer to its compression point. The presence of a loop-back receiver can be opportunistically used to also correct for modulator imperfections. A detailed discussion of the IQ modulator correction tech- niques used when a loop-back receiver is present is beyond the scope of this application note. However, the general procedure involves using the loop-back receiver to demodulate the I and Q bit streams. The demodulated constellation is then examined for evidence of IQ gain imbalance, imperfect quadrature, and/ or LO leakage. Once these imperfections have been identified, the I and Q data streams can be preprocessed so that the IQ modulator imperfections cancel out. For example, if the demodulated constellation from the loop-back receiver shows a rectangular constellation with the width of I being larger than the height of Q (see Figure 2B), gain adjust registers in the DAC can be used to either decrease the size of the I data stream or increase the size of the Q data stream. Likewise, the phase adjustment registers of the DAC can be used to slightly skew the I and Q channels so that the imperfect quadrature of the IQ modulator’s phase splitter is compensated. FACTORY CALIBRATION If a wireless transmitter does not use digital predistortion, it would be difficult to justify the cost of a loop-back receiver purely for the sake of the IQ modulator. In such situations, the two options that remain are: • Do not perform any correction of the IQ modulator’s imperfections. • Complete a one-time factory calibration and store the correction coefficients in nonvolatile memory. In recent years, the performance of IQ modulators has improved to such a degree that it is now feasible (depending on the modulation scheme) to design a transmitter without any need to provide correction for imperfections. For example, the ADL5375 from Analog Devices has gain and quadrature imbal- ances of 0.05 dB and 0.29°, respectively, at 900 MHz, with little or no degradation over temperature. As a result, in many appli- cations, it may be adequate to dispense with any correction algorithms. Table 1 shows the performance of this and other members of the Analog Devices IQ modulator family. |
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