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ADL5370 Arkusz danych(PDF) 8 Page - Analog Devices |
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ADL5370 Arkusz danych(HTML) 8 Page - Analog Devices |
8 / 8 page AN-1039 Application Note Rev. 0 | Page 8 of 8 COMPLEX MODULATION CONCLUSIONS While a detailed discussion is beyond the scope of this article, it is worth mentioning that all of the issues associated with modulator imperfections can be avoided with a slightly differ- ent transmit architecture. Many modern DACs incorporate complex modulators, that is, digital engines that convert baseband I and Q data up to a low intermediate frequency (IF). These signals, which are still in Cartesian I and Q format, drive the IQ modulator. Because modern IQ modulators, such as the ADL5375, have baseband input bandwidths of as high as 750 MHz, low IFs in the 100 MHz to 250 MHz range can be easily accommodated. When an IQ modulator is driven by such a signal, the output spectrum is essentially a single sideband spectrum similar to what is shown in Figure 3. While modern IQ modulators offer excellent out-of-the-box quadrature accuracy, IQ gain imbalance, and LO leakage, their performance can be improved further using calibration. If the transmitter incorporates a loop-back receiver as part of a digital predistortion scheme, the receiver can also be used to conti- nuously monitor and correct the imperfections of the IQ modulator. The post-calibration performance is only limited by the available compensation step sizes and the ability of the receiver to precisely measure the constellation degradation. In transmitters that do not contain a loop-back receiver, factory calibration is a reasonable alternative. A single calibration in the middle of a operating band most likely causes degradation at the band edges. As a result, calibration at multiple frequencies within a band is more effective. When temperature drift is factored in, factory calibration at the ambient temperature typically improves LO leakage and sideband suppression by around 10 dB to 15 dB. The lower sideband becomes the modulated carrier and is displaced from the LO by a frequency offset equal to the intermediate frequency. The imperfections of the IQ modulator now manifest themselves as out-of-band effects, which can be filtered away, resulting in in-band EVM, which is not affected by the IQ modulator’s imperfections. However, this approach comes at some cost. Care must be taken to filter out the LO leakage along with the undesired upper sideband. In contrast, a Nyquist filtered zero IF spectrum is completely free of spurious components apart from harmonics of the LO. In addition, as the frequency of the low IF increases, the distortion of the DAC and IQ modulator increases slightly. ©2009 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN08383-0-10/09(0) |
Podobny numer części - ADL5370 |
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Podobny opis - ADL5370 |
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