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ADL5593 Datasheet(PDF) 8 Page - Analog Devices

Part No. ADL5593
Description  Correcting Imperfections in IQ Modulators to Improve RF Signal Fidelity
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ADL5593 Datasheet(HTML) 8 Page - Analog Devices

   
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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)


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