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Modeling and analysis of aliasing image spurs problem in digital-RF-converter-based IQ modulators
Linköping University, Department of Electrical Engineering, Electronics System. Linköping University, The Institute of Technology.
Linköping University, Department of Electrical Engineering, Electronics System. Linköping University, The Institute of Technology.
2013 (English)In: ISCAS 2013, IEEE , 2013, 578-581 p.Conference paper, Published paper (Refereed)
Abstract [en]

In this work, we present an analytical study of aliasing image spurs problem in digital-RF modulators. The inherent finite image rejection ratio of this types modulators is conceptually discussed. A pulse amplitude modulation (PAM) model of the converter is used in the theoretical discussion. Behavioral level simulation of the digital-RF converter model is included. Finite image rejection is a limiting issue in this architecture, and Digital-IF mixing is used to alleviate the problem which is also reviewed and simulated.

Place, publisher, year, edition, pages
IEEE , 2013. 578-581 p.
Series
IEEE International Symposium on Circuits and Systems. Proceedings, ISSN 0271-4302
National Category
Signal Processing
Identifiers
URN: urn:nbn:se:liu:diva-100896DOI: 10.1109/ISCAS.2013.6571908ISI: 000332006800142ISBN: 978-1-4673-5760-9 (print)OAI: oai:DiVA.org:liu-100896DiVA: diva2:664207
Conference
IEEE International Symposium on Circuits and Systems (ISCAS 2013), 19-23 May 2013, Beijing, China
Available from: 2013-11-14 Created: 2013-11-14 Last updated: 2015-02-19Bibliographically approved
In thesis
1. On High-Speed Digital-to-Analog Converters and Semi-Digital FIR Filters
Open this publication in new window or tab >>On High-Speed Digital-to-Analog Converters and Semi-Digital FIR Filters
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

High-speed and high-resolution digital-to-analog converters (DACs) are vital components in all telecommunication systems. Radio-frequency digital-to-analog converter (RFDAC) provides high-speed and high-resolution conversion from digital domain to an analog signal. RFDACs can be employed in direct-conversion radio transmitter architectures. The idea of RFDAC is to utilize an oscillatory pulse-amplitude modulation instead of the conventional zero-order hold pulse amplitude modulation, which results in DAC output spectrum to have high energy high-frequency lobe, other than the Nyquist main lobe. The frequency of the oscillatory pulse can be chosen, with respect to the sample frequency, such that the aliasing images of the signal at integer multiples of the sample frequency are landed in the high-energy high-frequency lobes of the DAC frequency response. Therefore the high-frequency images of the signal can be used as the output of the DAC, i.e., no need to the mixing stage for frequency up-conversion after the DAC in the radio transmitter. The mixing stage however is not eliminated but it is rather moved into the DAC elements and therefore the local oscillator (LO) signal with high frequency should be delivered to each individual DAC element.

In direct-conversion architecture of IQ modulators which utilize the RFDAC technique, however, there is a problem of finite image rejection. The origin of this problem is the different polarity of the spectral response of the oscillatory pulse-amplitude modulation in I and Q branches. The conditions where this problem can be alleviated in IQ modulator employing RFDACs is also discussed in this work.

ΣΔ modulators are used preceding the DAC in the transmitter chain to reduce the digital signal’s number of bits, still maintain the same resolution. By utilizing the ΣΔ modulator now the total number of DAC elements has decreased and therefore the delivery of the high-frequency LO signal to each DAC element is practical. One of the costs of employing ΣΔ modulator, however, is a higher quantization noise power at the output of the DAC. The quantization noise is ideally spectrally shaped to out-of-band frequencies by the ΣΔ modulator. The shaped noise which usually has comparatively high power must be filtered out to fulfill the radio transmission spectral mask requirement.

Semi-digital FIR filter can be used in the context of digital-to-analog conversion, cascaded with ΣΔ modulator to filter the out-of-band noise by the modulator. In the same time it converts the signal from digital domain to an analog quantity. In general case, we can have a multi-bit, semi-digital FIR filter where each tap of the filter is realized with a sub-DAC of M bits. The delay elements are also realized with M-bit shift registers. If the output of the modulator is given by a single bit, the semi-digital FIR filter taps are simply controlled by a single switch assuming a current-steering architecture DAC. One of the major advantages is that the static linearity of the DAC is optimum. Since there are only two output levels available in the DAC, the static transfer function, regardless of the mismatch errors, is always given by a straight line.

In this work, the design of SDFIR filter is done through an optimization procedure where the ΣΔ noise transfer function is also taken into account. Different constraints are defined for different applications in formulation of the SDFIR optimization problem. For a given radio transmitter application the objective function can be defined as, e.g., the hardware cost for SDFIR implementation while the constraint can be set to fulfill the radio transmitter spectral emission mask.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 52 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1708
Keyword
DAC, RFDAC, SDFIR, FIR, semi-digital FIR filter, digital-to-analog converter, D/A converter, data converter, mixed-signal integrated circuits, mixer DAC, IQ modulator, transmitter
National Category
Signal Processing
Identifiers
urn:nbn:se:liu:diva-114274 (URN)10.3384/lic.diva-114274 (DOI)978-91-7519-122-5 (ISBN)
Presentation
2015-02-13, Visionen, Building B, Campus Valla, Linköping University, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2015-02-19 Created: 2015-02-16 Last updated: 2015-02-19Bibliographically approved

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Sadeghifar, Mohammad RezaWikner, Jacob

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