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Time-Interleaved Digital-to-Analog Converters for
UWB Signal Generation
Abstract-In this paper, a differentially encoded Time­
Interleaved Digital-to-Analog Converter (TIDAC) structure for
Ultra-Wideband (UWB) signal generation is presented. We show
that the output spectrum of the TIDAC is identical to the output
of a DAC running at a higher sampling rate. To support the
mathematical derivations, the UWB signal generator has been
implemented on FPGA hardware and measurements have been
taken for a sinusoidal wave, a Gaussian UWB pulse shape
and for a UWB OFDM signal. The measurements show that,
compared to the single-channel DAC, the mirror images around
the sampling frequency can be reduced by about 30 dB and the
analog interpolation filter design is simplified.
I. INTRODUCTION
Ultra Wide-Band (UWB) signal generation is an important
task within the design of UWB systems for data transmission
and positioning applications [1]. Thus, several approaches
have been proposed for wideband signal generation. For pulse
based UWB systems, the authors in [2], [3] have compared
different pulse shapes and their spectra. The applicability of
the studied pulses for communication scenarios has been presented.
However, most important is the generation of wideband
signals that comply with the spectral mask of the FCC [4] and
other standardization authorities. Generally, pulse based UWB
signals can be generated with a very simple analog circuit,
which contains step recovery diodes or other active elements.
However, for each pulse shape a different circuitry has to be
designed and optimized.
If the system should be universally applicable and flexible
in adaptation to changes in the environment (e.g. adapting to
mutual antenna coupling), then analog pulse generator circuits
appear not flexible enough. As an alternative, a Digital-to­
Analog Converter (DAC) can be used to generate different
wideband signals. Since technological limits have been pushed
recently (for DACs even further than for Analog-to-Digital
Converters (ADCs)) up to several GSafs of conversion rate
[5], direct broadband signal generation becomes feasible.
Moreover, the standardized Multi Band (MB)-OFDM signals
have to be generated with DACs anyway. The pulse shapes
generated by the DAC can be arbitrary, allowing quick adaptation
to different broadband systems, or an adaptive antenna
matching according to the scenario.
In this work we focus on time-interleaved DACs (TIDACs)
which consist of M DACs operating at low sampling rate.
1-4244-0521-1/07/$20.00 ©2007 IEEE
Christoph Krall 1,2 , Christian Voge1 2 , Klaus Witrisa1 2
lChristian Doppler Laboratory for Nonlinear Signal Processing
2 Signal Processing and Speech Communication Laboratory
Graz University of Technology
Inffeldgasse 1211, Graz, Austria
{christoph.krall, vogel, witrisal}@tugraz.at
366
Each of the DACs is used with a phase shifted clock which
results in time-shifted contribution to the output signal. The
main advantage of the TIDAC is that due to the phase shifted
signals, the output signal seems to be generated at a higher
sampling rate, given that the settling time of the individual
DACs is high.
For the TIDAC, the clock generation adds more implementation
complexity. However, this additional effort remains low
because only a multi phase clock generator has to be used to
clock the different DACs. For the two channel case, this can be
easily done by using differential signals. Hence, the additional
complexity is traded for flexibility in pulse shape adaptation
and generation which is needed in a demonstration environment.
Using a TIDAC on a mobile device may consume more
chip area than using a single DAC at the M -times higher
speed.
The TIDAC proposed in this paper distinguishes itself from
previous works as follows. The authors in [6] consider a
time-interleaved scheme that consists of two DACs that are
alternately switched on, i.e., either the first or the second DAC
contributes to the output signal. An improvement for the output
signal is achieved by mapping the data words with a return to
zero (RTZ) code which helps avoiding glitches in the output
signal [7]. However, this RTZ coding is not applicable to our
structure because the output signal combination in our scheme
is always the sum of the contributions of the individual DACs
in the TIDAC.
Another approach using TIDACs is shown in [8], where the
DACs are operated in a time-interleaved manner to achieve
a signal with less distortions due to aliasing effects. The
interleaving structure generates samples of the output signal
with four DACs operating in parallel. At the output the signal
is summed up and due to the overlapping of different samples
at low sampling rate in one sampling instance at high sampling
rate, an "averaging" (without normalization by the number of
samples) effect is achieved and a similar signal is generated at
the output. This scheme is depicted for a two-channel TIDAC
in Fig. 1. The individual signals from the DACs are phaseshifted
and the summed output is shown in the lowest picture.
The approach discussed in this paper uses a similar structure
with time-interleaved DACs, but uses a precoding scheme that
allows to generate exactly the same signal on an M -times

[7] R. Adams and K. Nguyen, "A 113-dB SNR Oversampling DAC with
Segmented Noise-Shaped Scrambling," IEEE 1. Solid-State Circuits,
vol. 33, no. 12, pp. 1871-1878, 1998.
[8] A. G. Venes, K. L. Miller, and P. Vorenkamp, "Method and System for
Time Interleaved Digital to Analog Conversion for a Cable Modem,"
u.S. Patent US 2006/0098823 AI, May, 2006.
[9] A. V. Oppenheim and R. W. Schafer, Discrete-Time Signal Processing,
2nd ed. Prentice Hall Inc., 1998.
371
[10] Xilinx, "Virtex-4 ML403 Embedded Platform," Xilinx Inc., Tech. Rep.,
May 2006. [Online]. Available: hUp:llwww.xilinx.comlml403
[11] "Stanford Research Systems CG635 Synthesized
Clock Generator," 2006. [Online]. Available:
hUp:llwww.thinksrs.comlproducts/CG635.htm
[12] ECMA, ECMA-368: High Rate Ultra Widebarul PHY and MAC
Standard. Geneva, Switzerland: ECMA (European Association for
Standardizing Information and Communication Systems), Jun. 2005.
[Online]. Available: hUp:llwww.ecma.chlecmal/STAND/ecma-368.htm