UWB CMOS Transmitters for UWB Communications

Rui Xu1 and Y. Jin2

1Qualcomm Inc. San Diego, California 92014 USA

2Skyworks Corporation Irvine, California 92617 U.S.A.

Meng Miao3 and Cam Nguyen4 3Intel Corporation

Chandler, Arizona 85226 U.S.A 4Department of Electrical and Computer Engineering

Texas A&M University College Station, Texas 77843-3128 USA

Abstract—Development of UWB CMOS transmitters using carrier and impulse techniques is presented. The carrier transmitter designed using a 0.18-μm CMOS process adopts a double-stage switching to enhance RF-power efficiency and reduce dc-power consumption and circuit complexity. Measurement results show that the generated UWB signal has variable 10-dB signal bandwidths from 0.5 to 4 GHz and tunable central frequency covering the entire UWB frequency range of 3.1 to 10.6 GHz. The impulse transmitter designed using a 0.25-μm CMOS process can generate and transmit both monocycle pulses from 140 to 350 ps and impulses from 100 to 300 ps.

Keywords-UWB transmitters; UWB communications

I. INTRODUCTION Ultra-wideband (UWB) technology has received significant

interests for unlicensed uses of UWB devices within the 3.1–10.6 GHz frequency band. UWB techniques are promising technology for short-range, high-date-rate communications.

Carrier UWB signals have been widely used in various communication applications [1]. They hold the advantage of more convenient spectrum management and less distortion through antennas [2]. In typical UWB transmitters, the generated carrier-based UWB voltage signal needs to be sent to a wideband power amplifier (PA) to achieve the required power level. This approach suffers from two major disadvantages: the design challenges of a UWB PA and low power efficiency in low pulse repetition frequency (PRF) situation.

Impulse UWB technique is attractive for high-data-rate, short-range communication systems. The pulse generator and antenna are two key components in both the transmitter and receiver in UWB impulse systems. Monocycle pulse has band-limited characteristics without DC component, facilitating its transmission through practical antenna, and is normally preferred. Meanwhile, pulse with tunable duration has both advantages of increased range and fine range resolution and is attractive for UWB systems [3]. The transmitted and received signals of UWB systems require antennas not only radiating energy efficiently but also having linear phase response.

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In this paper, we present carrier and impulse UWB transmitters designed using CMOS RFIC process. The CMOS carrier transmitter module covers the entire 3.1-10.6 GHz UWB band with variable bandwidth of 500 MHz to 4 GHz. It overcomes the problems of using a UWB PA and low-power efficiency mentioned earlier. The CMOS impulse transmitter subsystem is capable of radiating both tunable monocycle pulse (140-350 ps) and impulses (100-300 ps).

II. CARRIER UWB TRANSMITTER Figure 1(a) shows the block diagram of the carrier UWB

transmitter, consisting of a voltage-control oscillator (VCO), a buffer, a SPST switch, and two pulse generators. A double-stage switching scheme, using two pulse generators of wide and narrow pulses and two switches, is adopted in the proposed transmitter to remedy the switching speed limitation of the buffer. The CMOS pulse generator-SPST switch designed using a TSMC 0.18-μm CMOS process [4] was used in conjunction with an external frequency synthesizer to demonstrate the new carrier-based UWB transmitter. The CMOS chip’s microphotograph is displayed in Fig. 1(b). The die area of the entire circuit is 850 μm by 700 μm, including the input and output on-wafer pads. Under a 1.8-V supply voltage, the whole module consumes less than 1-mA DC current.

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Fig. 1 Block diagram (a) and microphotograph of the carrier UWB transmitter

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Fig. 2 Measured time-domain UWB signal with 0.5-ns duration (a) and spectrums of UWB signals covering the 3.1-10.6 GHz UWB band (b)

Figure 2(a) displays the time-domain waveform of a UWB

signal with 2-ns duration and 5-GHz center frequency. The signal amplitude is 2-Vp-p (peak-to- peak). By increasing the external biasing voltage for the pulse generator, the pulse width of the UWB signal can be reduced. When the pulse width is reduced to a certain value, the amplitude of the UWB signal, however, diminishes due to the fact that the SPST switch cannot be completely turned on any more. When the pulse duration is reduced to 0.5 ns, the according signal amplitude drops to 1Vp-p. Fig. 2(b) shows the measured spectrums of different UWB signals, obtained by varying the carrier frequency, demonstrating that the entire UWB band of 3.1-10.6

GHz can be achieved with the developed CMOS chip by using a multi-band signal source. To evaluate effects of coupling from the impulse signal applied to the SPST’s control terminals, we removed the LO input signal and directly observed the coupled signal at the output. The measured spectrum of the coupled signal shows it is mainly distributed below 1 GHz, signifying that the signal is out of the UWB band and can be easily removed using a filter.

III. IMPULSE UWB TRANSMITTER

NOR

Impulseforming

Tunabledelay cell

Referencecell

Square wavegeneration

A

B

Tuningdelay

Pulseshaping

C D

Output

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Fig. 3 Schematic (a) and photograph (b) of the CMOS UWB tunable monocycle pulse generator

The impulse UWB transmitter consists of a tunable monocycle-pulse generator and a uniplanar UWB antenna. Figure 3 shows the CMOS tunable monocycle pulse generator chip fabricated using a TSMC 0.25-μm CMOS process [5]. It integrates a tuning delay circuit, a square-wave generator, an impulse-forming circuit, and a pulse-shaping circuit in a single chip. The measured tunable pulse signals are shown in Fig. 4. Symmetric monocycle pulses with 0.3–0.6 V and 140–350 ps duration were achieved. It is noted that the tunable narrow impulse generated at node C consists of three parts: rising edge, tuning delay, and falling edge. For pulses with very narrow width, only part of the rising and falling edges is involved, resulting in impulse with much smaller amplitudes. Consequently, a tunable monocycle pulse is achieved at node D.

Figure 5 shows the CMOS tunable monocycle pulse generator chip mounted directly onto the edge of the UWB antenna without a feed line. The area occupied by the antenna aperture is only 1.2 in × 1.4 in. A quasi-microstrip antenna operating from 0.2 to more than 20 GHz is used as the receiving antenna for pulse transmission measurement of the UWB transmit module. Figure 6 shows the signals received

from the tunable impulse and monocycle pulse signals, shown in Fig. 4, transmitted by the UWB transmit module. The pulse-duration tunability is clearly visible in the received pulses. All the received signals have shape similar to the first derivative of the transmitted pulses, as expected from the designed antenna. Both the measured impulse and monocycle-pulse transmission results clearly demonstrate the workability of the developed CMOS-based tunable UWB transmit module.

IV. CONCLUSION Carrier and impulse CMOS UWB transmitters for UWB

communication systems have been presented. The implemented carrier CMOS transmitter module confirms the workability of the new transmitter as another simplified approach for carrier-based UWB signal generation. The developed impulse CMOS transmitter-antenna module with tunable pulse duration demonstrates its possibility for use in various UWB applications.

ACKNOWLEDGEMENT

This work was supported in part by the National Science Foundation and in part by the US Army - Corps of Engineers.

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Fig. 4 Impulse (a) and mono-cycle pulse (b) signals with tunable pulse duration

Fig. 5 Photograph of the UWB transmitter-antenna module

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Fig. 6 Received signals of the impulses (a) and monocycle pulses (b) transmitted by the UWB transmit module.

REFERENCES [1] Robert J. Fontana, “Recent system applications of short-pulse ultra-

Wideband (UWB) technology,” IEEE Trans. Microwave Theory and Tech., Vol. 52, no.9, pp. 2087-2104, Sept.2004.

[2] R.J. Fontana and J.F. Larrick, “Waveform adaptive ultra-wideband transmitter,” U.S. Patent 6026 125, Feb.15, 2000. [3] J. W. Han and C. Nguyen, “On the Development of a Compact Sub- Nanosecond Tunable Monocycle Pulse Transmitter for UWB Applications,” IEEE Trans. on Microwave Theory and Techniques, Vol. 54, No. 1, pp. 285-293, January 2006. [4] TSMC 0.18-μm CMOS Process, MOSIS Foundry, Marina del Rey, California, USA. [5] TSMC 0.25-μm CMOS Process, MOSIS Foundry, Marina del Rey, California, USA.

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