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An Ultra-Wideband CMOS Receiver Front-End
Abstract:
In this paperwe will present a CMOS receiver front-end for ultra-wideband (UWB) wireless communications based on multi-band OFDM Alliance (MBOA) / WiMedia standard proposal. By employing a direct conversion architecture, the RF front-end chip integrates a single-ended wideband low-noise amplifier (LNA) and two double-balanced downconversion mixers. The conversion of single ended LNA output to differential is embedded in the RF transconductance circuit of the mixer to reduce power consumptioin. The RFIC prototype fabricated in a 0.13µ CMOS technology and housed in a low-cost QFN package, delivers a conversion gain of 19.5-23.3 dB and a noise figure of 5.2-9.1 dB over the entire 3.1-10.6 GHz UWB band, and achieves an input IP3 of -10.4 dBm and an input IP2 of 32 dBm. The RF front-end occupies only 0.9 mm2 active chip area and consumes 28 mA from a 1.5 V supply.
Key words:
RF integrated circuit, receiver front-end, ultra-wideband (UWB), CMOS, direct-conversion, wideband low-noise amplifier, mixer.
I. INTRODUCTION
Ultra-wideband (UWB) is a radio technology that wasoriginalybased for military impulse radar applications bymeans of short “carrier-free” pulses. This thchnology has recently beendefined by the FCC as any signal occupying more than 500-MHz frequency band [1] for communication purpose. UWB technology has the promising ability to providehigh data rate at relatively low cost and low power consumption whencompared to the conventional narrow band wireless communications [2]. It is the foundation for replacingalmost every cable at home or in an office with a short-rangewireless connection that features hundreds of megabits of dataper second.
An exceptionally large spectrum of3.1-10.6 GHz for unlicensed use of UWB deviceshas been allocated by the FCC at averagepower spectral density limited to -41.3 dBm/MHz. Oneproposal for the UWB standard, that has received strongsupport from several academic and commercial organizations,is the multiband orthogonal frequency division multiplexing(MB-OFDM) scheme [3]. In this standard, the 7500-MHzavailable UWB spectrum is divided into fourteen 528 MHZsub-bands that are further organized into five band groups.Only the first group of three bands, corresponding to the lowerpart of the spectrum (3.1-4.8 GHz) and defined as “Mode 1”, iscurrently considered as mandatory for operation, while all theother band groups are left as optional for future expansion ofthe system capabilities.
The design of a receiver front-end is a challenge for the implementation of integrated UWBsystem that operates acrossa wide band with a sufficiently high gain and low noise figure(NF) while consuming moderate power. Further, due to thepossibility of strong interferers from devices such as WLANand Bluetooth that are operated in or close to the UWB band, high linearity is required for the UWB receivers. For UWB design the CMOS technologyis preferred to increase integrationlevel and, thus, to reduce cost. It is however quite challengingfor a CMOS implementation to cover a very broad frequencyband. recent efforts for the implementation of UWB devicesin CMOS thus focus on the mandatory mode [4][5].
Here we try to introduce an integrated and packaged receiverfront-end chip for MB-OFDM UWB wireless applications. This work is aimed at the entire 3.1-10.6 GHz UWB band toexploit the full potential of UWB technology while designed and implemented in a 0.13μm CMOS technology. In section IIthe proposed receiver front-end architecture and thesystem-level designconsiderations is described. Section III presents thedesign of the RF front-end building blocks including a 3-10GHz wideband low-noise amplifier (LNA) and a quadraturedown-conversion mixer with embedded single-to-differentialconversion. The measurement results from the RFIC prototypeare reported in section IV. Finally, inSection Vconclusions are drawn.
II. RECEIVER FRONT-END ARCHITECTURE
It is expectedWith a 528-MHz channel bandwidth thatflicker noise only affect the signal negligibly. Therefore, theproposed receiver front-end naturally uses a direct conversionarchitecture, which is suitable for a high level of integration toachieve a low-cost design. In Fig. 1,a block diagram of the receiverfront-end is shown, which consists of a widebandLNA and two mixers for quadrature down-conversion of theRF signal to zero-IF. The local oscillator (LO) signal isprovided externally by a frequency synthesizer chip withquadrature outputs.
As to avoid the needfor an off-chip wideband balun at the RF input,the LNA has a single-ended configuration which addssignal loss and cost. Adouble-balanced topology is chosen for the down-conversion mixers, to reject LO tone in theoutput spectrum and to reduce second-order inter-modulationproducts falling in the signal band. It is clear that the single-endedLNA output must be converted to differential as to enable theuse of a double-balanced mixer. This function is merged into the mixer circuit instead of designing a standalonebalun circuit to perform the needed single-to-differentialconversion, suchthat it is done at no extra cost of power consumption.
The LNA output is coupled to the transconductance input ofthe I and Q down-conversion mixers through a buffer, whichalso forms a high-pass filter by the use of a spiral inductor in asource follower topology. This simple filter provides improvedout-of-band interference rejection at minimal degradation tothe overall system noise figure. Furthermore, low-frequency distortioncomponents are filtered out before down-conversion which are generated by the second-order nonlinearitiesin the LNA. Otherwise,they may partly leak through the mixer to the output.
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III. CIRCUIT DESIGN
A. Low-Noise Amplifier
The LNA as a key component in the receiver front-end, needsto feature matched input impedance, flat wideband gain, goodlinearity, and minimum possible noise figure over the entireUWB band, yet at relatively low power consumption. In Fig. 2a simplifiedschematic of the designed UWB LNA is shown. The LNA uses inductive degeneration in a common-sourcetopology to generate an equivalent 50- resistance needed tomatch the source impedance, i.e. a conventional narrowbanddesign, while exploiting the impedance property of a
