A low power transceiver architecture for the 2.4 GHz ISM band using a 1.0 V supply is presented. It employs a transformer to convert the 100 Ω antenna impedance to almost 1 kΩ and so facilitates a low power transmitter and receiver. The simulated post-layout output power of the differential class-E power amplifier is 2.0 dBm with a drain efficiency of 28.4%. The direct-conversion receiver achieves a very low power consumption of 420 μW and a noise figure of 15.0 dB.Ministerio de Ciencia e Innovación TEC2009-08447Junta de Andalucía TIC-0281

Delgado Restituto, Manuel

Masuch, Jens

Rodríguez Vázquez, Ángel Benito

English

idUS. Depósito de Investigación Universidad de Sevilla

2010 Asia Pacific Conference on Circuits and Systems (APCCAS 2010)6 - 9 December 2010, Kuala Lumpur, MalaysiaTransformer based front-end for a low power2.4 GHz transceiverJens Masuch, Manuel Delgado-Restituto and A´ngel Rodrı´guez-Va´zquezInstitute of Microelectronics of Seville (IMSE-CNM-CSIC) - University of SevilleEmail: {masuch,mandel,angel}@imse-cnm.csic.esAbstract—A low power transceiver architecture for the2.4 GHz ISM band using a 1.0 V supply is presented. It employs atransformer to convert the 100 Ω antenna impedance to almost1 kΩ and so facilitates a low power transmitter and receiver.The simulated post-layout output power of the differential class-Epower amplifier is 2.0 dBm with a drain efficiency of 28.4%. Thedirect-conversion receiver achieves a very low power consumptionof 420 µW and a noise figure of 15.0 dB.I. INTRODUCTIONBody area networks (BAN) using miniature sensors areexpected to allow for a lot of new applications of wirelesscommunication. They range from entertainment and automa-tion to health care or human interface devices. In many of theseapplications, the need for ultra-low power consumption isprioritary since network nodes are supplied by small batteriesor even rely on autonoumus energy scavenging techniques.Facilitating this trend, the Bluetooth SIG has recently pub-lished a low energy extension that allows short range commu-nication with very low power consumption [1]. Bluetooth lowenergy also operates in the 2.4 GHz ISM band but with fewerchannels at an increased spacing of 2 MHz. The modulationtype is frequency shift keying (FSK) with a data rate of1 Mbps and a modulation index of 0.5±10%. The standarddemands for receiver sensitivity of -70 dBm or better. Thistranslates into a tolerable noise figure (NF) of 24 dB assuminga bandwidth of 1 MHz and a signal-to-noise ratio (SNR) of20 dB.This paper presents a transceiver architecture intended forthe new standard. In section II the transceiver architecture ispresented while section III describes the circuit level imple-mentation of the blocks. Finally, simulation results are givenin section IV.II. ARCHITECTUREIn order to increase the battery life time of the sensor nodethe power consumption of the transceiver must be reducedas much as possible. As a consequence the direct conversionarchitecture is often used for this kind of short-range wirelessapplications [2]–[4]. There is just one step of frequencyconversion and so there are few blocks working at the radiofrequency.The block diagram of the proposed low power transceiveris shown in Fig. 1. The low requirements on the noiseperformance imposed by the Bluetooth low energy standardallow to use a passive receiving path. Signal amplification isPassive mixer PA 0°90°BBoutIBBoutQTx-dataLOQLOIQVCOFig. 1. Block diagram of the proposed transceivershifted completely into the baseband where it can be realizedwith less power consumption. The noise figure of this approachis defined by the losses in the passive down-conversion andthe noise performance of the baseband amplifier.A quadrature voltage controlled oscillator (QVCO) gener-ates the I and Q phases of the carrier frequency and directlydrives the switches of the passive mixer. In this setup thecapacitive load of the mixer is tuned out by the inductor ofthe LC-QVCO [4], [5].The transmitting path is also reduced to the minimum num-ber of blocks. The FSK modulation defined by the Bluetoothlow energy standard allows direct modulation of the VCO.The required frequency deviation of ±225 . . . 275 kHz canbe realized using small-sized PMOS transistors as varactors[6]. Direct modulation of the QVCO makes an up-conversionmixer unnecessary and the constant envelope transmit signalallows an efficient nonlinear power amplifier.Switching type class-E power amplifiers are known fortheir very good efficiency. Losses are minimized because thecurrent peaks in the switching transistor occur when the drain-source voltage is almost zero. Moreover, class-E configurationachieves high efficiency because the drain voltage can ideallyrise up to 3.56 times the supply voltage [7]. However, in orderto operate the power amplifier in class-E mode for low outputpower levels of about 0 dBm the load resistance seen by thePA must be approximately 1 kΩ. Therefore, a transformer isused to boost the antenna impedance. The transformer alsoprovides single-ended to differential conversion if a single-ended antenna is used. Differential PAs generate smallercurrent peaks on the supply line than single-ended PAs, andsuch peaks appear at twice the operating frequency. Thisfacilitates the decoupling of the PA from the remainder of978-1-4244-7456-1/10/$26.00 ©2010 IEEE 2882010 Asia Pacific Conference on Circuits and Systems (APCCAS 2010)6 - 9 December 2010, Kuala Lumpur, MalaysiaTABLE ITRANSFORMER PARAMTERSParameter PA/mixer coil antenna coilNumber of turns 6 2Windings in parallel 1 2Serial inductance 11.6 nH 1.56 nHSerial resistance 18.0 Ω 3.2 ΩCoupling factor 0.85Area 0.116 mm2 (340µm x 340µm)the circuit.The impedance conversion of the transformer is also benefi-cial for the receiver design because it provides passive voltagegain and the switches of the passive mixer can have a higheron-resistance.III. IMPLEMENTATIONThe transceiver front-end has been designed in a 130 nmRF CMOS process using 6 metal layers (1 thick top metal)and MIM capacitors. The supply voltage is set to 1.0 V.A. TransformerImpedance transformation is achieved using a step-up trans-former with a 2-turn coil at the antenna port and a 6-turn coilat the internal port In order to minimize the insertion lossthe coupling factor k between the coils has to be maximized[8]. Therefore, the coil connected to the antenna consists oftwo parallel windings of two turns each. This also reduces theserial resistance in this coil. On the internally connected coil ahigh inductance is needed to support the high impedance levelon this port. The transformer is drawn as a planar structureusing the thick top metal layer 6 and the thin metal layer 5 ofthe process.Characterization of the electrical behavior has been withthe 2.5-D electromagnetic solver Momentum. The resulting s-parameter set was then fitted to a direct form model of thetransformer [8] with the most important parameters presentedin Table I.B. QVCO with direct modulationThe QVCO is based on a previous work [9] using parallelcoupling. Thanks to the high differential inductance of 10.2 nHin each tank it achieves an amplitude of 0.2 V at the fouroutput I/Q phases with a very low current consumption ofapproximately 350 µA.During transmission mode only the in-phase component ofthe carrier is needed and therefore one core of the QVCO isdeactivated. In this mode the QVCO is also used to directlygenerate the FSK modulation of the carrier. The high tankinductance leads to a differential capacitance of only 170 aFthat has to be switched. Such a small switchable capacitancecan be implemented using almost minimum sized PMOStransistors as varactors with digital control voltage [6].In order to obtain a positive and negative frequency devia-tion from the carrier simply two differential varactors are usedvminusmod[0]mod[1]vplusFig. 2. PMOS varactor pair used for direct modulation of the VCORFINLOITx-enableantennaoff-chipon-chipRx-enable4 parallel PA stagesTx/Rx switchM1 M2M3 M4M5M6C1CCCCESD ESDFig. 3. Tx/Rx switch with class-E power amplifierin parallel (Fig 2). Setting both modulation inputs (mod[0:1])to VDD leads to the higher differential capacitance and henceto a negative frequency deviation. On the other hand, settingboth inputs to ground results in a positive frequency shift. Theintermediate state with one input at ground and one at VDD isneeded to tune the VCO to the center frequency within a phaselocked loop (PLL). During transmission the PLL is opened toallow direct modulation of the VCO.The accuracy of the frequency shift heavily depends on theprocess variation, i.e. the offset frequency varies as much as±25%. Therefore, an array of 5 differential varactor pairs hasto be added to the LC tank in order to select the pair with thedesired frequency shift by calibration. Within the industrialtemperature range of -40°C. . . 85°C the variation fortunatelyis much smaller (≈ ±2%) so that no further calibration versustemperature is needed.C. Power amplifierThe proposed differential power amplifier is shown in Fig. 3together with the Tx/Rx-switch. The thin oxide output tran-sistor M1 and M2 are driven by inverters and thus operate asswitches. If all parallel stages are actived the power amplifierworks in class-E mode and achieves its peak efficiency andhighest output power level. The concept of 4 stages in parallelhas the advantage that for a reduced output power setting alsothe power consumption in the drivers is reduced.The Tx/Rx switch uses thick oxide transistors in order towithstand the class-E voltage waveforms of up to 2.4 V at2892010 Asia Pacific Conference on Circuits and Systems (APCCAS 2010)6 - 9 December 2010, Kuala Lumpur, MalaysiaLOI/QRFINmixeroutputCfbRfbCfbRfb-++-CLPBBoutI/QFig. 4. Passive current-driven mixer with baseband transimpedance amplifiertheir drains. Also in case of an ESD event the voltage at thispoints can rise up to three times higher than at the antennainput due to the impedance conversion of the transformer.D. ReceiverIn a direct-conversion receiver the low frequency 1/f noise isof special concern because the down-converted signal appearsaround DC. Since the noise density is proportional to the DCcurrent a passive mixer with zero bias current achieves verygood noise performance and a low 1/f corner frequency [10].Therefore, a passive current driven mixer as shown in Fig. 4is used in the receiver.The noise performance of the passive mixer itself is ba-sically defined by its conversion gain. Due to its passivebehavior the conversion gain is always negative and directlyincreases the noise figure of the receiver. The conversion gainis improved if the transistors work as switches with a low on-resistance. This can be achieved by using wide transistors withlarge over drive voltage. However, larger overdrive voltageleads to an increased power consumption in the driving QVCOor even requires a buffering stage. Low on-resistance can alsobe achieved using wide transistors leading to an increasedparasitic coupling capacitance from the mixer input to theQVCO. This would make the QVCO more susceptible tofrequency pulling at high RF input levels. Therefore, in orderto limit power consumption and VCO pulling the implementedpassive mixer is driven by a low amplitude of 0.2 V and usesrelatively small transistors.The output of the passive mixer is loaded by the basebandtransimpedance amplifier and by a MIM-capacitor (CLP =20pF ) to remove the RF components. The amplifier presentsa low impedance load (≈ 500kΩ) to the mixer and convertsthe input current to an output voltage with a gain of Rfb =15kΩ. Parallel feedback capacitors Cfb form together withthe freedback resistors Rfb the channel filter with a low passcorner frequency of 650 kHz.The operational amplifier contains a differential PMOS pairfor amplification followed by a buffering stage to drive the loadwhich is mainly formed by the feedback network. Each stageworks with a bias current of approximately 100 µA resulting ina total current consumption of 210 µA per baseband amplifierincluding biasing and common mode feedback. The commonmode voltage at the input and output of the baseband amplifierFig. 5. Layout of the transceiver frontend test chip (die size 1.4 x 1.1 mm)TABLE IISIMULATED POST-LAYOUT PERFORMANCE OF THE POWER AMPLIFIERParameter ValueSupply voltage 1.0 Vnumber of active stages 1 2 4DC supply current incl. driver 2.5 mA 4.3 mA 6.2 mAPA drain efficiency 16.0% 24.8% 28.4%Output power (fundamental tone) -4.2 dBm -0.1 dBm 2.0 dBmis set to 0.2 V in order to allow sufficient overdrive voltagein the Rx-switches (M5, M6 in Fig. 3).IV. SIMULATION RESULTSThe complete frontend was simulated with a differential100 Ω antenna including parasitics extracted from the layout(Fig. 5) using the SpectreRF simulator. ESD structures and aQFN36 package model have also been included in the simula-tion to account for losses in the antenna IOs. At the basebandoutputs the amplifiers are loaded by 100 fF capacitors toaccount for the load imposed by subsequent stages.The simulated power amplifier performance is summarizedin Table II. It shows a peak drain efficiency at the highest out-put level setting of 28.4%. Including the power consumptionof the QVCO with all its biasing current steered to one of thetwo cores, the overall efficiency of the transmitter is expectedto be 24.1%.The spectrum of the differential baseband output voltageis shown in Fig. 6 at an input level of -70 dBm. Thisinput level coincides with the required sensitivity stated in100 1k 10k 100k 1M 10M 100M−110−100−90−80−70−60−50offset from LO frequency [Hz]differential voltage at BBoutI/Q [dBV]Fig. 6. Differential baseband output voltage at an input power of -70 dBm2902010 Asia Pacific Conference on Circuits and Systems (APCCAS 2010)6 - 9 December 2010, Kuala Lumpur, Malaysia100 1k 10k 100k 1M 10M 100M10152025303540offset from LO frequency [Hz]double−sideband Noise Figure [dB]Fig. 7. Noise Performance of the receiverthe Bluetooth low energy standard. It leads to a signal-to-noise ratio (SNR) of 26.2 dB in each baseband channel andtherefore leaves plenty of margin for the FSK demodulator.The overall noise figure is plotted in Fig. 7. Below the 1/fcorner frequency of 20 kHz the baseband amplifier dominatesthe noise performance while at higher frequencies both thepassive mixer and the baseband amplifier equally contributeto the total noise.Fig 8 shows the simulation of the receiver linearity athigh input levels excluding the baseband amplifier. The inputreferred 1-dB compression point and 3rd order intercept pointare -15.3 dBm and -5.7 dBm respectively. Therefore, the RFsignal has to be attenuated to reach the Bluetooth specificationof -10 dBm as the maximum input signal power. Such aattenuation can be simply implemented by a transistor by-passing a portion of the current at the input of the mixer. Thereceiver performance is summarized in Table III.−40 −35 −30 −25 −20 −15 −10 −5 0−55−50−45−40−35−30−25−20input power at antenna port [dBm]mixer output power [dBm]  single−tone 1st order output1dB/dB tangent1dB compression point(a)−40 −35 −30 −25 −20 −15 −10 −5 0−120−100−80−60−40−200input power at antenna port [dBm]mixer output power [dBm]  two−tone 1st order outputtwo−tone 3rd order output1dB/dB tangent3dB/dB tangent3rd order intercept point(b)Fig. 8. Linearity of the receiver (without baseband amplifier): (a) 1dBcompression point, (b) 3rd order intercept pointTABLE IIISIMULATED POST-LAYOUT PERFORMANCE OF THERECEIVERParameter ValueSupply voltage 1.0 VRX current consumption 420 µAPassive conversion gain -13.9 dBBaseband voltage gain 32.6 dBintegrated Noise Figure 15.0 dB(10. . . 500kHz)1dB compression point (input referred)* -15.3 dBm3rd order intercept point (input referred)* -5.7 dBm* without baseband amplifierV. CONCLUSIONSA low-power transceiver architecture has been presentedthat is capable of conforming the Bluetooth low energy spec-ification. The antenna impedance is up-converted by a trans-former which allows the design of an efficient class-E poweramplifier for low output power levels and a supply of 1.0 V.In the receiver the power consumpton has been minimized asmuch as possible while sacrifycing noise performance. Stillthe performance is sufficient for wireless sensor networks orbody area networks.ACKNOWLEDGEMENTSThis work has been supported by the Spanish Ministryof Science & Innovation under grant TEC2009-08447, theJunta de Andalucı´a under grant TIC-02818 and the 2007-2013FEDER Programme.REFERENCES[1] Bluetooth Specification, Volume 6 (Low Energy Controller), BluetoothSIG Std., Rev. 4.0, Dec 2009.[2] S. Hae-Moon et al., “A Low Power Fully CMOS Integrated RFTransceiver IC for Wireless Sensor Networks,” Very Large Scale In-tegration (VLSI) Systems, IEEE Transactions on, vol. 15, no. 2, pp.227–231, 2007.[3] A. C. W. Wong et al., “A 1 V Wireless Transceiver for an Ultra-Low-Power SoC for Biotelemetry Applications,” Solid-State Circuits, IEEEJournal of, vol. 43, no. 7, pp. 1511–1521, 2008.[4] C. Bernier et al., “An ultra low power 130nm CMOS direct conversiontransceiver for IEEE802.15.4,” in Radio Frequency Integrated CircuitsSymposium, 2008. RFIC 2008. IEEE, 2008, pp. 273–276.[5] B. W. Cook, A. Berny, A. Molnar, S. Lanzisera, and K. S. J. Pister,“Low-Power 2.4-GHz Transceiver With Passive RX Front-End and 400-mV Supply,” Solid-State Circuits, IEEE Journal of, vol. 41, no. 12, pp.2757–2766, 2006.[6] R. B. Staszewski, H. Chih-Ming, D. Leipold, and P. T. Balsara, “A firstmultigigahertz digitally controlled oscillator for wireless applications,”Microwave Theory and Techniques, IEEE Transactions on, vol. 51,no. 11, pp. 2154–2164, 2003.[7] P. Reynaert and M. Steyaert, RF Power Amplifiers for Mobile Commu-nications. Dordrecht, The Netherlands: Springer, 2006.[8] J. R. Long, “Monolithic transformers for silicon RF IC design,” Solid-State Circuits, IEEE Journal of, vol. 35, no. 9, pp. 1368–1382, 2000.[9] J. Masuch and M. Delgado-Restituto, “Low power 2.4 GHz quadraturegeneration for Body Area Network applications,” in Circuits and Sys-tems, 2010. ISCAS 2010. IEEE International Symposium on, 2010, pp.493–496.[10] E. Sacchi et al., “A 15 mW, 70 kHz 1/f corner direct conversion CMOSreceiver,” in Custom Integrated Circuits Conference, 2003. Proceedingsof the IEEE 2003, 2003, pp. 459–462.291

Transformer based front-end for a low power 2.4 GHz transceiver

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Transformer based front-end for a low power 2.4 GHz transceiver

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