RF Transmitter With Cartesian Feedback

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UNIVERSITY OF MICHIGAN EECS 522 FINAL PROJECT: RF TRANSMITTER WITH CARTESIAN FEEDBACK1RF transmitter with Cartesian feedbackAlexandra Holbel, Fu-Pang Hsu, and Chunyang Zhai, University of Michigan Abstract—This paper presents a 2.4 GHz transmitter withCartesian Feedback for use in Bluetooth applications. TheCartesian feedback is used to improve the linearity of the system,allowing the power amplifier to be designed for efficiency. TheCartesian feedback loop consists of two independent I and Qfeedback loops that are coupled only through their phase offsetwhich can be compensated.The upconversion anddownconversion mixers use a doubly balanced Gilbert celltopology with strong linearity requirements for the downconverting mixers. The transmitter is realized by the 0.13umIBM CMOS process.Index Terms— Cartesian feedback, Class A power amplifier,RF transmitterI. INTRODUCTIONTare ubiquitous as our lives becomeincreasingly data oriented. Devices are constantly incommunication with other technology and the ability totransfer wirelessly is important. This paper presents a RFtransmitter for the 2.4 GHz band to be applied to Bluetoothtechnology. As power increases, distortion out of the poweramplifier increases, so we discuss Cartesian feedback as aneffective method for linearizing the transmitter.Cartesian feedback was chosen as the method of feedbackbecause of its simplicity and robustness to a poor poweramplifier model. It is not necessary to characterize the poweramplifier completely for a Cartesian feedback system. Otherpossible feedback methodologies include predistortion, powerbackoff, and envelope elimination and restoration [1]. Theidea behind predistortion is to code in predistorted signals thatare chosen based on the input signal. The disadvantage to thismethod is that it increases complexity and requires a verygood power amplifier model, which is difficult to derive.RANSMITTERSIPower backoff is a simple approach, but reduces powerefficiency. Finally, envelope elimination and restorationinvolves envelope matching in the power amplifier. In thiscase, the phase matching is critical. Additionally, this is adifficult method to implement with power efficiency.Therefore, Cartesian feedback provides an excellent option forimplementing the feedback system.II. THEORY OF OPERATIONOn a basic level, a transmitter consists of an upconversionmixer and a power amplifier attached to an antenna. Tolinearize the power amplifier, feedback can be wrappedaround it. However, at the high frequency output of the poweramplifier, it is difficult to stabilize a feedback loop.Therefore, we can take advantage of the basebandcharacteristic of the inputs and downconvert the output back tothe baseband frequency for the feedback. Figure 1 shows theblock diagram of the system.A Cartesian feedback system is chosen as opposed to apolar feedback system to take advantage of the decoupling ofamplitude and phase. In a polar feedback loop, the phase andamplitude are closely linked which makes the designcomplicated. For a Cartesian feedback system, the quadraturenature of the systems is taken advantage of and the phase andamplitude are decoupled. [1] Instead, two independent butidentical I and Q loops are designed. In reality, the I and Qloops are not entirely decoupled. Phase misalignment will stillcouple the signal between the two channels if not properlycorrected. The phase alignment system is outside the scope ofthis paper.There are many papers discussing theimplementation of phase alignment systems so for the purposeof this paper it is assumed that it can be implemented and thephase misalignment is measured and compensated . 1 Block Diagramcos(ωt)

UNIVERSITY OF MICHIGAN EECS 522 FINAL PROJECT: RF TRANSMITTER WITH CARTESIAN FEEDBACKIII. DESIGN IMPLEMENTATIONA. MixersThe mixer is an essential component in an RF system and akey building block for modern wireless communicationintegrated circuits. In the transmitter, the mixer is used toupconvert the baseband signal to RF for transmission andanother mixer is used to downconvert the RF signal forbaseband feedback. The same mixer is used for both casesand the linearity requirement for the downconversion mixerdrives the design [2].The RF mixer is a three port, nonlinear device. Through thenonlinearity of the MOSFET, two input signals with differentfrequencies can be multiplied to generate a sum and differenceterm in frequency. While a passive mixer has better linearityand no power consumption, it suffers from conversion lossand high noise figure. While the linearity is important, thatcould be compensated for by attenuating the output of thepower amplifier to a level that is within the linearity of thedownconversion mixer. Therefore, a doubly balanced Gilbertcell topology (Figure 2) was chosen. The doubly balancedtopology was chosen for its port to port isolation, higherlinearity, and improved suppression of even order spuriousproducts [3].LO RF LO LO- IF-2As expected from mixer design, the output consists of a sumand a difference term in frequency:.Either a lowpass filter or bandpass filter can be used on theoutput of the downconversion mixer to acquire the desiredsignal of the two. On the upconversion signal, the poweramplifier passes only the RF signal and no filtering isrequired. The RF signal is at 2.4GHz, the LO frequency is at2.41GHz and the baseband IF frequency is at 10MHz.B. Power AmplifierThe power amplifier is the other main building block of a RFtransmitter. The power amplifier consumes a large share ofthe power budget in most transmitters but the linearity of thepower amplifier also determines the output capacity. A linearpower amplifier provides a clean output spectrum whichrepresents the ability to transmit data at the highest possiblerate for a given channel bandwidth [6]. In this work, thepower amplifier operates as close to saturation as possible tomaximize the power efficiency while trading off linearity. Alinearization technique is presented in the next section toachieve a moderately linear power amplifier in this nearsaturated region.A two stage Class A power amplifier with pseudodifferential architecture is chosen (Figure 3). The pseudodifferential architecture helps minimize the coupling of RFenergy into sensitive nodes and the power and ground lineswhich helps alleviate concerns of crosstalk and coupling whenthe power amplifier is on the same die as the linearizationsystem [1].RF--out Fig. 2 Mixer SchematicThe switching quad stage is driven by differential LO signalsand a transconductance stage that amplify the differential RFsignals by g m. A biasing stage provides the bias points for thetransistors. A buffering stage on the output consists of adifferential source follower which can transform a high outputimpedance to a lower impedance in order to provide enoughcurrent for driving the next stage [4]. The source degenerationinductors of the mixing stage enhance the 1dB compressionpoint but decrease the conversion gain.In the steady state, the switching quad stage transistors areoperating in subthreshold region in order to minimize thepower consumption and only conduct when LO power isprovided [5]. They were sized smaller to minimize theparasitic capacitance that decreases conversion gain andincreases noise. However, there is also the tradeoff that sizingthem too small makes it too difficult to drive the transistor.Therefore, the sizing was optimized for those constraints. Thetransconductance stage transistors are biased in saturation inorder to provide high gm and sized accordingly.VbiasVbiasin VbiasVbiasVbiasFig. 3 Power Amplifier SchematicVbiasin-An inductor is used at the output instead of a resistor toimprove the output peak amplitude and increase the powerefficiency. Cascode transistors stabilize the amplifier as wellas add gain to the output. One tradeoff in this design isheadroom. Because the headroom is limited, tail currentsources are eliminated in this architecture and therefore thepower amplifier lacks common mode rejection. As stated in[1], it is important to decouple the ground terminals of the firstand second stage of the power amplifier on the die to reducethe possibility of common mode oscillation.

UNIVERSITY OF MICHIGAN EECS 522 FINAL PROJECT: RF TRANSMITTER WITH CARTESIAN FEEDBACKC. Loop FilterThe loop filter is designed with slow rolloff compensation[7]. The slow rolloff compensation technique approximatesthe transfer function:Compression Point and IIP3 are shown in Figure 6. Table 1lists complete simulation results.Mixer 1dB Compression and IIP330R101dB Compression Point0-10-20-30-40-40-30-20-10Input Power (dBm)010Fig. 6 Mixer P1dB and IIP3 PlotTABLE 1MIXER SUMMARYSpecificationResults1dB Compression (inputreferred)IIP3-8.02 dBmRF to IF Isolation77.63 dBLO to IF Isolation47.50 dBLO to RF Isolation95.88 dBConversion Gain4.65 dB-20Noise Figure21.73 dB-40-30Power Consumption3.64 mWFig. 4 Ladder NetworkBode Diagram8060Magnitude (dB)IIP3CC/αPhase (deg)Output Power (dBm)20which is chosen for its ability to stabilize the system. Thistransfer function gives a 10dB/decade rolloff and 45 degreesof phase which will be added to the dominate pole of thepower amplifier that follows and still have a healthy phasemargin. This transfer function is realized by placingalternating poles and zeros. Schematically, this consists of aladder network of increasing R and C blocks (Figure 4) inparallel. Each “rung” of the ladder multiplies the previous Rand C value by a chosen alpha. A bode plot of the system(Figure 5) shows the added poles and zeros which give anapproximate 10dB/decade rolloff and the phase boost whichbrings the phase up to 45 degrees.αRαC31.57 dBm40200-60-903104105106107108109101010Frequency (rad/sec)Fig. 5 H(s) Bode PlotIV. SIMULATION RESULTSEach block was tested independently and then the entiresystem was simulated both open and closed loop to determinethe performance.A. Mixer ResultsThe mixer was simulated to test linearity and proper mixing.The isolation was excellent but the power was fairly high,likely due to the buffer output stage. A plot of the 1dBB. Power Amplifier ResultsThe power amplifier was simulated to test efficiency anddistortion. A power added efficiency (PAE) of 48% wasachieved and considered acceptable for a Class A design. Aplot of the 1dB Compression Point is show in Figure 7. Table2 lists complete simulation results.C. System ResultsAfter simulating the individual blocks, the entire system wassimulated both open loop and closed loop to compare results.The I and Q input signals were identical sine waves offset by

UNIVERSITY OF MICHIGAN EECS 522 FINAL PROJECT: RF TRANSMITTER WITH CARTESIAN FEEDBACKPA 1dB Compression2.4GHz signal (Figure 9d). This suggests that instead ofsweeping an ideal circle in the IQ plane, we are insteadsweeping an elliptical shape where the amplitude changes intime (Figure 9c). The loop was then closed and resimulated.The output in Figure 9f demonstrates that closing the loopdecreased the amplitude distortion on top of the 2.4GHzsignal. Some amplitude distortion still existed, but the outputmuch more closely approximates the ideal circle sweep andonly slightly resembles and ellipse (Figure 9e).Another indication of this distortion reduction is visible inthe input signal to the power amplifier. Figure 8a shows theinput in an open loop. The signal is already slightly distortedas it has passed through the upconversion mixer. In Figure 8b,we see the input in the closed loop. In this case, the input isfurther distorted to something closer to a triangle wave, but itgives a much cleaner sine wave output. This is a predistortionto compensate for the distortion in the power amplifier.The 1dB Compression Point and max output power werealso simulated. The results are seen in Figure 10 and Table 3respectively. As expected, the gain from the closed loopsystem was decreased, but the 1dB Compression Point waspushed out further. Likewise, the max output power is slightlyless, but this was expected.40Output Power (dBm)30201001dB Compression Point-10-20-30-20-100Input Power (dBm)1020Fig. 7 Power Amplifier P1dB PlotTABLE 2POWER AMPLIFIER SUMMARYSpecificationResultsPower Added Efficiency(PAE)1 dB Compression (inputreferred)Max Output Power48%-1.89 dBm(a)12 dBm90degrees. This gives an ideal output that sweeps a constantamplitude circle in the Cartesian coordinate system (Figure9a) with an output that has constant amplitude at 2.4GHz(Figure 9b). When simulating open loop, the results indicatedamplitude distortion as indicated by the wave on top of the(b)(a)4(d)(c)Fig. 8 System Output(b)Fig. 9 System Input(f)(e)

UNIVERSITY OF MICHIGAN EECS 522 FINAL PROJECT: RF TRANSMITTER WITH CARTESIAN FEEDBACK5A comparison to existing literature is also seen in Table 3.Our system is comparable in output power achieved andlinearity.Noise and PAE were not measured for the system since H(s)was implemented with ideal components and would thereforenot give accurate results for a real system.D. LayoutThe layout is shown in Figure 11. It is DRC and LVS clean.The layout was designed to be symmetric with the poweramplifier in the middle and the mixers on either side. The areaof the layout is 1.15mm2.1dB Compression Point for System40Output Power (dBm)3530Open LoopFig. 11 Layout25ACKNOWLEDGMENT20We would like to thank Professor David Wentzloff andProfessor Joel Dawson for their advice on the project.Closed Loop1510REFERENCES50-30[1]-20-10Input Power (dBm)0[2]10[3]Fig. 10 System P1dB PlotV. CONCLUSION[4]This paper effectively demonstrates a RF transmitter at 2.4GHz with Cartesian feedback to improve linearity. Resultsindicated a reduction in distortion for the closed loop systemcompared to the open loop system. Future work wouldinclude implementing the loop filter with nonidealcomponents, improving the linearity of the downconversionmixer, and implementing a phase alignment system.[5][6][7]APPENDIX[8]The schematic, layout, and test bench files are stored ]J.L. Dawson, Feedback Linearization of RF Power Amplifiers. UnitedStates: Kluwer Academic, 2004.T. H. Lee, The Design of CMOS Radio-Frequency Integrated Circuits,Cambridge, U.K.: Cambridge Univ. Press, 1998.Sulivan, P.J.; Xavier, B.A.; Ku, W.H.; , "Low voltage performance of amicrowave CMOS Gilbert cell mixer," Solid-State Circuits, IEEEJournal of , vol.32, no.7, pp.1151-1155, Jul 1997Lin, C.-S.; Wu, P.-S.; Chang, H.-Y.; Wang, H.; , "A 9-50-GHz Gilbertcell down-conversion mixer in 0.13-μm CMOS technology," Microwaveand Wireless Components Letters, IEEE , vol.16, no.5, pp. 293- 295,May 2006Hanil Lee; Mohammadi, S.; , "A 500μW 2.4GHz CMOS SubthresholdMixer for Ultra Low Power Applications," Radio Frequency IntegratedCircuits (RFIC) Symposium, 2007 IEEE , vol., no., pp.325-328, 3-5 June2007Palaskas, Y.; Taylor, S.S.; et al; “A 5 GHz class-AB power amplifier in90 nm CMOS with digitally-assisted AM-PM correction,” CICC, 2005.J.K. Roberge, Operational Amplifiers: Theory and Practice. New York:Wiley, 1975.D. Chowdhury; et al, “A 2.4GHz mixed signal polar power amplifierwith low-power integrated filtering in 65nm CMOS,” CICC, 2010.L. Perraud; et al, “A direct-conversion CMOS transceiver for the802.11a/b/g WLAN standard utilizing a Cartesian feedback transmitter,”JSSC, 2004.TABLE 3SYSTEM SUMMARYSpecification1 dB Compression (inputreferred)Max Output PowerArea[8][9]Open LoopClosed Loop-10 dBm-31dBm-12 dBm-5 dBm21.8 dBm8 dBm24 dBm22 dBm--1.15 mm2 1.15 mm2

linearize the power amplifier, feedback can be wrapped around it. However, at the high frequency output of the power amplifier, it is difficult to stabilize a feedback loop. Therefore, we can take advantage of the baseband characteristic of the inputs and downconvert the output back to the baseband frequency for the feedback. Figure 1 shows the

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