Measuring System Efficiency in Multi-Mode PA

Proceedings of the 1st European Wireless Technology Conference

Measuring System Efficiency in Multi-Mode PA Modules for Mobile Phones Paul N. Whatmough1, Brian J. Minnis2, Paul A. Moore3 RF Systems and Architecture Group, NXP Semiconductors Research Redhill, Surrey, RH1 5HA, UK 1

[email protected] 2 [email protected] 3 [email protected]

Abstract—A multi-mode GSM/EDGE/UMTS power amplifier (PA) module, comprising a highly-efficient CMOS hybrid powersupply modulator combined with a BiCMOS power amplifier, is used to quantify the relative efficiency enhancements that can be achieved by three different supply-tracking regimes: polar modulation, envelope tracking and average envelope tracking. The power efficiency of the PA module is measured for the given standards over a 20 dB range of output power levels. A demonstrated improvement in PA efficiency was observed with all supply tracking regimes, the highest with polar modulation, where PA efficiency > 60% is achieved. However, after taking into account power dissipated in the supply modulator, the envelope tracking technique was found to offer the best PA module efficiency, improving the nominal fixed supply efficiency by up to 25% for the EDGE standard.

II. EXPERIMENTAL EVALUATION SYSTEM The experimental evaluation system consists of a digital baseband signal generator, digital-to-analogue converters (DACs), analogue reconstruction filters, RF up-conversion and finally the PA and SM ICs (Fig. 1). Modulated I and Q baseband signals are generated in software at the native sample rate, and interpolated up to 60 MHz before being loaded into the pattern memory of a National Instruments (NI) PXI-6562 digital pattern generator. This streams interleaved 14 bit I and Q samples over a Low Voltage Differential Signalling (LVDS) interface to a custom PCB containing an Altera Stratix-II field-programmable gate array (FPGA). Here, the samples are de-interleaved and the envelope component extracted before being scaled and offset to generate the supply modulation signal. All three digital streams are then passed to AD8138 14-bit DACs, clocked at 60 MHz, followed by a discrete component 3-pole Chebychev reconstruction filter with a 42 MHz -3 dB bandwidth. The baseband analogue I and Q signals are then applied to a vector modulator for upconversion onto an RF carrier at 870 MHz. The analogue baseband envelope signal is then applied to the supply modulator. The supply modulator is operated from a DC power supply set to 4.45 V, which represents the handset battery.

I. INTRODUCTION Power efficiency has become a major competitive challenge for commercial mobile terminal designs. The radio frequency (RF) parts of the transmitter often dominate power consumption during operation. The challenge is especially severe for non-constant envelope modulation schemes that give rise to time-varying envelopes with significant peak-toaverage ratio (PAR). Traditionally, these signals are amplified using a backed-off power amplifier (PA), which offers relatively poor power efficiency [1]. The use of powersupply modulation at the PA collector/drain enables an improved balance of linearity and efficiency. However, the efficiency of the supply modulator (SM) circuitry is critical to realising an overall system efficiency improvement. Also, spurious-emission requirements imposed by the cellular standards are particularly demanding and without the help of a costly RF surface acoustic wave (SAW) filter, make the design of the modulator even more challenging. This paper presents an experimental measurement system for a GSM/EDGE/UMTS PA module that incorporates a multi-standard PA and SM, allowing the relative evaluation of three approaches to the efficiency enhancement of multi-mode PAs: polar modulation (also known as envelope elimination and restoration), envelope tracking (ET) and average envelope tracking (AET). These techniques are described in detail in [2]. Following in section II is a description of the experimental evaluation system, including the PA and SM circuits. Section III presents the measurement results, focussing on EDGE for the sake of brevity.

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Fig. 1. Experimental Evaluation System Hardware

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III. MEASUREMENT RESULTS A. RF Performance RF output spectrum performance was measured with an Agilent PSA spectrum analyzer. Fig. 2 shows the output spectra for an EDGE burst at 29 dBm output power, without digital pre-distortion, for two of the tracking regimes and a benchmark case where VCC is fixed. The fixed-VCC system performs best in terms of spectral shape, while the addition of ET has the effect of raising the noise floor by approximately 5 dB. Despite this, ET barely decreases the close-in mask margin. The polar modulation transmitter narrowly fails to meet the EDGE spectral mask requirements around the 400 kHz offset, indicating the need for some pre-distortion. Similar results were obtained for UMTS, although due to the increased envelope bandwidth, the ET and polar transmitters become particularly sensitive to delay mismatch between RF and envelope paths [6]. All the regimes satisfied the EVM requirements for the given standards. However polar modulation offers only a slender margin to the specification for both UMTS and EDGE,

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whereas the EVM improved somewhat for envelope tracking compared to the fixed-VCC case, due to the PA being held at a roughly constant gain compression point, as was also observed in [7]. GSM phase error is below 1° for both fixedVCC and AET regimes.

EDGE Spectral Emissions mask

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Fig. 2. Measured EDGE RF Output Spectrum

B. System Efficiency In considering the implications of RF efficiency, it is important to note that statistically for the given radio standards, the RF output power is seldom at the specified maximum. Hence, it is important to measure efficiency at reduced output powers as well as at full power. Accordingly, the system efficiency was measured across a 20 dB range of transmitter output powers for polar, ET and AET tracking regimes and also the nominal fixed-VCC case, with GSM, EDGE and UMTS modulated signals. Polar 60

PA Collector Efficiency (%)

The PA is a single-chip multi-mode design in BiCMOS technology, covering a bandwidth of 824-915 MHz [3]. It features embedded gain control, can be operated in linear (Class-B), or saturated (inverse Class-F) modes and accommodates power-supply modulation. The output stage is implemented as a balanced, push-pull design to maximise power supply rejection over a single ended design, to help relax the SM output noise requirements. The design was dimensioned for 35 dBm peak output power at 3.6 V but the prototype achieves around 33.5 dBm at 4 V, mainly due to a small mismatch in the reactive part of the load impedance. The SM design is a hybrid configuration of a linear regulator and DC/DC converter [4]. The -3dB bandwidth is around 50 MHz when driving low-ohmic loads and it offers 3.2 W peak output power with an RMS ripple of < 4 mV. The IC has been fabricated in a 0.25 μm CMOS process with an additional gate oxide permitting 6 V transistors. The switching converter stage can be disabled for super-low-noise operation using only the linear regulator. This is necessary to minimise spectral emissions for polar modulation, but comes at the cost of reduced efficiency, as described later. The interface between the SM IC and the PA IC is a very low impedance node and achievable supply-modulation bandwidth between the two is thus limited by parasitic effects associated with the PCB tracks. An improved solution would be to integrate both circuits on the same IC [5]. To measure the efficiency of the PA module and principal components, it is necessary to measure the power flow between the battery (DC power supply), SM and PA and also the RF power at the PA input and output ports. Currents (IBAT, ICC) and voltages (VBAT, VCC) were measured using Tektronix TCP312 wideband current probes and passive voltage probes respectively and then digitised by 60 MHz, 12-bit NI PXI5105 signal acquisition hardware. An Agilent N1912A dualchannel wide-video-bandwidth power meter is used to measure the RF powers.

ET 40

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Fig. 3. Instantaneous RF PA efficiency with EDGE modulation at 20 dBm (dotted lines show efficiency averaged over the entire modulation burst)

Looking first at the PA efficiency for the various system configurations, Fig. 3 shows the instantaneous output stage collector efficiency for EDGE modulation at 20 dBm output power. This plot is a clear indication of the achievable efficiency enhancement in the PA alone. The lowest trace in the figure is for fixed-VCC, where the PA is connected directly to the battery and is the benchmark against which the other regimes are critiqued. The efficiency of such a backed-off linear PA is low, with measured average efficiency of 16%.

The AET trace represents the case where VCC is lowered to the point where the peaks of the amplitude modulation (AM) are just approaching 1 dB gain saturation, effectively recovering much of the efficiency lost due to the reduction in average output power to 20 dBm from the maximum value of 29 dBm. Consequently, the average efficiency improves considerably, although the figure of 33% is still limited by poor instantaneous efficiency during AM troughs. Moving to ET, the peak efficiency is the same as the AET case, but efficiency during AM back-off has increased, since the supply modulation attempts to maintain approximately 1 dB gain compression at all times. In this case, the efficiency lost due to the AM on the input signal is partially recovered, with the outcome that average efficiency rises to 40%. Finally, polar modulation has by far the best PA efficiency (average 61%) by virtue of the deep gain compression held throughout the AM range. However, polar modulation also demands the greatest signal bandwidths and the fact that the PA is operating in compression places even more stringent limits on the SM output noise. This is so severe that only a linear regulator can be used for polar modulation, which has strong implications on system efficiency, as shown later. Having examined instantaneous PA efficiency, average efficiency is considered next, specifically over the top 20 dB of the power control range (Fig. 4), below which efficiency enhancement ceases to be worthwhile due to the very low powers involved. An improvement in PA efficiency is clearly observed for all the supply tracking techniques over the fixedVCC PA. The measurements show AET is effective in maintaining good average PA efficiency at reduced output powers. ET, which is capable of tracking the instantaneous modulation envelope, shows a fixed improvement over AET, with PA efficiency of > 38% over the top 10 dB of the output power range. Finally, the potential benefit of polar modulation is demonstrated by the peak efficiency of > 60% and average efficiency of > 54% over the top 10 dB. The efficiency of the SM is a strong function of the input modulation signal statistics. This is partly due to its hybrid circuit topology whereby the switching converter part delivers most of the power content at frequencies below 200 kHz with high efficiency, whereas the linear regulator part principally delivers power at frequencies above 200 kHz, with lower efficiency. Fig. 5 shows the measured SM efficiency for the three tracking regimes in EDGE mode. The AET regime, which, in the steady state requires zero bandwidth, demonstrates the best efficiency of up to 80%, as the switching converter provides all the output power. At the other bandwidth extreme, polar modulation achieves the lowest efficiency as the linear regulator now provides a significant proportion of the output signal. As the hybrid modulator cannot be made clean enough for polar modulation, it is important to also measure the case of polar modulation using a pure linear regulator. As is to be expected, this case produces the lowest modulator efficiency of up to 51%, largely cancelling the efficiency gains achieved in the PA. Fig. 6 gives the measured overall system efficiency of the PA and SM combined for EDGE, over the top 20 dB of the

power control range. Compared with the benchmark fixedVCC result, polar modulation with the hybrid SM offers by far the best efficiency result at all power levels, notwithstanding the fact that this configuration would not meet the required limits on spurious emissions. By contrast with a pure linear regulator, the polar configuration achieves an efficiency significantly worse than a fixed-VCC linear PA and hence has little merit on efficiency grounds alone. The ET regime using the hybrid SM, however, improves PA module efficiency by 25%, as compared to fixed-VCC. This advantage does not extend to the maximum power setting since the losses in the SM still outweigh the gains in the PA at this level. In the case of AET, only a small efficiency gain is seen of a few percent, over an even more narrow range of output powers. 80 RF PA Collector Efficiency (%)

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Fig. 5. Measured CMOS Supply Modulator Efficiency for EDGE 50

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IV. CONCLUSIONS Measurements have been presented of an experimental PA module that incorporates a BiCMOS RF PA and CMOS SM designed for the GSM, EDGE and UMTS standards. The system measurements cover the ET, AET and polar modulation supply tracking regimes, as well as benchmark fixed-VCC results, over a power-control range of 20 dB. The RF output spectral mask was satisfied for all configurations, without digital pre-distortion, except in the case of polar modulation. Requirements for EVM were satisfied for all standards, although polar modulation offered only a slim margin against the specification. As far as efficiency is concerned, polar modulation offers the greatest benefit to PA efficiency but this does not translate easily into improved system efficiency. Severe noise requirements mean that a hybrid modulator cannot yet be used for polar modulation and the need to use a linear regulator negates all the efficiency gains in the PA. ET and to a lesser degree AET have been demonstrated to show a modest benefit over a fixed-VCC PA for the EDGE standard, albeit at the cost of increased complexity. The improvement in efficiency for UMTS was generally insignificant, mainly as a consequence of increased back-off in both the multimode PA and hybrid SM. Further circuit adaptation in the PA and SM between the different standards could potentially improve this situation. For standards with notably higher PAR, such as UMTS Long Term Evolution (LTE), the benefit of supply tracking is expected to become markedly more significant.

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Fig. 8. PA Module Efficiency (RF PA and SM) for GSM

ACKNOWLEDGMENT The authors wish to thank Mark van der Heijden, Peter Blanken, Ravi Karadi and Henk-Jan Bergveld for their support and project contributions.

The system efficiency results for UMTS are presented in Fig. 7. Practical limitations associated with the multi-standard PA and SM mean that efficiency for UMTS at the maximum transmit power of 26 dBm is already degraded compared to GSM and EDGE modes. This, and a further small reduction in SM efficiency due to the lower power and wider bandwidth of the UMTS envelope signal, means that the potential for efficiency enhancement in UMTS mode is only marginal, as shown in the measurement results. Polar modulation with a hybrid SM offers the greatest increase in PA module efficiency of around 16% over fixed-VCC. But, as was observed to a lesser degree with the EDGE standard, polar modulation with a linear regulator actually degrades efficiency compared to the nominal case. ET offers a PA module efficiency increase of around 10%. For completeness, the measured GSM system efficiency case is presented in Fig. 8. Since the PA is operating in saturation, the noise from a hybrid modulator would cause the RF output to fail the spurious emissions requirements for the standard and hence is not yet a viable option. However, the graph shows that if a sufficiently clean converter could be designed, the improvement in system efficiency could be as much as 75% in the middle of the power range. This would be very worthwhile indeed.

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[3]

[4]

[5]

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S. C. Cripps, “RF Power Amplifiers for Wirelesss Communications”, Artech House Publishers, 1999 B. J. Minnis, P. A. Moore, P. N. Whatmough, P. G. Blanken, M. P. van der Heijden, “Accurate Efficiency Analysis of PA Supply-Tracking Regimes in Mobile Transmitters”, paper accepted for publication in IEEE Trans. Circuits and Systems I, Feb. 2008 M. P. van der Heijden, I. Volokhine, P. N. Whatmough, “Multi-Mode Push-Pull BiCMOS Power Amplifier Design for the 824-915 MHz Cellular Band”, IEEE Topical Symposium on Power Amplifiers for Wireless Comm. Dig. 2008, Orlando, FL, USA Jan. 2008 P. G. Blanken, R. Karadi, H. J. Bergveld, “A 50 MHz Bandwidth Multi-Mode PA Supply Modulator for GSM, EDGE and UMTS Application”, paper accepted for IEEE RFIC Symposium, Atlanta, USA, June 2008 F. Wang, D. F. Kimball, D. Y. C. Lie, P. M. Asbeck, L. E. Larson, “A Monolithic High-Efficiency 2.4-GHz 20-dBm SiGe BiCMOS Envelope-Tracking OFDM Power Amplifier”, IEEE. J. Solid-State Circuits, 42, 6, pp. 1553-1556, 2006 F. Wang, A. H. Yang, D. F. Kimball L.E. Larson P. M. Asbeck, “Design of Wide-Bandwidth Envelope-Tracking Power Amplifiers for OFDM Applications”, IEEE Trans. Microwave Theory and Techniques, 53, 4, pp. 1244-1255, 2004 D. Y. C. Lie, J. D. Popp, F. Wang, D. Kimball, L. E. Larson, “Linearization of Highly-Efficient Monolithic Class E SiGe Power Amplifiers with Envelope-Tracking (ET) and Envelope-Eliminationand-Restoration (EER) at 900MHz”, IEEE Dallas Circuits and Systems Workshop on System-on-Chip, 2007