An Automatic Antenna Tuning System Using Only RF-Signal Amplitudes E.L

TCAS-II 5055 1 An Automatic Antenna Tuning System using only RF-Signal Amplitudes E.L. Firrao, A.J. Annema, and B. Nauta Abstract— The operating environment of mobile phones impedances. fluctuates continuously, due to changing handling conditions and Note that by using an automatic tuner, the bulky and nearby objects. The resulting fluctuations in antenna impedance expensive isolators that are widely applied to absorb reflected cause both a decrease in link quality and a higher standing wave power become redundant [4]. ratio, that requires more robust and hence less efficient PA implementations. In this paper an automatic antenna tuner The paper is arranged as follows. Section II presents an system for handheld applications is presented that uses two series analysis of the changing antenna impedance and of RF front reactances combined with 3 simple RF-peak detectors to sense end performance. The automatic antenna tuning system is both reactive and real impedance mismatches. The control loop described in section III. Measurements on demonstrator only requires low-frequency electronics which makes it low cost, implementations are shown in section IV. The conclusions are low power and relatively easy to integrate. Measurements on a summarized in section V. demonstrator system show correct behaviour for VSWR up to 10. Index Terms—Impedance matching, antenna tuning, high II. ANTENNA IMPEDANCE AND FRONT END PERFORMANCE efficiency power amplifiers, wireless communication systems. It is well known from literature that the antenna impedance is affected by any nearby impedance, such as the user’s hand I. INTRODUCTION and/or head [1]-[3]. For this paper, many measurements of the Mobile devices are widely used today, with as main antenna impedance were done to clearly identify the antenna technology drivers --- for the RF front end --- better impedance range and its sensitivity to the environment. Two performance at lower power consumption and lower cost. The types of widely used antennas were used: a planar inverted-F antennas used are typically narrow band, small, high-Q antenna (PIFA) and a ceramic antenna. A large number of devices that are detuned by any nearby impedances formed by typical handling positions for the two antennas were e.g. hands, the head, tables and more [1]-[3]. RF front ends considered for measurements of impedances and sensitivities. are typically designed under the assumption of a 50 Ω antenna For both types of antenna the measurements show impedance, for which maximum efficiency, talk-time, quality significant detuning of the antenna mainly due to a dramatic of the link, and maximum life-time are obtained. Due to change of the imaginary part of the impedance. The real part detuning of the narrow band antenna, significant degraded of the antenna impedance appears to be much less sensitive to efficiency and life-time can be observed [2]-[4]. handling conditions. This paper presents an automatic antenna tuner system that The impedance Zload of the antenna roughly lies in the range can tune fluctuations in antenna impedances, resulting in 30Ω < Re(Zload ) < 100Ω and 0 < Im()Zload < 100 jΩ . The RF higher (overall) efficiency and lower demands on robustness front end performance depends heavily on the antenna aspects for front end RF blocks. Lower robustness impedance. Figure 1 shows a simplified block diagram for the requirements translate into lower demands on design margins RF front end of a single band mobile phone. A single band that may be exchanged for higher front end efficiency. A front end is considered here only for simplicity reasons; number of requirements on antenna tuners can readily be extending the analysis towards multi-band systems is straight formulated: forward. The area covered by the reflection coefficient of the • Environmental fluctuation for antennas occur on a antenna is shown in the Smith-chart at the right hand side of time scale of ms which is slow compared to the RF Figure 1 for the GSM 900 and GSM 1800 bands; the signals in the GHz range [5]. Consequently a maximum VSWR for this range is about 10. relatively slow control loop is sufficient. In a typical design [7]-[8] the RF power amplifier is assumed • The insertion loss of an antenna tuner must be to be loaded with a real impedance while the antenna small for efficiency reasons. impedance is assumed to be 50 Ω. Transformation of the • The demands on the electronics in the antenna assumed 50 Ω to a lower real impedance level required by the tuner system must be low to simplify PA is not addressed in this paper as it is merely an extra implementations and to minimize the power (fixed) matching stage that is irrelevant in the context of consumption of the control loop itself. automatic antenna tuners. In the remainder of this paper we • The antenna tuner system must be able to correct assume therefore --- without loss of generality --- that the PA for a wide range of (heavily detuned) antenna includes a matching network that ideally sees 50 Ω loads. The authors are with the IC-Design Group, CTIT, University of Twente, Because the phase shift of each front end block is strongly Enschede, The Netherlands. E.L. Firrao is now with Thales, Hengelo, The design dependent, any (design-specific, fixed) extra phase Netherlands. This work was funded by NXP-semiconductors. Copyright (c) 2008 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending an email to [email protected] TCAS-II 5055 2 shift θ must be taken into account. Consequently the • Adaptation methods based on analytic computation impedance of the antenna as seen by the PA equals the that assume that the impedance can be measured antenna impedance with some fixed (design dependent) exactly [9]. Usually this requires impedance excess phase shift 2θ. The area covered by the possible measurement at RF-frequencies and a antenna reflection coefficients on a Smith chart at the antenna computationally expensive control loop. connector then translates into a full circle at the RF power • Adaptation methods that use the sign of the phase amplifier output, see the leftmost Smith-chart in Figure 1. difference to tune a reactive component to zero antenna (saw) [4][10][11]. These methods usually employ RF-phase LNA RF power filter detectors [4][12] or Foster-Seeley discriminators amplifier (PA) switch output [10][11] and usually are zero-crossing methods: the RF in transmission matching PA line 30+j100 Ω optimum then is a zero phase difference signal. network 100+j100 Ω • Adaptation methods that minimize a pre-specified phase shift θ figure of merit e.g. reflected power or VSWR. These Pout=Pmax are mainly gradient descent like methods [13]-[15] or Pout=Pmax-10dB trial&error methods [16][17]. 30 Ω 100 Ω Figure 1 The RF front end: the antenna impedance seen by the PA equals the The disadvantage of exact computation of the state of the transformed antenna impedance with a design-dependent excess phase shift θ. adaptive matching network is that the antenna impedance must be measured exactly, which usually involves voltage and An RF power amplifier is designed to deliver power in a current measurements at RF-frequencies. This in turn puts certain resistive load, represented here by the centre of the high demands on detection circuits and hence results in Smith chart. With other load impedances, load-pull analysis relatively high power consumption for the detection circuitry. [7] shows that the maximum output power is lower than the Gradient decent methods are not pursued because of the maximum by an amount Pdrop : need for the derivation of some sensitivity measure of ⎛1+ Γ 2 ⎞ ⎛1+ Γ ⎞ (1) matching network components. Adaptation based on direct 10log⎜ antenna ⎟ ≤ P []dB ≤10log⎜ antenna ⎟ ⎜ 2 ⎟ drop ⎜1− Γ ⎟ information usually is much more efficient in terms of control- 1− Γantenna ⎝ antenna ⎠ ⎝ ⎠ loop complexity. The trial&error methods are not where Γantenna is the (complex) antenna reflection implemented because any knowledge based method can coefficient. For an antenna VSWR=10, see Figure 1, the outperform them in antenna tuner systems. corresponding reflection coefficient |Γantenna| = 0.818 and Pdrop. To avoid computational expensive control mechanisms, in is then 7dB to 10dB. Note that the 2 boundaries in (1) this work we use a zero-crossing adaptation algorithm, while correspond to a imaginary and real Γantenna, respectively. For for power efficiency reasons and cost reasons we use quasi- an antenna VSWR of 2 the power drop can be already 3 dB DC measurable properties of RF signals (signal amplitudes) to which is significant for mobile applications. In Figure 3 a - measure the RF antenna impedance. The impedance 1.5dB contour is included. To be able to transmit at full power measuring electronics and the (strongly related) control levels as defined by (e.g. GSM) standards, without algorithm are introduced below. compensating impedance variations with automatic antenna tuners, the PA must be designed for much higher than A. The concept maximum nominal power levels which significantly decreases The concept of the automatic antenna tuning system is shown efficiency and increases costs [7]. in Figure 2. Tuning is done in two steps: first the imaginary part of the load impedance is tuned to (almost) zero using a III. AUTOMATIC ANTENNA TUNERS series (or shunt) reactance, then the resulting real part is An automatic antenna tuner is a system able to transform transformed to the target real value (i.e. 50 Ω) with a tunable the unpredictable or badly defined load impedance to a well “transformer”. This two-step sequential tuning fits well to the defined impedance (usually 50Ω) automatically, and it is low-power impedance measurement method that is introduced mainly made up of a tunable matching network, an impedance later in this paper.

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