WO2009032408A1 - Rf circuit with control unit to reduce signal power under appropriate conditions - Google Patents

Rf circuit with control unit to reduce signal power under appropriate conditions Download PDF

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Publication number
WO2009032408A1
WO2009032408A1 PCT/US2008/070327 US2008070327W WO2009032408A1 WO 2009032408 A1 WO2009032408 A1 WO 2009032408A1 US 2008070327 W US2008070327 W US 2008070327W WO 2009032408 A1 WO2009032408 A1 WO 2009032408A1
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WO
WIPO (PCT)
Prior art keywords
signal
power
output signal
operable
circuit
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/US2008/070327
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English (en)
French (fr)
Inventor
David M. Gonzalez
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
NXP USA Inc
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Freescale Semiconductor Inc
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Filing date
Publication date
Application filed by Freescale Semiconductor Inc filed Critical Freescale Semiconductor Inc
Priority to KR1020107004436A priority Critical patent/KR101502644B1/ko
Priority to JP2010522998A priority patent/JP5489172B2/ja
Publication of WO2009032408A1 publication Critical patent/WO2009032408A1/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/06—Receivers
    • H04B1/16—Circuits
    • H04B1/1607—Supply circuits
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/02—Transmitters
    • H04B1/04—Circuits

Definitions

  • the disclosed subject matter is in the field of radio frequency (RF) circuits and, more particularly, battery-powered RF circuits for use in applications requiring linear power amplifier performance.
  • RF radio frequency
  • RF circuits are prevalent in a larger number of applications including wireless or mobile telecommunications where one or more rechargeable batteries frequently provide the source of power for the RF circuit.
  • RF circuits are well known to include transmitter units that produce an RF signal that is provided as an input to a power amplifier.
  • the power amplifier amplifies and/or modulates the received signal to generate an RF output signal that is converted into electromagnetic waves and propagated into space via an antenna.
  • a battery powered RF device includes a voltage regulator or power supply circuit that receives the voltage produced by the battery as its input and generates one or more supply voltages and/or reference voltages for the RF circuit.
  • a significant component of any RF transmitter is the power amplifier.
  • the power amplifier produces the RF output signal that is propagated into space by means of an antenna.
  • Linear power amplifier performance is important in conjunction with certain types of modulation schemes, including modulation schemes designed to provide high rates of data transfer including, as a popular example, the Global System for Mobile communication (GSM) Enhanced Data rates for GSM Evolution (EDGE) modulation, referred to herein as GSM-EDGE or simply EDGE.
  • GSM Global System for Mobile communication
  • EDGE Enhanced Data rates for GSM Evolution
  • the output signal may begin to compress.
  • signal compression occurs, the spectral purity of the signal begins to degrade and the linear operation of the power amplifier may degrade simultaneously.
  • Even marginally non-linear power amplifiers may produce signals having characteristics that fail to meet certain specified requirements.
  • Parameters most at risk of becoming non-compliant during a low battery condition in an EDGE modulation implementation are the Alternate Channel Power Ratio (ACPR) and the Error Vector Magnitude (EVM).
  • ACPR Alternate Channel Power Ratio
  • EVM Error Vector Magnitude
  • FIG. 1 is a block diagram of selected elements of an embodiment of an RF system including a control unit
  • FIG. 2 is circuit diagram of selected elements of an embodiment of the control unit of FIG. 1.
  • a disclosed RF circuit is operable to be powered by a supply voltage.
  • the supply voltage may be provided by a battery that outputs a battery voltage.
  • the battery voltage is applied to the RF circuit so that the supply voltage equals the battery voltage.
  • the RF circuit includes a power amplifier that is powered by the supply voltage.
  • the power amplifier receives an RF input signal and produces an RF output signal.
  • the RF circuit further includes a detector that generates a detector output signal.
  • the detector output signal is indicative of the RF output signal power.
  • An offset unit adjusts the detector output signal when a low battery voltage condition is encountered. The low battery voltage condition occurs when the supply voltage is below a specified threshold.
  • the disclosed RF circuit may include a transmitter operable to generate the RF input signal.
  • the transmitter may be implemented as a transceiver that is operable as a receiver as well.
  • the adjusted control signal is provided to and received by the transmitter.
  • the transmitter may adjust the power level of the transmitter output signal, which is provided to the power amplifier as the RF input signal, based on the value of the adjusted control signal.
  • the disclosed RF circuit may include a coupler that receives the RF output signal.
  • the coupler produces a first coupled signal, which is provided to an antenna, and a second coupled signal, which is provided to an input of the detector.
  • the detector may be an IA/ converter that produces a control current indicative of the RF output signal power and, in some embodiments, a control current that is proportional to the logarithmic RF output signal power (expressed in dBm).
  • the control unit is operable to supplement the control current based at least in part on a difference between the battery voltage and a nominal value of the battery voltage.
  • the supply voltage is generated from and/or determined by the battery voltage.
  • the control circuit is operable to supplement the control current based on a difference between an actual battery voltage and a nominal battery voltage.
  • the offset unit may be operable to adjust the control signal to reduce the RF output signal power sufficiently to ensure linear operation of the power amplifier despite the low supply voltage condition. In some embodiments, for example, the offset unit is operable to adjust the adjusted detector output signal to reduce the RF output signal power sufficiently to ensure compliance with ACPR and EVM requirements as specified under EDGE.
  • the disclosed RF circuit thus monitors the battery voltage and reduces the power of the RF output signal during low battery voltage conditions.
  • a low battery voltage condition occurs when the battery voltage drops below a specified value.
  • the power of the RF output signal is decreased, in dBm, by an amount proportional to the amount by which the battery voltage is below nominal.
  • empirical data indicates that the RF output signal power needs just 1.5 dB of attenuation when the battery voltage drops from 3.5 V to 3.0 V. The decrease in RF output signal power in exchange for improved linear performance of the power amplifier may be acceptable in certain environments. Accounting for reference voltage variation and circuit tolerances, it may be desirable to achieve 2 dB of power attenuation when the battery voltage drops from 3.5 to 3.0 V.
  • a disclosed transceiver or transmitter system suitable for use in an RF application includes a power amplifier that receives an RF input signal and produces an RF output signal having an RF output signal power.
  • a battery provides a supply voltage to the power amplifier.
  • a transmitter provides the RF input signal to the power amplifier. The system reduces the RF output signal power when it detects a battery voltage below a nominal voltage.
  • the system may determine a difference between the battery voltage and a nominal voltage.
  • the RF output signal power reduction in dBm, may be a linear or nonlinear function of the difference. In embodiments where the nominal voltage is approximately 3.5 V, the RF output signal power reduction may be in the range of approximately 3 db/V to approximately 4 db/V.
  • the disclosed system may include a control unit that receives an input signal that indicates the RF output signal power.
  • the system may generate a control signal based on the RF output signal power and the battery voltage.
  • the system may be operable to reduce the RF output signal power by providing the control signal to the transmitter.
  • the transmitter is operable to reduce a power level of the RF input signal when the cut back circuit supplements the detector output signal.
  • the control unit includes a detector that produces a detector signal based on the RF output signal power and an offset unit that supplements the detector signal when the battery voltage is below the nominal voltage.
  • the detector may generate a detector current indicative of the RF output signal power.
  • the offset circuit sinks an offset current proportional to a difference between the battery voltage and the nominal voltage.
  • the offset current supplements the detector current.
  • the supplemental detection current is drawn through a feedback resistor of an operational amplifier of the detector to produce the control signal.
  • a disclosed RF circuit includes a power amplifier that receives a supply voltage equal to or otherwise derived from a battery voltage.
  • the RF circuit is further operable to produce an RF output signal.
  • a control unit determines the battery voltage and the RF output signal power.
  • the control unit may reduce the RF output signal power when a combination of the battery voltage and the RF output signal power are indicative of nonlinear power amplifier operation, for example, when the battery voltage drops below a nominal value or specified threshold and the power of the RF output power is high.
  • the RF circuit further includes a transceiver that provides an RF input signal to the power amplifier. The control signal may be provided to the transceiver, in which case, reducing the RF output signal power may be achieved by adjusting the power of the RF input signal.
  • FIG. 1 selected elements of an embodiment of an RF system 100 are depicted to emphasize the use of a control unit 121 to reduce or otherwise regulate the power of the RF output signal 106 when the system 100 is in an operational state that jeopardizes the linear operation of the system 100.
  • the RF output signal power is reduced when a supply voltage drops below a specified threshold to avoid non-linear operation that may occur when the RF output signal amplitude approaches or exceeds the supply voltage.
  • the power reduction may occur when the battery voltage drops below a specified threshold, e.g., a nominal value of the battery voltage.
  • EDGE modulation for example, employs 8 phase shift keying (8PSK) for the upper five of its nine modulation and coding schemes.
  • 8PSK 8 phase shift keying
  • Faithful EDGE operation requires a spectrally pure RF output signal. When the RF output signal amplitude approaches the supply voltage, signal compression degrades the spectral purity and jeopardizes compliance with at least two linearity specifications, namely, ACPR and EVM.
  • RF system 100 The elements of RF system 100 shown in FIG. 1 include a power amplifier 102 that receives an RF input signal 104 from a transmitter 140 and produces an RF output signal 106. Although labeled as a transmitter, transmitter 140 may be implemented as an integrated transmitter/receiver, i.e., a transceiver.
  • RF system 100 as depicted further includes a battery 150 that generates a battery voltage (V BAT )- In the depicted embodiment, the battery 150 is connected directly to the power terminals of power amplifier 102 so that the supply voltage for power amplifier 102 is VBAT-
  • V BAT battery voltage
  • RF output signal 106 as shown in FIG. 1 is delivered to a coupler 108, which couples a first portion 109 of RF output signal 106 to antenna 110 for transmission. Coupler 108 also couples a second portion of RF output signal 106, referred to herein as RF sample signal 112, to a detector unit 120 of the control unit 121.
  • Control unit 121 produces a control signal 135 that RF system 100 uses to regulate the power of RF output signal 106.
  • control of RF output signal power is achieved pre-amplification, by controlling the power of the RF input signal 104 generated by transmitter 140.
  • control signal 135 is delivered to transmitter 140.
  • Transmitter 140 controls the power of RF input signal 104 based, at least in part, on the magnitude or other characteristic of control signal 135.
  • the detector unit 120 of control unit 121 includes a log detector 119 that produces a detector current (I DET ) 123 based on the power of RF output signal 106.
  • Control unit 121 also includes an offset unit 130 that produces an offset current (l O s) 131 based on the battery voltage V BAT - The two currents are added together to obtain a control current I CON 124.
  • I CON 124 is converted to control signal 135, a voltage, by a current-to-voltage block 125.
  • Offset unit 130 receives the battery voltage V BAT and a regulated voltage V REG as its inputs.
  • V REG may be produced from V BAT by a conventional switch-mode or linear regulator 115.
  • V REG is a relatively stable voltage that provides a reference signal.
  • the ratio of V REG variation to V BAT variation is 10% or less.
  • Offset unit 130 produces the offset current l O s 131 based on the values of V BA ⁇ and V REG . Under the assumption that V REG is relatively invariant, l O s 131 is largely a function of the battery voltage V BA ⁇ - Specifically, as described in greater detail with respect to FIG.
  • the magnitude of los 131 increases as V BA ⁇ drops below a specified threshold.
  • Offset unit 130 controls the magnitude of l O s 131 to cause an RF signal power reduction during low battery voltage operation.
  • the reduction in power of RF output signal 106 caused by los 131 is sufficient to preserve the linearity of power amplifier 102 and the spectral purity of RF output signal 106 during low battery voltage operation so that linearity parameters including ACPR and EVM remain compliant with the applicable specifications during low battery voltage operation.
  • detector unit 120 includes a detector 119 that produces detector current (I DET ) based on the power of sample signal 112.
  • Detector 119 is referred as log detector 119 in embodiments where I DET is logarithmically related to power of sample signal 112. Because the power of sample signal 112 reflects the power of RF output signal 106, the magnitude of I DET 123 is indicative of the power of output signal 106.
  • I DET detector current
  • IA/ converter 125 includes an operational amplifier (op amp) 206 and a feedback resistor Rf 204 connected between the output of op amp 206 and an inverting input of op amp 206.
  • the non-inverting input of op amp 206 is grounded through a connection to an analog ground (V AG )-
  • V AG analog ground
  • the value of offset current l O s 131 is indicative of a difference between the battery voltage V BAT and the regulated voltage V RE G- AS a regulated voltage, V RE G is relatively stable across a wide range of V BAT values.
  • the variation in V REG may be approximately 3% or less.
  • V BAT may vary much more significantly, but it is desirable to maintain operation across the widest possible range of V BAT -
  • offset unit 130 facilitates linear operation of RF system 100 even when V BAT falls well below a nominal value.
  • offset unit 130 includes PMOS transistors 231 through 234, NMOS transistors 241 through 244, npn bipolar transistors 251 and 252, constant current sources 261 and 262, voltage dividers 271 and 272, and a bias resistor 281 having a resistance of R BIAS , all connected as shown.
  • Voltage divider 271 which has a ratio of K1 , produces a voltage K1 *V REG at the base terminal of transistor 251.
  • Voltage divider 272, which has a ratio of K2 produces a voltage K2*V BAT at the base terminal of npn transistor 252.
  • IBIAS [(K1 * V REG ) - (K2 * V BA ⁇ )] / RBIAS
  • T 252 is in series with T 232 so that the collector current I252 of T252 equals source/drain current I 2 32- T231 and T232 are configured as a current mirror in which I232 is mirrored in T231 so that I 231 equals I 232 , assuming transistors T 23 2 and T231 are both saturated.
  • T 24 i is in series with T 23 i so that l 24 i equals I231 - T 24 i and T 242 are configured as a current mirror so that l 24 i is mirrored in T 242 as I 242 .
  • I 242 is equal to I 252 , the collector current Of T 252 .
  • T 251 is in series with T 233 so that the collector current I 251 of T 251 equals I 233 , the source/drain current Of T 233 .
  • I 233 is mirrored in T 234 so that I 234 equals I 233 , assuming T 233 and T 234 are both saturated.
  • T 234 is in series with the parallel combination of T 242 and T 243 so that I 234 equals the sum of I 242 and I 243 .
  • I 234 is equal to the sum of I 242 and I 243 .
  • T 241 and T 242 are, however, configured as a current mirror so that I 242 equals I 241 .
  • I 234 therefore, represents the collector current of T 252
  • I 242 represents the collector current of T 251
  • I 243 represents the difference between the two collector currents.
  • T 243 and T 244 are configured as a current mirror so that l O s, the source/drain current of T 244 , is the mirror of I 243 .
  • los is equal in magnitude to the difference in the collector currents of T 252 and T 253 .
  • the emitter currents of T 2 5i and T 252 are approximately equal to their respective collector currents.
  • the offset current ⁇ os is substantially equal to twice the bias current I BIAS and, therefore:
  • offset unit 130 sinks los when V BA ⁇ is below its nominal value and the magnitude of los is proportional to the difference between V BA ⁇ and its nominal value.
  • l O s increases I CON , which flows through feedback resistor Rf 204 and increases the voltage of output signal 135 (los * Rf)-
  • V BAT equals or exceeds the nominal value, los drops to zero since negative drain current cannot flow through NMOS transistor 244.
  • ⁇ os is zero, the voltage of control signal 135 is determined solely by I DET 123.
  • offset unit 130 increases the voltage of control signal 135 to reduce RF signal power when battery voltage is low.
  • V BA ⁇ is nominal
  • the control unit prohibits non-zero values of los and the detector circuit is solely responsible for generating control signal 135. Since higher values of control signal 135 produce lower RF signal power, the RF signal power will be attenuated based on V BAT when V BAT is low.
  • Offset unit 130 as shown does not illustrate a disable mechanism. In some cases, it may be desirable to disable offset unit 130 so that it does not sink any offset current even when V BA ⁇ is low. If, for example, the RF output signal power were not sufficiently high to jeopardize the linearity of the RF signal, it may be desirable to disable offset unit 130 to prevent it from reducing the RF signal power when doing so is not necessary. In some cases, this objective may be achieved via software control that is not visible in FIG. 2. In other cases, RF system 100 may include hardware not shown in FIG. 2 to prevent offset unit 130 from drawing current during low power operation of the power amplifier. For example, control unit 121 as shown in FIG.
  • offset unit 130 may be effectively disabled when RF signal power is low.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Transmitters (AREA)
  • Amplifiers (AREA)
PCT/US2008/070327 2007-08-31 2008-07-17 Rf circuit with control unit to reduce signal power under appropriate conditions Ceased WO2009032408A1 (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
KR1020107004436A KR101502644B1 (ko) 2007-08-31 2008-07-17 적절한 상태 하에 신호 전력을 줄이는 제어 유닛을 갖는 ｒｆ 회로
JP2010522998A JP5489172B2 (ja) 2007-08-31 2008-07-17 適切な条件の下で信号電力を低減する制御ユニットを有するｒｆ回路

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US11/849,124 US7941110B2 (en) 2007-07-23 2007-08-31 RF circuit with control unit to reduce signal power under appropriate conditions
US11/849,124 2007-08-31

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WO2009032408A1 true WO2009032408A1 (en) 2009-03-12

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JP (1) JP5489172B2 (https=)
KR (1) KR101502644B1 (https=)
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US20090029659A1 (en) 2009-01-29
KR20100047301A (ko) 2010-05-07
US7941110B2 (en) 2011-05-10
JP5489172B2 (ja) 2014-05-14
JP2010538540A (ja) 2010-12-09

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