Shuley Nakamura Vladimir Dvorkin INTRODUCTION Minimizing size cost cru
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Application Note October 2002 Cost Coupling Methods Power Detectors Replace Directional Couplers
Shuley Nakamura Vladimir Dvorkin INTRODUCTION Minimizing size cost crucial wireless applications such cellular telephones. components typical cellular telephone transmit channel consist power amplifier, power controller, directional coupler diplexer. Some more recent power amplifiers incorporate directional coupler their module, reducing component count board area. Most power amplifiers, however, require external directional coupler. Unfortunately, directional couplers come price sometimes performance loss. While cost issue, long lead-time wide variations coupling loss other concerns facing cell phone designers. directional coupler commonly used (Murata LDC21897M190-078) unidirectional (forward) dual band. input frequency signals (897.6MHz ±17.5MHz) coupling factor 19dB ±1dB. second input higher frequency signals (1747.5MHz ±37.5MHz) coupling factor 14dB ±1.5dB. Murata LDC21897M190-078 directional coupler housed 0805 package requires external termination resistor. When signal passed through inputs, small portion signal, equal difference between POUT coupling factor, appears coupling output. remainder signal goes corresponding signal output. typical feedback configurations, coupled output passed through 33pF coupling capacitor shunt resistor (Figure 1a). Linear Technology developed coupling scheme power controllers power detectors which lower cost, more readily available features tighter tolerance. This coupling method eliminates termination resistor, shunt resistor 33pF coupling capacitor used traditional coupling schemes. Instead, 0.4pF capacitor series resistor replace directional coupler external components (Figure 1b)1 Alternate Coupling Solutions with Power Controller
Method DC401B demo board designed demonstrate performance tapped capacitor coupling method (Figure signal coupled back LTC4401-1 input through 0.4pF capacitor series resistor shown Figure signal directly diplexer from power amplifier. component count reduced two.
0.4pF series capacitor must have tolerance ±0.05pF less. tolerance directly affects much signal coupled back power controller input. ultralow ESR, high microwave capacitors with tight tolerances desired. 600S0R4AW250XT 0.4pF capacitor with ±0.05pF tolerance. This capacitor comes small 0603 package. series resistor 49.9 (AAC CR16-49R9FM) with tolerance.
Method second solution implements 4.7nH shunt inductor. inductor compensates parasitic shunt capacitance associated with input power controller. Consequently, improves power control voltage range sensitivity. dual-band applications, inductor value chosen increase sensitivity frequency band over other. Using inductor requires that capacitor placed between input
registered trademarks Linear Technology Corporation.
Note This method been tested with LTC4401-1 following Hitachi power amplifiers: PF08107B, PF08122B, PF08123B.
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Application Note
inductor. This capacitor provides impedance path signal. 33pF capacitor used shown Figure each frequencies tested, reactance 33pF capacitor lower than inductor's. This method uses same 0.4pF capacitor resistor implemented Method Murata film type inductor, LQP15MN4N7B00D, comes 0402 package ±1nH tolerance. 33pF capacitor 06035A330JAT1A, comes 0603 package tolerance. Tight tolerance shunt inductor 33pF capacitor critical.
POWER CONTROLLER 33pF
Figure DC401B Demo Board
POWER AMPLIFIER
an91 F01a
DIRECTIONAL COUPLER
DIPLEXER
ANTENNA
Theory Operation 0.4pF capacitor resistor form voltage divider with input impedance power controller. voltage divider ratio varies over frequency. Reactance capacitors inversely proportional frequency. Thus, frequency increases, reactance decreases fixed capacitance. Similarly, reactance increases capacitance decreases. tenth picofarad greatly impacts reactance because value coupling capacitor small. This tight tolerance absolutely crucial. Small changes capacitance will change reactance consequently, voltage divider ratio. Table shows reactance various components 900MHz, 1800MHz 1900MHz.
Table Reactance Variations over Frequency
Frequency (MHz) 1800 1900
Figure Typical Cellular Phone Coupling Solution
POWER CONTROLLER
0.4pF
POWER AMPLIFIER
DIPLEXER
ANTENNA
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Figure Capacitive Coupling Method
POWER CONTROLLER
33pF
0.3pF 0.4pF
0.4pF
4.7nH
Component Value
0.5pF 33pF 4.7nH
POWER AMPLIFIER
DIPLEXER
ANTENNA
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Figure Capacitive Coupling Method
resistor value determined series capacitor value additional shunt placement parasitics. When shunt inductor utilized, smaller capacitor used, yielding less loss main line. shunt inductor method tuned particular frequency band expense other frequency bands. second coupling
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Application Note
method, example, tuned band frequencies. coupling loss this method closely resembles coupling loss directional coupler (Figure 3b).
1600 1400 PCTLMAX VOLTAGE 1200 WITH 0.4pF 49.9 1000 WITH 0.4pF 4.7nH POUT (dBm) WITH DIRECTIONAL COUPLER
Considerations There several factors consider when using either coupling method, such board layout loading main line. Conservative parts placement necessary order minimize distance between output line input power controller. Parasitic effects also greatly alter feedback network characteristics. With good layout techniques tight tolerance components, this directional coupler substitute used over GSM, band frequencies. Test Setup Measurement Three different coupling methods were tested using DC401A DC401B demo boards. DC401A demo board triple-band directional coupler served control board. coupling factor 19dB 900MHz 14dB 1800MHz 1900MHz. DC401B used test capacitive coupling methods described earlier (Figure Each these demo boards contains LTC4401-1 power controller Hitachi PF08123B triple-band power amplifier. component layout boards identical, except components that make coupling scheme. measurement interest coupling loss. method measuring coupled signal select output power level compare PCTL voltages applied each three coupling methods. Figure shows what typical PCTL waveform looks like. Only maximum level amplitude (maximum PCTL voltage) adjusted each measurement. PCTL waveform generated Linear Technology's ramp shaping program, LTRSv2.vxe programmed onto DC314A demo board. DC314A digital demo board provides regulated power supplies, control logic 10-bit generate SHDN signal power control PCTL signal. Input power applied each power amplifier channel 0dBm. nominal battery voltage 3.6V used. Figure illustrates test setup.
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Figure GSM900 PCTL POUT
1600 1400 PCTLMAX VOLTAGE 1200 1000 WITH 0.4pF 49.9 WITH 0.4pF 4.7nH POUT (dBm)
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WITH DIRECTIONAL COUPLER
Figure DCS1800 PCTL POUT
1600 1400 PCTLMAX VOLTAGE 1200 1000 WITH 0.4pF 49.9 WITH 0.4pF 4.7nH POUT (dBm)
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WITH DIRECTIONAL COUPLER
Figure PCS1900 PCTL POUT
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Application Note
higher PCTL voltage indicates less coupling loss (i.e., more signal being coupled back). Having little coupling loss problem higher power levels because PCTL value exceed maximum voltage that output. Having much coupling loss make achieving lower output power levels difficult. Using PCTL voltage less than 18mV recommended, since output will unstable. Thus, minimum output power, POUT, limited PCTL 18mV. 900MHz (GSM900), PCTL voltage measurements were taken following output power levels: 5dBm, 10dBm, 13dBm, 20dBm, 23dBm, 30dBm 33dBm. 1800MHz (DCS1800) 1900MHz (PCS1900), PCTL measurements were recorded following output powers: 0dBm, 5dBm, 10dBm, 15dBm, 20dBm, 25dBm 30dBm. Figures relate output power applied PCTL voltage each coupling method. general, capacitive coupling solutions have more coupling loss than directional coupler. full output range achieved using both coupling methods. Coupling Solution LTC5505 Power Detector tapped capacitor method also utilized systems using LTC5505 power detector. example, circuit Figure shunt inductor implemented input tune parasitic shunt capacitance power detector package (5-pin ThinSOTTM) actual operating frequency. Using shunt inductor improves sensitivity LTC5505-2 factor 4dB. operating between 3GHz 3.5GHz, shunt inductor recommended because bond wire inductance compensates input parasitic capacitance. blocking capacitor (C4) needed, because LTC5505-2 internally biased. Figure illustrates example dual band mobile phone transmitter power control with LTC5505-2 capacitive instead directional coupler. 0.3pF capacitor (C1) followed resistor (R1) forms tapping circuit with about 20dB loss cellular band (900MHz) 18dB loss (1900MHz) band referenced LTC5505-2 input pin. best coupling accuracy, should have tight tolerance (±0.05 pF).
LEVEL AMPLITUDE
STEP AMPLITUDE INITIAL OFFSET STEP TIME 12µs RISE TIME LEVEL TIME
AMPLITUDE
FALL TIME
ZERO TIME
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Figure Typical PCTL Ramp Waveform
INPUT
LTC5505-2 RSSI OUTPUT
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SHDN
Figure LTC5505-2 Application Diagram with Shunt Inductor
Note Consult factory more applications information power detectors.
ThinSOT trademark Linear Technology Corporation.
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Application Note
0.3pF
MODULE 0.1µF VOUT Li-ION BAND CELL BAND DIPLEXER
LTC5505-2
SHDN
MOBILE PHONE
BSEL
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Figure LTC5505-2 Power Control Application Diagram with Capacitive
AGILENT E4433B SIGNAL GENERATOR AGILENT 3631A CABLE ATTENUATOR RUNNING LTRSv2.VXE SERIAL CABLE DC314A-A SHDN CONNECTOR DC401 CONNECTOR 20dB ATTENUATOR COAXIAL CABLE
AGILENT HP8594E* SPECTRUM ANALYZER
CABLE TRIGGER INPUT
*HP85722B HP85715B MEASUREMENT PERSONALITIES
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Figure PCTL Measurement Test Setup
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49.9 4.7nH (OPT) 0.1µF 330pF 1000pF 330pF 33pF (OPT) 0.4pF 100pF POUT_GSM POUT_DCS SHDN PIN_DCS PF08123B VCTL
an91
Application Note
VBATT LTC4401-1 PIN_GSM VAPC INPUT (0dBm) 15pF INPUT (0dBm)
VBATT VDD1 POUT_GSM POUT_DCS VDD2
0.1µF PCTL 33pF
LFDP20N0020A
33pF
OUPUT
RAMP
RAMP
SHDN
SHDN
BSEL
BSEL
Figure DC401B Schematic
Application Note
Conclusion Laboratory measurements have shown that capacitive coupling method effective means coupling output signal. coupling capacitors with tight tolerances used, coupling factor will consistent. other hand, directional coupler's coupling factor vary 1.5dB. total number components decreases series resistor capacitor used. Cost will also reduced. capacitive coupling scheme been shown work with LTC4401-1 power controller Hitachi PF08123B power amplifier. This scheme applied power controllers (LTC1757A, LTC1758, LTC1957, LTC4400, LTC4401, LTC4402 LTC4403) supported power amplifiers, well power detectors. When used with different power controller power amplifier combinations, capacitor resistor values need adjusted. Decreasing coupling capacitor increasing series resistor will increase coupling loss. Linear Technology currently supports Anadigics, Conexant, Hitachi, Philips RFMD power amplifiers. DC401B demo boards available upon request.
Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
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Application Note
PARTS LIST
REFERENCE C11, (OPT) (OPT)
(Demo Board DC401B)
QUANTITY PART NUMBER 0603YC104MAT1A 06035A101JAT1A 06035A150JAT1A 06035A330JAT1A EMK212BJ105MG-T 06033C102KAT1A 06035A331JAT1A 600S0R4AW LQP15MN4N7B00 CR16-49R9FM LTC4401-1 PF08123B LFDP21920MDP1A048 DESCRIPTION 0.1µF Capacitor 100pF Capacitor 15pF Capacitor 33pF Capacitor Capacitor 1000pF Capacitor 330pF Capacitor 0.4pF ±0.5pF Capacitor 4.7nH 0402 ±0.1nH Inductor 49.9 1/16W Chip Resistor SOT-23-6 Power Control Power Amplifier Dual Wideband Diplexer VENDOR Taiyo Yuden Murata Hitachi Murata
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408) 432-1900 FAX: (408) 434-0507
an91f LT/TP 1002 PRINTED
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2002
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