Low Cost Coupling Methods for RF Power Detectors Replace Directional Couplers

Application Note 91
October 2002
Low Cost Coupling Methods for
RF Power Detectors Replace
Directional Couplers
by Shuley Nakamura and Vladimir Dvorkin
INTRODUCTION
Minimizing size and cost is crucial in wireless applications
such as cellular telephones. The key components in a
typical GSM cellular telephone RF transmit channel consist of an RF power amplifier, power controller, directional
coupler and diplexer. Some of the more recent RF power
amplifiers incorporate a directional coupler in their module, reducing component count and board area. Most
power amplifiers, however, require an external directional
coupler. Unfortunately, directional couplers come at a
price and sometimes a performance loss. While cost is an
issue, long lead-time and wide variations in coupling loss
are other concerns facing cell phone designers.
The directional coupler commonly used (Murata
LDC21897M190-078) is unidirectional (forward) and dual
band. One input is for low frequency signals (897.6MHz
±17.5MHz) and has a coupling factor of 19dB ±1dB. The
second input is for higher frequency signals (1747.5MHz
±37.5MHz) and has a coupling factor of 14dB ±1.5dB. The
Murata LDC21897M190-078 directional coupler is housed
in a 0805 package and requires an external 50Ω termination resistor.
When a signal is passed through one of the inputs, a small
portion of RF signal, equal to the difference between POUT
and the coupling factor, appears at the coupling output.
The remainder of the signal goes to the corresponding
signal output. In typical RF feedback configurations, the
coupled RF output is passed through a 33pF coupling
capacitor and 68Ω shunt resistor (Figure 1a).
Linear Technology has developed a coupling scheme for
LTC RF power controllers and RF power detectors which
is lower cost, more readily available and features tighter
tolerance. This coupling method eliminates the 50Ω termination resistor, 68Ω shunt resistor and 33pF coupling
capacitor used in traditional coupling schemes. Instead, a
0.4pF capacitor and 50Ω series resistor replace the directional coupler and its external components (Figure 1b)1 .
Alternate Coupling Solutions for use with an LTC
Power Controller
Method 1
The DC401B demo board was designed to demonstrate
the performance of the tapped capacitor coupling method
(Figure 2). RF signal is coupled back to the LTC4401-1 RF
input through a 0.4pF capacitor and 50Ω series resistor as
shown in Figure 1b. The RF signal is fed directly to the
diplexer from the power amplifier. The component count
is reduced by two.
The 0.4pF series capacitor must have a tolerance of
±0.05pF or less. The tolerance directly affects how much
RF signal is coupled back to the power controller RF input.
ATC has ultralow ESR, high Q microwave capacitors with
the tight tolerances desired. The ATC 600S0R4AW250XT
is a 0.4pF capacitor with ±0.05pF tolerance. This capacitor
comes in a small 0603 package. The series resistor is
49.9Ω (AAC CR16-49R9FM) with 1% tolerance.
Method 2
The second solution implements a 4.7nH shunt inductor.
The inductor compensates for the parasitic shunt capacitance associated with the RF input on the power controller.
Consequently, it improves the power control voltage range
and sensitivity. In dual-band applications, the inductor
value is chosen to increase the sensitivity of one frequency
band over the other. Using an inductor requires that a
capacitor be placed between the RF input pin and the
, LTC and LT are registered trademarks of Linear Technology Corporation.
Note 1: This method has been tested with the LTC4401-1 and the
following Hitachi power amplifiers: PF08107B, PF08122B, PF08123B.
AN91-1
Application Note 91
inductor. This capacitor provides a low impedance path
for the RF signal. A 33pF capacitor is used as shown in
Figure 1c. At each of the frequencies tested, the reactance
of the 33pF capacitor is lower than the inductor’s.
This method uses the same 0.4pF capacitor and 50Ω
resistor implemented in Method 1. The Murata film type
inductor, LQP15MN4N7B00D, comes in a 0402 package
and has ±1nH tolerance. The 33pF capacitor is an AVX
06035A330JAT1A, comes in a 0603 package and has 5%
tolerance. Tight tolerance for the shunt inductor and 33pF
capacitor is not critical.
VPC
U1
POWER
CONTROLLER
33pF
RF
68Ω
Figure 2. DC401B Demo Board
U2
POWER
AMPLIFIER
U3
DIRECTIONAL
COUPLER
U4
DIPLEXER
TO
ANTENNA
50Ω
an91 F01a
Figure 1a. Typical Cellular Phone Coupling Solution
VPC
U1
POWER
CONTROLLER
RF
50Ω
0.4pF
U2
POWER
AMPLIFIER
U4
DIPLEXER
TO
ANTENNA
an91 F01b
Figure 1b. Capacitive Coupling Method 1
Theory of Operation
The 0.4pF capacitor and 50Ω resistor form a voltage
divider with the input impedance of the LTC power controller. The voltage divider ratio varies over frequency. Reactance for capacitors is inversely proportional to frequency.
Thus, as frequency increases, the reactance decreases for
a fixed capacitance. Similarly, reactance increases as
capacitance decreases. A tenth of a picofarad greatly
impacts the reactance because the value of the coupling
capacitor is so small. This is why tight tolerance is absolutely crucial. Small changes in capacitance will change
the reactance and consequently, the voltage divider ratio.
Table 1 shows the reactance of various components at
900MHz, 1800MHz and 1900MHz.
Table 1. Reactance Variations over Frequency
VPC
U1
POWER
CONTROLLER
33pF
RF
Frequency (MHz)
50Ω
4.7nH
U2
POWER
AMPLIFIER
U4
DIPLEXER
0.4pF
Component Value
TO
ANTENNA
an91 F01c
Figure 1c. Capacitive Coupling Method 2
AN91-2
900
1800
1900
0.3pF
590Ω
295Ω
279Ω
0.4pF
442Ω
221Ω
210Ω
0.5pF
354Ω
177Ω
167Ω
33pF
5.4Ω
2.7Ω
2.5Ω
4.7nH
27Ω
53Ω
55Ω
The resistor value is determined by the series capacitor
value and additional shunt and placement parasitics. When
a shunt inductor is utilized, a smaller capacitor can be
used, yielding less loss in the main line. The shunt inductor
method is tuned to a particular frequency band at the
expense of other frequency bands. The second coupling
Application Note 91
method, for example, is tuned to DCS band frequencies.
The coupling loss for this method closely resembles the
coupling loss of the directional coupler (Figure 3b).
1600
PCTLMAX VOLTAGE (V)
1400
WITH DIRECTIONAL COUPLER
1200
WITH 0.4pF AND 49.9Ω
1000
WITH 0.4pF AND 4.7nH
800
600
0
Test Setup and Measurement
5
10
15
20
25
POUT (dBm)
30
35
AN91 F03a
Figure 3a. GSM900 PCTL vs POUT
1600
1400
PCTLMAX VOLTAGE (V)
There are several factors to consider when using either
coupling method, such as board layout and loading in the
main line. Conservative parts placement is necessary in
order to minimize the distance between the TX output 50Ω
line and the RF input pin on the power controller. Parasitic
effects can also greatly alter the feedback network characteristics. With good layout techniques and use of tight
tolerance components, this directional coupler substitute
can be used over GSM, DCS and PCS band frequencies.
400
200
WITH DIRECTIONAL COUPLER
1000
WITH 0.4pF AND 49.9Ω
800
600
WITH 0.4pF AND 4.7nH
400
0
0
5
10
15
20
POUT (dBm)
25
30
AN91 F03b
Figure 3b. DCS1800 PCTL vs POUT
1600
1400
Three different coupling methods were tested using the
DC401A and the DC401B demo boards. The DC401A RF
demo board has a triple-band directional coupler and
served as the control board. The coupling factor is 19dB at
900MHz and 14dB at 1800MHz and 1900MHz. The DC401B
was used to test the two capacitive coupling methods
described earlier (Figure 8).
Each of these demo boards contains an LTC4401-1 power
controller and a Hitachi PF08123B triple-band power
amplifier. The component layout of the two boards is
identical, except for the components that make up the
coupling scheme.
1200
200
PCTLMAX VOLTAGE (V)
Considerations
WITH DIRECTIONAL COUPLER
1200
1000
WITH 0.4pF AND 49.9Ω
800
600
WITH 0.4pF AND 4.7nH
400
A key measurement of interest is coupling loss. One
method of measuring coupled RF signal is to select an RF
output power level and compare the PCTL voltages applied
in each of the three coupling methods. Figure 4 shows
what a typical PCTL waveform looks like. Only the maximum level amplitude (maximum PCTL voltage) is adjusted for each measurement. The PCTL waveform is
generated by Linear Technology’s ramp shaping program,
LTRSv2.vxe and is programmed onto the DC314A demo
board. The DC314A digital demo board provides regulated
power supplies, control logic and a 10-bit DAC to generate
the SHDN signal and the power control PCTL signal. Input
power applied to each power amplifier channel is 0dBm. A
nominal battery voltage of 3.6V is used. Figure 7 illustrates
the test setup.
200
0
0
5
10
15
20
POUT (dBm)
25
30
AN91 F03c
Figure 3c. PCS1900 PCTL vs POUT
AN91-3
Application Note 91
A higher PCTL voltage indicates less coupling loss (i.e.,
more RF signal is being coupled back). Having too little
coupling loss can be a problem at higher power levels
because the PCTL value may exceed the maximum voltage
that the DAC can output. Having too much coupling loss
can make achieving lower output power levels difficult.
Using a PCTL voltage less than 18mV is not recommended, since the RF output will be unstable. Thus, the
minimum output power, POUT, is limited by PCTL = 18mV.
At 900MHz (GSM900), PCTL voltage measurements were
taken at the following output power levels: 5dBm, 10dBm,
13dBm, 20dBm, 23dBm, 30dBm and 33dBm. At 1800MHz
(DCS1800) and 1900MHz (PCS1900), PCTL measurements were recorded for the following output powers:
0dBm, 5dBm, 10dBm, 15dBm, 20dBm, 25dBm and 30dBm.
Figures 3a, 3b and 3c relate the output power to the applied
PCTL voltage for each coupling method. In general, the
capacitive coupling solutions have more coupling loss
than the directional coupler. The full output range was
achieved using both coupling methods.
Coupling Solution for LTC5505 Power Detector 2
The tapped capacitor method can also be utilized in
systems using the LTC5505 power detector. For example,
in the circuit in Figure 5, a shunt inductor is implemented
at the RF input pin to tune out the parasitic shunt capacitance of the power detector package (5-pin ThinSOTTM)
and the PCB at the actual operating frequency. Using a
shunt inductor improves the sensitivity of the LTC5505-2
by a factor of 2dB to 4dB. If operating between 3GHz to
3.5GHz, the shunt inductor is not recommended because
the bond wire inductance compensates for the input
parasitic capacitance. A DC blocking capacitor (C4) is
needed, because Pin 1 of the LTC5505-2 is internally DC
biased.
Figure 6 illustrates an example of dual band mobile phone
transmitter power control with an LTC5505-2 and a capacitive tap instead of a directional coupler. A 0.3pF
capacitor (C1) followed by a 100Ω resistor (R1) forms a
tapping circuit with about 20dB loss at cellular band
(900MHz) and 18dB loss at PCS (1900MHz) band referenced to the LTC5505-2 RF input pin. For best coupling
accuracy, C1 should have tight tolerance (±0.05 pF).
MAX LEVEL AMPLITUDE
STEP
AMPLITUDE
0V
AMPLITUDE
MAX LEVEL TIME
INITIAL
OFFSET
STEP
TIME
RISE
TIME
FALL
TIME
12µs
ZERO
TIME
AN91 F04
Figure 4. Typical PCTL Ramp Waveform
C1
C4
5
1
RF INPUT
2
L1
SHDN
3
VCC
C2
LTC5505-2
4
R1
RSSI OUTPUT
C3
AN91 F05
Figure 5. LTC5505-2 Application Diagram with a Shunt Inductor
ThinSOT is a trademark of Linear Technology Corporation.
AN91-4
Note 2: Consult factory for more applications information on LTC
power detectors.
Application Note 91
C1
0.3pF
R1
100Ω
Tx PA MODULE
RF 1
2
SHDN
3
CELL BAND
5 VCC
DIPLEXER
C2
0.1µF
LTC5505-2
Li-ION
PCS BAND
4 VOUT
VPC
MOBILE PHONE DSP
BSEL
AN91 F06
Figure 6. LTC5505-2 Tx Power Control Application Diagram with a Capacitive Tap
AGILENT
E4433B
RF SIGNAL
GENERATOR
AGILENT
HP8594E*
SPECTRUM
ANALYZER
AGILENT
3631A
5V, 3A
SMA CABLE
3dB ATTENUATOR
PC
RUNNING
LTRSv2.VXE
SMA CONNECTOR
SERIAL CABLE
DC314A-A
SHDN
SMA CABLE
EXT
TRIGGER
INPUT
DC401
SMA CONNECTOR
20dB ATTENUATOR
COAXIAL CABLE
*HP85722B AND HP85715B FOR DCS AND GSM MEASUREMENT PERSONALITIES
AN91 F07
Figure 7. PCTL Measurement Test Setup
AN91-5
AN91-6
BSEL
SHDN
RAMP
VBATT
BSEL
SHDN
RAMP
VBATT
C1
0.1µF
C2
100pF
L1
4.7nH
(OPT)
3
2
1
SHDN
PCTL
C4
33pF
VIN
VPC
GND
LTC4401-1
RF
U1
C13
33pF
(OPT)
VCTL
POUT_DCS
POUT_GSM
6
VDD2
PF08123B
PIN_DCS
VAPC
PIN_GSM
3
VDD1
C7
330pF
Figure 8. DC401B Schematic
7
C3
15pF
8
DCS
INPUT
(0dBm)
4
1
2
C9
0.1µF
5
6
GSM
INPUT
(0dBm)
C10
1µF
POUT_GSM
POUT_DCS
5
3
1
C6
1000pF
4
C8
330pF
P5
LFDP20N0020A
P1 GND GND GND
2
4
6
P2
U3
C5
1µF
R1
49.9Ω
5
C11
33pF SMA
C12
0.4pF
an91 F08
RF
OUPUT
Application Note 91
Application Note 91
Conclusion
Laboratory measurements have shown that the capacitive
coupling method is an effective means of coupling the RF
output signal. If coupling capacitors with tight tolerances
are used, the coupling factor will be consistent. On the
other hand, a directional coupler’s coupling factor can
vary up to 1.5dB. The total number of components decreases if the series resistor and capacitor are used. Cost
will also be reduced.
The capacitive coupling scheme has been shown to work
with the LTC4401-1 power controller and Hitachi PF08123B
power amplifier. This scheme can be applied to all LTC
power controllers (LTC1757A, LTC1758, LTC1957,
LTC4400, LTC4401, LTC4402 and LTC4403) and supported power amplifiers, as well as LTC power detectors.
When used with different power controller and power
amplifier combinations, the capacitor and resistor values
may need to be adjusted. Decreasing the coupling capacitor or increasing the series resistor will increase the
coupling loss. Linear Technology currently supports
Anadigics, Conexant, Hitachi, Philips and RFMD power
amplifiers. DC401B demo boards are available upon request.
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
AN91-7
Application Note 91
PARTS LIST
REFERENCE
(Demo Board DC401B)
QUANTITY
PART NUMBER
DESCRIPTION
VENDOR
C1, C9
2
0603YC104MAT1A
0.1µF 16V 20% X7R Capacitor
AVX
C2
1
06035A101JAT1A
100pF 50V 5% NPO Capacitor
AVX
C3
1
06035A150JAT1A
15pF 50V 5% NPO Capacitor
AVX
C4, C11, C13 (OPT)
2
06035A330JAT1A
33pF 50V 5% NPO Capacitor
AVX
C5, C10
2
EMK212BJ105MG-T
1µF 16V 20% X5R Capacitor
Taiyo Yuden
C6
1
06033C102KAT1A
1000pF 25V 10% X7R Capacitor
AVX
C7, C8
2
06035A331JAT1A
330pF 50V 5% NPO Capacitor
AVX
C12
1
600S0R4AW 250 XT
0.4pF ±0.5pF NPO Capacitor
ATC
L1 (OPT)
1
LQP15MN4N7B00
4.7nH 0402 ±0.1nH Inductor
Murata
R1
1
CR16-49R9FM
49.9Ω 1/16W 1% Chip Resistor
AAC
U1
1
LTC4401-1
SOT-23-6 RF Power Control IC
LTC
U2
1
PF08123B
Power Amplifier SMT IC
Hitachi
U3
1
LFDP21920MDP1A048
Dual Wideband Diplexer SMT IC
Murata
AN91-8
Linear Technology Corporation
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