Application Note

Application Note
WCDMA/GSM/GPRS/Polar EDGE
Power Amplifier Module
with Integrated Power Control
Revision 0
RELEVANT PRODUCTS
•
AWT6223
INTRODUCTION
GENERAL DESCRIPTION
This WEDGE Power Amplifier Module supports GSM
type dual, tri and quad band applications for GMSK
and 8-PSK modulation schemes using a polar
architecture, as well as WCDMA in the IMT band.
The WEDGE module includes an internal reference
voltage and integrated power control scheme for
use in GMSK and 8-PSK operation, which facilitates
fast and easy production calibration and reduces
the number of external components required to
complete a power control function. This integrated
control loop reduces the development time
associated with optimizing loop filters to meet time
mask and switching transient requirements, as it is
completely self-contained.
The application circuit below shows the relative
ease with which this amplifier can be designed into
a GSM transmit front-end. All of the RF ports for this
device are internally matched to 50Ω.
The RF inputs can interface to transmit VCO’s with
the addition of simple attenuators. These can be
used to set the input drive to the PA and is generally
good practice to help minimize any possible load
pulling effects at the VCO, PA interface. The RF
outputs can interface directly to an antenna switch
module to complete the front-end solution.
The logical control inputs, TX_EN and BS, are both
1.8 V and 3 V logic compliant. The TX_EN is used to
enable the amplifier typically with the TX burst. The
BS is used to select which amplifier is enabled.
The module size is a competitive 6 mm x 8 mm and
with the few external components required, it is well
suited for a small form factor transmit front-end
solution.
4.7uF ++
10nF++
SUPPLY VOLTAGE FROM
DC-DC CONVERTER***
22pF**
22
CEXT2
1
IMT RF INPUT
2
WCDMA BIAS MODE
27pF ++
3
WCDMA ENABLE
27pF ++
4
DCS/PCS RF INPUT
5
BAND SELECT
27pF ++
6
TX ENABLE
BATTERY
VOLTAGE
27pF
++
7
4.7uF ++
2.7pF **
22nF **
DAC OUTPUT
9
10K *
WCDMA_OUT
VMODE
GND
VEN
DCS/PCS_OUT
DCS/PCS_PIN
GND
BS
AWT6223R
TX_EN
GND
VBATT
CEXT3
CEXT1
GND
21
10nF ++
19
18
WCDMA RF OUTPUT
DCS/PCS RF OUTPUT
17
16
15
VRA MP
GND
14
13
27pF*
10
GSM_IN
GSM_OUT
12
VCC_GSM
11
*
22pF**
20
1nF**
8
GSM850/900 RF INPUT
V CC_WCDMA
WCDMA_IN
Filtering may be required to filter noise from baseband.
** This component should be placed as close to the device pin as possible.
*** If the final design uses a DC-DC Converter, otherwise connect Pin 21 directly to VBATT.
++ These components are recommended as good design practice for improving noise rejection
characteristics. The values specified are not critical as they may not be required in the final
application.
Figure 1: Recommended Application Circuit
05/2006
GSM850/900 RF OUTPUT
WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
POWER CONTROL
The scheme used is a closed loop method that
requires only the application of an analog voltage to
the VRAMP pin to set the output power. This can be
applied directly from a standard DAC output. The
method used does not require any power or current
sensing. Setting the VRAMP voltage, in turn sets the
collector voltages of the power amplifiers to a
multiple of the VRAMP voltage using a pre-determined
formula. This collector voltage is regulated in a
voltage control loop as shown below. The amplifier’s
bias is held constant while the collector voltage is
adjusted to set the power. The relationship between
the output power and collector voltage is described
by Equation 1.
POUT (Watts ) =
(2 ⋅ VCC − VSAT ) 2
8 ⋅ RLOAD
Equation 1
where V CC , V SAT are the collector voltage and
saturation voltage of the transistor respectively. This
expression shows how the power variation due to
VBATT is limited due to voltage control loop.
Under extreme conditions, as the battery voltage
degrades, it is important to maintain the control loop
bandwidth, so the collector voltage quickly follows
VRAMP. This is done by adjusting VRAMP, such that:
V RAMP ≤ 0.38 ⋅ V BATT + 0.20 ≤ 1.6V
Equation 2
The effect of the loop bandwidth slowing can be seen
most clearly in the switching transients
measurement. This adjustment can be incorporated
in the software of the final application, so that
performance is enhanced under low voltage
conditions. Another advantage of this control scheme
is the improved noise performance due to individual
stages being held in compression, thus improving
the overall receive band noise performance.
Figure 2: Voltage Control Loop
2
Application Note - Rev 0
05/2006
WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
DESIGN RECOMMENDATIONS
1. RF DECOUPLING
To comply with any potential radiated contribution
around the PAM mostly related to PCB design it is
good practice to place two RF decoupling ceramic
capacitors on pins 7 and 22 of the AWT6223, as
close to the PAM as possible. Also, provision should
be made to bypass VBATT at the battery connector
with the same level of decoupling. This can be
accomplished by placing two ceramic capacitors
similar to the one at VBATT pin 7 as close as possible
to the VBATT pin of the battery connector. The grounding
of any bypass capacitors is critical to achieve the
expected improvements. All GND for decoupling
capacitors should be connected to the main GNDplane as directly as possible preferably using
microvias, see PCB cutout in Figure 3.
MICROVIA
COMPONENTS
SIGNAL
SIGNAL
GND
Buried or through VIA
Figure 3: Example of GND Connection from Pad
Using Microvia and Buried Via
The values of the capacitors chosen depend on their
location, grounding and physical size. These factors
determine the resonant frequency at which the
decoupling is most effective. One cap should be
optimized for the GSM850/900 band performance,
the other for DCS/PCS and IMT. The VBATT line to the
AWT6223 pin 7 should be kept as short as possible
and preferably shielded inside the PCB between
two GND layers (known as stripline) in order to
suppress radiation originating from this line.
2. INPUT POWER
The GSM quad-band part of the AWT6223 operates
over the 0 to 5 dBm range and has been optimized
for best typical performance at 3 dBm. ANADIGICS
recommends a series capacitor and resistive
attenuator to be placed between the VCO and PAM.
Some transceiver designs require a capacitor
between the VCO output of the chipset and the
resistive attenuator on the input of the PAM because
the VCO output has a DC offset. The attenuator helps
prevent load pulling of the VCO by further improving
the return loss and also gives the flexibility to optimize
the input drive to the PAM, which will depend on the
transceiver output power and losses prior to the
PAM. In order to avoid potential problems of strong
RF signals radiating or coupling to other parts of
the circuit, we suggest that the attenuator is placed
as close to the transceiver chip VCO output as
possible.
3.
MATCHING BETWEEN PA AND ANTENNA
SWITCH MODULE (ASM)
Provision should be made for matching the ASM to
50 ohms. Though the ASM usually specifies 50
ohms impedance, the actual impedance on the PCB
is dependent on the transmission lines at the PA
interface, and any vias and buried layer routing. This
is rarely 50 ohms. A T-network or π-network is
recommended for the matching as this gives
sufficient flexibility. The component values used for
the matching are dependent on the impedance
presented by the ASM used.
4. ANALOG RAMP VOLTAGE INPUT
At the GSM/EDGE VRAMP input pin 9 an analog voltage
to set the output power can be applied directly from
a standard DAC output. In order to smooth this DAC
output staircase shaped signal, it is recommended
to make provision for an RC filter. The values for this
filtering depend on DAC bit resolution and sample
rate.
5. WCDMA PA OUTPUT
The majority of current WCDMA design solutions
using ANADIGICS WCDMA PAMs do not require an
isolator or circulator in the front end design in order
to reject any reverse signal from the duplex filter
and antenna due to mismatch. The impedance of
the duplex filter often changes in the upper TX
frequency area where it is about to cut off and filter
out TX signals in order to separate TX & RX as the
upper TX frequency is closest to the RX frequency
area. The load deviates from the ideal 50 Ω and
increases the mismatch seen at the WCDMA PA
output. The WCDMA PA stage would in most cases
be able to address this, by decreasing output power
by a relative small level of approximately 1 dB.
Application Note - Rev 0
05/2006
3
WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
6. PCB LAYOUT GROUNDING
To have the best performance good grounding is
crucial. “Local ground planes” only connected to the
board GND plane using a few microvias is not
adequate. All GND planes must be connected to the
main GND layer (one designated inner layer of the
PCB) using a lot of through vias. Besides being the
reference for the all RF and other signals, the GND
plane is also used to distribute the heat dissipated
by the PA and should therefore by sufficient size and
with many through vias to spread the heat to other
copper layers. In order to establish a good ground
connection for the PA, it is necessary to assign an
area on the first inner layer to GND. Microvias will go
from the large GND pad under the PA to area on the
first inner layer and buried or through vias will go the
rest of the way to the ground plane in the center of
the board, see Figure 4.
GND Land area for shielding
VIAS
Figure 5: Example of Via Placement at Shielding
Landpattern Close to PA
PA
Components
Signal
Signal
GND
Power
GND
Signal
Keypad
Figure 4: Example of PCB Stack-up with
Microvias from Top and Bottom Layers, Buried
Vias from Layers 2 to 7, and through Vias from
Layers 1 to 8
For more information regarding GND underneath
the PAM, please refer to the “Package Outline”
chapter and the “Soldering Guidelines for Module
PCB Mounting - Application Note”. Generally, it is
good practice to establish shielding around the PAM.
For the shielding to be effective, a good GNDconnection is also required. Therefore a lot of vias
4
should be used, where the shielding - whether it is
metallized plastic or a traditional shielding can - is
attached to the PCB surface, see Figure 5.
7. RF TRACKS
Keep RF tracks as short as possible and with as
few corners and bends as possible. Make openings
in the ground plane on the layer just under all 50 Ω
pads on the PAM and the FEM (Front End Module) or
ASM (Antenna Switch Module). The short distance
between the component layer and the first inner layer
is very small compared to the pad size. This means
that the pads terminate the transmission line in low
impedance much different from that of the track. The
result of this is mismatch, which again can result in
up to a couple of dB loss of signal. As a rule,
ANADIGICS normally uses a clearance around
microstrips and striplines of at least the same width
as that of the track, i.e. if the track width is 12 mil or
0.3 mm, the clearance on both sides of the track to
any copper area is also 12 mil or 0.3 mm.
Generally, keep reference clock signals, digital
signals or analog I and Q signals away from RF
tracks and VBATT connections. Make sure that these
different signal connections are not running directly
underneath or above RF tracks or VBATT.
Application Note - Rev 0
05/2006
WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
SETUP RECOMMENDATIONS
1. TIMING
In order to meet the ETSI specified GSM power time
template and switching transients, the sequence of
events outlined in Figure 6 is recommended. The
timing on BS is not critical; it just needs to be enabled
and settled prior to TX_EN going high (approx. 2
µs). The PA “forward isolation 1” parameter and the
ASM isolation is met outside burst. The PA “forward
isolation 2” parameter ensures the time template is
met during the burst with sufficient margin. The
timing of the TX_EN is critical to ensure the
application has sufficient margin for meeting the
burst timing requirement.
GSM TRANSMITTED POWER vs TIME
dBm
-28
-18
0
V
543
t ( s)
VBATT
BS
TX_EN = HIGH
VRAMP
Raised cosine for
optimum spectral
efficiency
y
TX_EN = HIGH
y
Start VRAMP approx 18us
prior to useful part of burst
t
y
End VRAMP approx 12us
before end of guard period
y
TX_EN = LOW
12µs
Figure 6: Timing Recommendations
Application Note - Rev 0
05/2006
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WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
The change in the power ramping profile could
possible conflict with the timing of the PA. Since the
signals like TX_EN are usually used to improve the
isolation, it can be quite close to the VRAMP ramping
signal. If one of the control signals for instance
controlling the ASM conflicts with the VRAMP signal, it
usually results in a very poor switching spectrum
performance. In order to verify this, all the control
signals for ASM, TX_EN, BS, etc. should be
compared with the V RAMP profile using an
oscilloscope. The timing can affect the switching
spectrum and the isolation. In Figure 7 and Figure 8
the yellow line [1] is the VRAMP signal and the blue
line [2] is TX_EN and the figures shows an example
of how logic signals should be kept clear of the power
ramping signal VRAMP.
ramping with regard to switching spectrum when
developing a new platform. This is usually not a
complicated task, and can be done manually in a
few hours. The power ramping affects switching
spectrum, and have to meet the Power Time
Template specified by ETSI. Generally, the ramp
profile should be as close as possible to a raised
cosine waveform. In Figures 9 and 10 is an example
of a smooth shaped power ramping profile resulting
in good switching spectrum performance, see Figure
11. The example is based on measurement results
from a radio demo platform based on an ANADIGICS
PAM and a standard tranceiver. ANADIGICS
baseband emulator provides the baseband signals
for this platform.
2. POWER RAMPING
Since different PAMs have different control
characteristics, it is necessary to optimize the power
Figure 7: Close in of the Timing With a Proper
Distance Between the Logic TX_EN Signal and the
Analog Power Ramping Signal VRAMP
Figure 9: Rising Edge of a Smooth Power Ramping
Profile Example Resulting in Good Switching
Spectrum
Figure 8: Total Timing Period of TX_EN & VRAMP With
a Proper Distance Between the Logic TX_EN Signal
and the Analog Power Ramping Signal VRAMP
Figure 10: Falling Edge of a Smooth Power Ramping
Profile Example Resulting in Good Switching
Spectrum
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WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
(+2.4 V) at the VMODE pin places the PA in Low Bias
Mode. Applying a logic low (0 V) to both the VEN and
V MODE pins places the amplifier in shutdown
(standby) mode. The Bias-mode switching of
HELPTM can also be used with a DC-to-DC converter
controlling the VCC_WCDMA pin 21 to achieve further
efficiencies at power levels under 7 dBm, though
using HELP TM alone offers the best balance of
improved performance, efficient use of board space,
and low bill of materials cost.
EVALUATION BOARD
Figure 11: Corresponding Switching Spectrum
Result from the Total Power Ramping Profile
Shown in Figures 9 and 10
3. OUTPUT MATCHING OF THE PAM
Different PAMs will have different load contours. This
means that they, in order to function at optimum
performance, need to have the output matching
circuitry tuned. The way to do this is by using the
load contours provided by ANADIGICS and a network
analyzer and then tune the matching network on the
PA output toward the desired performance. The
optimum matching network will usually be a
compromise between output power, current
consumption, and conducted harmonics. In most
cases, 50 Ω is the best choice. In extreme cases,
the switching spectrum can be affected if a poor
matching solution is selected. This case is caused
by the poor matching solution resulting in limiting
the output power to the point of conflict with the ETSI
specifications and forces the PAM close to
saturation.
The evaluation board is a multilayer board using
GETEK substrate, which is similar to FR4, but has a
more controlled dielectric constant. The board
thickness is 1.57 mm. All the routing is on the top
layer (1) of a 4 layer board with a distance of 0.36
mm to the ground plane, which is on layer 2. All RF
routing has been sized to present a 50 Ω impedance.
WCDMA MODE RECOMMENDATION
In order to maximize performance the ANADIGICS
HELPTM WCDMA Bias Control functionality should
be used, which enables the WCDMA PA to operate
in Low and High Bias Modes thereby optimizing
current consumption. Applying a logic level at VEN
pin 3 or VMODE pin 2 corresponding to the desired
PA Bias control mode controls this feature. Operation
in the High Bias Mode allows the PA to exceed the
system performance requirements at output power
levels from +16 dBm to +28.5 dBm. For an output
power less than +16 dBm, the Low Bias Mode should
be used to minimize quiescent current while
maintaining system performance. Setting the VEN
logic high (+2.4V) and logic low (0 V) at the VMODE pin
places the PA in High Bias Mode, and a logic high
Application Note - Rev 0
05/2006
7
WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
Figure 12: Evaluation Board Layout
Figure 13: Evaluation Board Structure
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WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
CEXT2
21
VCC_WCDMA
2
20
WCDMA_OUT
VEN
3
19
GND
DCS/PCS_IN
4
18
DCS/PCS_OUT
BS
5
17
GND
WCDMA_IN
1
VMODE
22
GND
TX_EN
6
16
GND
VBATT
7
15
CEXT3
CEXT1
8
14
GND
VRAMP
9
13
GND
GSM_IN
10
12
GSM_OUT
11
VCC_GSM
Figure 14: Pinout (X- Ray View)
Table 1: Pin Description
PIN
NAME
1
WCDMA_IN
2
VMODE
3
DESCRIPTION
PIN
NAME
DESCRIPTION
WCDMA RF Input
12
GSM_OUT
WCDMA Mode Control
Voltage
13
GND
Ground
V EN
WCDMA Shutdown
14
GND
Ground
4
DCS/PCS_IN
DCS/PCS RF Input
15
CEXT3
Bypass
5
BS
Band Select Logic Input
16
GND
Ground
6
TX_EN
TX Enable Logic Input
17
GND
Ground
7
VBATT
Battery Supply
18
8
CEXT1
Bypass
19
GND
9
VRAMP
Analog signal used to
control the GSM output
power
20
WCDMA_OUT
10
GSM_IN
GSM850/900 RF Input
21
VCC_WCDMA
11
VCC_GSM
VCC test point for GSM
secton. Do not connect.
Do not ground.
22
CEXT2
GSM850/900 RF Output
DCS/PCS_OUT DCS/PCS RF Output
Application Note - Rev 0
05/2006
Ground
WCDMA RF Output
WCDMA Supply Voltage
Bypass
9
WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
PACKAGE OUTLINE
Figure 16: Package Outline - 22 Pin 6 mm x 8 mm x 1 mm Surface Mount Package
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WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
Figure 17: Recommended PCB Metal
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WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
Figure 18: Recommended PCB Solder Mask
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WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
Figure 19: Recommended Stencil Aperture
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WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
NOTES
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WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
NOTES
Application Note - Rev 0
05/2006
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WCDMA/GSM/GPRS/Polar EDGE Power Amplifier Module
ANADIGICS, Inc.
141 Mount Bethel Road
Warren, New Jersey 07059, U.S.A.
Tel: +1 (908) 668-5000
Fax: +1 (908) 668-5132
URL: http://www.anadigics.com
E-mail: [email protected]
IMPORTANT NOTICE
ANADIGICS, Inc. reserves the right to make changes to its products or to discontinue any product at any time without
notice. The product specifications contained in Advanced Product Information sheets and Preliminary Data Sheets are
subject to change prior to a product’s formal introduction. Information in Data Sheets have been carefully checked and are
assumed to be reliable; however, ANADIGICS assumes no responsibilities for inaccuracies. ANADIGICS strongly urges
customers to verify that the information they are using is current before placing orders.
WARNING
ANADIGICS products are not intended for use in life support appliances, devices or systems. Use of an ANADIGICS
product in any such application without written consent is prohibited.
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