CN-0144: Broadband Low Error Vector Magnitude (EVM) Direct Conversion...

CN-0144: Broadband Low Error Vector Magnitude (EVM) Direct Conversion...
Circuit Note
CN-0144
Devices Connected/Referenced
Circuits from the Lab™ tested circuit designs address
common design challenges and are engineered for
quick and easy system integration. For more information
and/or support, visit www.analog.com/CN0144.
ADF4350
Fractional-N PLL IC with Integrated VCO
ADL5385
Wideband Transmit Modulator
ADP150
Low Noise 3.3 V LDO
ADP3334
Low Noise Adjustable LDO
Broadband Low Error Vector Magnitude (EVM) Direct Conversion Transmitter Using
LO Divide-by-2 Modulator
EVALUATION AND DESIGN SUPPORT
CIRCUIT FUNCTION AND BENEFITS
Circuit Evaluation Boards
ADF4350 Evaluation Board (EVAL-ADF4350-EB1Z)
ADL5385 Evaluation Board (ADL5385-EVALZ)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
This circuit is a complete implementation of the analog portion of
a broadband direct conversion transmitter (analog baseband in,
RF out). RF frequencies from 68.75 MHz to 2.2 GHz are supported
through the use of a PLL with a broadband integrated voltage
controlled oscillator (VCO). Unlike modulators that use a
divide-by-1 LO stage (as described in CN-0134), harmonic
filtering of the LO is not required.
ADP150
5.5V
ADP3334
5.5V
1µF
1µF
1µF
1µF
3.3V
5.0V
VVCO
VDD
16
17
VVCO
28
10
DVDD AVDD
I/Q SMA INPUTS
26
4
6
32
CE PDB RF VP SDV DD
VPS1, VPS2
1nF 1nF
FREF IN
RFOUTB+ 14
VVCO
IBBN
RFOUTB– 15
1 CLK
2 DATA
ZBIAS
ZBIAS
3 LE
SPI-COMPATIBLE SERIAL BUS
ADL5385
IBBP
29 REF IN
51Ω
LOIP
RFOUTA+ 12
ADF4350
1nF
22 RSET
4.7kΩ
LOIN
RFOUTA– 13
DIVIDE-BY-2
QUADRATURE
PHASE
SPLITTER
RFOUT
1nF
VTUNE 20
QBBP
180Ω
CPOUT
7
QBBN
330nF
22nF
SW 5
8
SDGND AGND
31
9
AGNDVCO
11 18
21
DGND
27
I/Q SMA INPUTS
08835-001
CPGND
10nF
82Ω
Figure 1. Direct Conversion Transmitter (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. C
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CN-0144
Circuit Note
The circuit shown in Figure 1 utilizes the ADF4350, a fully
integrated fractional-N PLL IC, and the ADL5385 wideband
transmit modulator. The ADF4350 provides the local oscillator
(the LO is twice the modulator RF output frequency) signal for
the ADL5385 transmit quadrature modulator, which upconverts
analog I/Q signals to RF. Taken together, the two devices
provide a wideband baseband I/Q-to-RF transmit solution.
The ADF4350 is powered off the ultralow noise 3.3 V ADP150
regulator for optimal LO phase noise performance. The ADL5385
is powered off a 5 V ADP3334 LDO. The ADP150 LDO has an
output voltage noise of only 9 µV rms, integrated from 10 Hz to
100 kHz, and helps to optimize VCO phase noise and reduce
the impact of VCO pushing (equivalent to power supply
rejection). See CN-0147 for more details on powering the
ADF4350 with the ADP150 LDO.
The ADL5385 uses a divide-by-2 block to generate the quadrature
LO signals. The quadrature accuracy is, thus, dependent on the
duty cycle accuracy of the incoming LO signal (as well as the
matching of the internal divider flip-flops). Any imbalance in
the rise and fall times causes even order harmonics to appear, as
evident on the ADF4350 RF outputs. When driving the modulator
LO inputs differentially, even-order cancellation of harmonics
is achieved, improving the overall quadrature generation. (See
“Wideband A/D Converter Front-End Design Considerations:
When to Use a Double Transformer Configuration.” Rob
Reeder and Ramya Ramachandran. Analog Dialogue, 40-07.)
Because sideband suppression performance is dependent on the
modulator quadrature accuracy, better sideband suppression is
achievable when driving the LO input ports differentially vs.
single-ended. The ADF4350 has differential RF outputs
compared to a single-ended output available on most
competitor PLL devices with integrated VCO.
A sweep of sideband suppression versus RF output frequency
is shown in Figure 2. In this sweep, the test conditions were as
follows: baseband I/Q amplitude = 1.4 V p-p differential sine
waves in quadrature with a 500 mV dc bias; baseband I/Q
frequency (fBB) = 1 MHz; LO = 2 × RFOUT. A simplified
diagram of the test setup is shown in Figure 3. A modified
ADL5385 evaluation board was used because the standard
ADL5385 board does not allow a differential LO input drive.
0
ADF4350 AS LO SOURCE
DIFFERENTIAL CONNECTION
DATA SHEET SPECIFICATION
–10
–20
–30
–40
–50
–60
–70
0
500
1000
1500
FREQUENCY (MHz)
2000
2500
08835-002
CIRCUIT DESCRIPTION
The ADF4350 output match consists of the ZBIAS pull-up and, to
a lesser extent, the decoupling capacitors on the supply node. To
get a broadband match, it is recommended to use either a resistive
load (ZBIAS = 50 Ω) or a resistive in parallel with a reactive
load for ZBIAS. The latter gives slightly higher output power,
depending on the inductor chosen. Use an inductor value of 19 nH
or greater for LO operation below 1 GHz. The measured results
in this circuit were performed using ZBIAS = 50 Ω and an
output power setting of 5 dBm. When using the 50 Ω resistor,
this setting gives approximately 0 dBm on each output across
the full band, or 3 dBm differentially. The ADL5385 LO input drive
level specification is −10 dBm to +5 dBm; therefore, it should be
possible to reduce the ADF4350 output power to save current.
SIDEBAND SUPPRESSION (dBc)
To achieve optimum performance, the only requirement is that
the LO inputs of the modulator be driven differentially. The
ADF4350 provides differential RF outputs and is, therefore, an
excellent match. This PLL-to-modulator interface is applicable
to all I/Q modulators and I/Q demodulators that contain a
2XLO-based phase splitter. Low noise LDOs ensure that the
power management scheme has no adverse impact on phase
noise and error vector magnitude (EVM). This combination of
components represents industry-leading direct conversion
transmitter performance over a frequency range of 68.75 MHz to
2.2 GHz. For frequencies above 2.2 GHz, it is recommended to
use a divide-by-1 modulator, as described in CN-0134.
Figure 2. Sideband Suppression, RFOUT Swept from 68.75 MHz to 2200 MHz
This circuit achieves comparable or improved sideband
suppression performance when compared to driving the
ADL5385 with a low noise RF signal generator, as used in the
data sheet measurement. Using the differential RF outputs of
the ADF4350 provides even-order harmonic cancellation and
improves modulator quadrature accuracy. This impacts sideband
suppression performance and EVM (error vector magnitude).
A single carrier W-CDMA composite EVM of better than 2%
was measured with the circuit shown in Figure 1. The solution
thus provides a low EVM broadband solution for frequencies
from 68.75 MHz to 2.2 GHz. For frequencies above 2.2 GHz,
a divide-by-1 modulator block should be used, as described in
CN-0134.
A complete design support package for this circuit note can be
found at http://www.analog.com/CN0144-DesignSupport.
Rev. C | Page 2 of 4
Circuit Note
CN-0144
R&S AMIQ
IP
RFOUTA+
QP
QN
AD5385 EVALUATION BOARD
ADAPTED TO ACCEPT
RFOUT
DIFFERENTIAL LO INPUTS
ADF4350
EVALUATION BOARD
RFOUTA–
IN
LOIP
SPECTRUM
ANALYZER
LOIN
08835-003
5V
POWER SUPPLY
Figure 3. Sideband Suppression Measurement Test Setup (Simplified Diagram)
COMMON VARIATIONS
Functional Block Diagram
The PLL-to-modulator interface described in this circuit note is
applicable to all I/Q modulators that contain a 2XLO-based
phase splitter. It is also applicable to 2XLO-based I/Q
demodulators such as the ADL5387.
The CN-0144 contains the function block diagram of the described
test setup in Figure 3.
Equipment Needed
Windows® XP, Windows Vista (32-bit), or Windows 7 (32-bit) PC
with USB Port, the EVAL-ADF4350EB1Z, and the ADL5385EVALZ circuit evaluation boards, the ADF4350 programming
software, power supplies, I-Q signal source, such as a Rhode &
Schwarz AMIQ, and a spectrum analyzer. See the CN-0144 and the
UG-109 user guide for evaluation board EVAL-ADF4350EB1Z
and the ADF4350 and ADL5385 data sheets.
FURTHER IMPROVEMENTS WITH FILTERING
The sideband suppression of this circuit can be further improved
by filtering the LO signal before the LOIP and LOIN pins of the
ADL5385. Filtering attenuates harmonic levels so as to minimize
errors in the quadrature generation block of the ADL5385. At
some frequencies, this can result in improvements over 10 dB.
However, using a filter will limit the bandwidth of the circuit.
See Figure 4 for narrowband results.
0
–10
Getting Started
This circuit note contains a description of the circuit, the schematic,
and a block diagram of the test setup. The user guide UG-109
details the installation and use of the EVAL-ADF4350 evaluation
software. The UG-109 also contains board setup instructions
and the board schematic, layout, and bill of materials. The
ADL5385-EVALZ board schematic, block diagram, bill of
materials, layout, and assembly information is included in the
ADL5385 data sheet. See the ADF4350 and ADL5385 data sheet
for device information.
–20
–30
–40
–50
–60
NO FILTER
WITH FILTER
–70
–80
700
800
900
1000
1100
RFOUT (MHz)
1200
1300
08835-004
The CN-0144 uses the EVAL-ADF4350EB1Z and the
ADL5385-EVALZ boards for evaluation of the described circuit,
allowing for quick setup and evaluation. The EVAL-ADF4350EB1Z
board uses the standard ADF4350 programming software,
contained on the CD that accompanies the evaluation board.
After setting up the equipment, use standard RF test methods to
measure the sideband suppression of the circuit.
SIDEBAND SUPPRESSION (dBc)
CIRCUIT EVALUATION AND TEST
Setup and Test
Figure 4. Sideband Suppression Comparison With and Without a Harmonic Filter
The LO signal was passed through a low-pass filter with a 3 dB
point at approximately 2600 MHz. This results in a usable
output frequency up to approximately 1300 MHz.
Rev. C | Page 3 of 4
CN-0144
Circuit Note
LEARN MORE
Data Sheets and Evaluation Boards
CN0144 Design Support Package:
http://www.analog.com/CN0144-DesignSupport
ADF4350 Data Sheet
ADIsimPLL Design Tool
ADL5385 Data Sheet
ADIsimPower Design Tool
ADL5385 Evaluation Board
ADIsimRF Design Tool
ADP150 Data Sheet
Brandon, David, David Crook, and Ken Gentile. AN-0996
Application Note, The Advantages of Using a Quadrature
Digital Upconverter (QDUC) in Point-to-Point Microwave
Transmit Systems. Analog Devices.
ADP3334 Data Sheet
ADF4350 Evaluation Board
CN-0134, Broadband Low EVM Direct Conversion Transmitter.
Analog Devices.
CN-0147, Using the ADP150 LDO Regulators to Power the
ADF4350 PLL and VCO. Analog Devices.
Nash, Eamon. AN-1039 Application Note, Correcting
Imperfections in IQ Modulators to Improve RF Signal
Fidelity. Analog Devices.
Reeder, Rob, and Ramya Ramachandran. “Wideband
A/D Converter Front-End Design Considerations:
When to Use a Double Transformer Configuration.”
Analog Dialogue, 40-07.
REVISION HISTORY
10/12—Rev. B to Rev. C
Added Further Improvements with Filtering Section ..................3
11/10—Rev. A to Rev. B
Changes to Circuit Note Title ..........................................................1
Added Evaluation and Design Support Section ............................1
Changes to Figure 3 ...........................................................................3
Added Circuit Evaluation and Test Section ...................................3
8/10—Rev. 0 to Rev. A
Changes to Circuit Function and Benefits Section .......................1
Changes to Circuit Description Section .........................................2
Added Common Variations Section ...............................................3
3/10—Revision 0: Initial Version
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©2010–2012 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN08835-0-10/12(C)
Rev. C | Page 4 of 4
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