CN-0134: Broadband Low Error Vector Magnitude (EVM) Direct Conversion...

CN-0134: Broadband Low Error Vector Magnitude (EVM) Direct Conversion...
Circuit Note
CN-0134
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/CN0134.
ADF4350
Fractional-N PLL IC with Integrated VCO
ADL5375
Wideband Transmit Modulator
ADP150
Low Noise 3.3 V LDO
ADP3334
Low Noise Adjustable LDO
Broadband Low Error Vector Magnitude (EVM) Direct Conversion Transmitter
EVALUATION AND DESIGN SUPPORT
CIRCUIT FUNCTION AND BENEFITS
Circuit Evaluation Boards
CN-0134 Evaluation Board (CFTL-CN0134-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 500 MHz to 4.4 GHz are
supported through the use of a PLL with a broadband
integrated voltage controlled oscillator (VCO). Harmonic
filtering of the LO from the PLL ensures excellent quadrature
accuracy.
ADP150
1µF
5.5V
1µF
ADP3334
1µF
3.3V
VVCO
16
17
VVCO
FREF IN
28
10
DVDD AVDD
1µF
5.0V
VDD
I/Q SMA INPUTS
26
4
6
32
CE PDB RF VP SDV DD
1nF 1nF
29 REF IN
51Ω
VPS1, VPS2
RFOUTB+ 14
2 DATA
SPI-COMPATIBLE SERIAL BUS
VVCO
IBBN
RFOUTB– 15
1 CLK
ZBIAS
3 LE
ADF4350
ZBIAS
LOIP
RFOUTA+ 12
22 RSET
LOIN
RFOUTA– 13
4.7kΩ
VTUNE 20
CPOUT
CPGND SDGND AGND AGNDVCO
31
9
11 18
21
180Ω
7
SW 5
8
ADL5375
IBBP
22nF
DGND
330nF
QUADRATURE
PHASE
SPLITTER
RFOUT
QBBP
10nF
QBBN
82Ω
27
I/Q SMA INPUTS
08659-001
5.5V
Figure 1. Direct Conversion Transmitter (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. B
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Fax: 781.461.3113
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Circuit Note
08659-002
CN-0134
Figure 2. Evaluation Board for CN-0134 Direct Conversion Transmitter
Low noise LDOs ensure that the power management scheme
has no adverse impact on phase noise and EVM. This
combination of components represents industry-leading direct
conversion transmitter performance over a frequency range of
500 MHz to 4.4 GHz
CIRCUIT DESCRIPTION
The circuit shown in Figure 1 utilizes the ADF4350, a fully
integrated fractional-N PLL IC, and the ADL5375 wideband
transmit modulator. The ADF4350 provides the local oscillator
(LO) signal for the ADL5375 transmit quadrature modulator,
which upconverts analog I/Q signals to RF. Taken together, the
two devices provide a wideband baseband IQ to RF transmit
solution. The ADF4350 is powered off the ultralow noise 3.3 V
ADP150 regulator for optimal LO phase noise performance.
The ADL5375 is powered off a 5 V ADP3334 LDO. The
ADP150 LDO has an output voltage noise of only 9 µV rms
and helps to optimize VCO phase noise and reduce the impact
of VCO pushing (equivalent to power supply rejection).
Filtering is required on the ADF4350 RF outputs to attenuate
harmonic levels so as to minimize errors in the quadrature
generation block of the ADL5375. From measurement and
simulation, the odd order harmonics contribute more than even
order harmonics to quadrature error and, if attenuated to below
−30 dBc, results in sideband suppression performance of −40 dBc
or better. The ADF4350’s 2nd harmonic (2H) and 3rd harmonic
(3H) levels are as given in the data sheet and shown in Table 1.
To get the 3rd harmonic below -30 dBc, approximately 20 dB of
attenuation is required.
Table 1. ADF4350 RF Output Harmonic Levels Unfiltered
Harmonic Content
(Second)
Harmonic Content
(Third)
Harmonic Content
(Second)
Harmonic Content
(Third)
−13 dBc
Fundamental VCO
output
Fundamental VCO
output
−20 dBc
Divided VCO output
−10 dBc
Divided VCO output
−19 dBc
This circuit gives four different filter options to cover four
different bands. The filters were designed for a 100 Ω differential input (ADF4350 RF outputs with appropriate matching)
and 50 Ω differential output (ADL5375 LOIN differential
impedance). A Chebyshev response was used for optimal filter
roll-off at the expense of increased pass-band ripple.
The filter schematic is shown in Figure 3. This topology allows
the use of either a fully differential filter to minimize
component count, a single-ended filter for each output, or a
combination of the two. It was determined that for higher
frequencies (>2 GHz) two single-ended filters gave the best
performance because the series inductor values are twice the
value compared to a fully differential filter and, hence,
the impact of component parasitics is reduced. For lower
frequencies (<2 GHz), a fully differential filter provides
adequate results.
Rev. B | Page 2 of 5
Circuit Note
CN-0134
Table 2. ADF4350 RF Output Filter Component Values (DNI = Do Not Insert)
Frequency Range
(MHz)
ZBIAS
L1
(nH)
L2
(nH)
C1a
(pF)
C1c
(pF)
C2a
(pF)
C2c
(pF)
C3a
(pF)
C3c
(pF)
a.
500–1300
27 nH|| 50 Ω
3.9
3.9
DNI
4.7
DNI
5.6
DNI
3.3
b.
850–2450
19 nH || (100 Ω in position C1c)
2.7
2.7
3.3
100 Ω
4.7
DNI
3.3
DNI
c.
1250–2800
50 Ω
0Ω
3.6
DNI
DNI
2.2
DNI
1.5
DNI
d.
2800–4400
3.9 nH
0Ω
0Ω
DNI
DNI
DNI
DNI
DNI
DNI
0
NO FILTERING
FILTER B: 850MHz TO 2450MHz
–10
–20
SSB (dBc)
The filter should be designed with a cutoff approximately 1.2 to
1.5 times the highest frequency in the band of interest. This
allows margin in the design, as typically the cutoff will be lower
than designed due to parasitics. The effect of PCB parasitics can
be simulated in an EM simulation tool for improved accuracy.
–30
–40
–50
–60
–70
800
1000
1200
1400
1600
1800
2000
2200
2400
FREQUENCY (MHz)
08659-004
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. Note that it is
possible to place the parallel resistor as a differential component
(i.e. 100 Ω) in position C1c to minimize board space. This is
done in filter type c, described in Table 2.
Figure 4. Sideband Suppression for Filter b, 850 MHz to 2450 MHz
3.3V
ZBIAS
L1
RFOUTA+ 13
ZBIAS
RFOUTA– 12
ADF4350
C1a
C1c
L1
C1a
MAGNITUDE ERROR
(I/Q ERROR PHASE)
C2a
L2
C2c
L2
C2a
C3a
1nF
9
LOIP
10
LOIN
ERROR
VECTOR
MEASURED
SIGNAL
C3c
1nF
C3a
PHASE ERROR
(I/Q ERROR PHASE)
ADL5375
0
IDEAL SIGNAL
(REFERENCE)
Figure 3. ADF4350 RF Output Filter Schematic
I
As can be seen from Table 2, at lower frequencies below
1250 MHz, a 5th order filter is required. For 1.25 GHz to
2.8 GHz, 3rd order filtering is sufficient. For frequencies above
2.8 GHz, no filtering is required, as the harmonic levels are
sufficiently low to meet sideband suppression specifications.
08659-005
0.1µF
Q
120pF
08659-003
120pF
Figure 5. EVM Plot
A sweep of sideband suppression vs. frequency is shown in
Figure 4 for the circuit using Filter b (850 MHz to 2450 MHz).
In this sweep, the test conditions were the following:
baseband I/Q amplitude = 1 V p-p differential sine waves in
quadrature with a 500 mV (ADL5375-05) dc bias; baseband I/Q
frequency (fBB) = 1 MHz.
Rev. B | Page 3 of 5
CN-0134
Circuit Note
Table 3. Single-Carrier W-CDMA Composite EVM Results Comparing Filter vs. No Filter on ADF4350 RF Outputs
(Measured As Per 3GPP Specification Test Model 4)
Composite EVM No LO
Filtering
3.50%
3.40%
3.30%
Frequency (MHz)
2140
1800
900
Composite EVM with LO Filtering,
Filter C
1.80%
1.50%
0.90%
Error vector magnitude (EVM) is a measure of the quality of
the performance of a digital transmitter or receiver and is a
measure of the deviation of the actual constellation points from
their ideal locations, due to both magnitude and phase errors.
This is shown in Figure 5.
EVM measurements are given in Table 3 comparing results with
and without the filter. In this case the baseband I/Q signals were
generated using 3GPP test model 4 using a Rhode and Schwarz
AMIQ I/Q Modulation Generator with differential I and Q
analog outputs. Filter b was also used. A block diagram of the
test setup for EVM is given in Figure 6.
In addition to the improvement in sideband suppression and
EVM, there is also a performance benefit to driving the
ADL5375 LO inputs differentially. This improves modulator
OIP2 performance by 2 dB to 5 dB, compared with singleended LO drive. Note that most external VCOs only come with
a single-ended output, so using the differential outputs on the
ADF4350 provides a benefit over an external VCO in this case.
Figure 7 shows sideband suppression results using an 850 MHz
to 2450 MHz filter (filter b).
10
Adjacent channel leakage ratio (ACLR) is a measure of the
power in adjacent channels relative to the main channel power
and is specified in dBc.
SSB #10 +5dBm
SSB #10 +2dBm
SSB #10 –1dBm
SSB #10 –4dBm
0
–10
SSB (dBc)
The LO phase noise and the linearity of the modulator are the
main contributors to ACLR. The ACLR test setup is the same
as for EVM with the exception that coaxial filters were placed
on the I/Q outputs of the signal generator to reduce aliasing
products.
Modulator Output Power
(dBm)
−7
−7
−7
–20
–30
–40
–60
0
500
1000
1500
2000
2500
3000
3500
FREQUENCY (MHz)
R&S AMIQ GEN.
I+
I–
Q+
Figure 7. Sideband Suppression Results for 850 MHz to 2450 MHz Filter b
SPECTRUM ANALYZER
[R&S FSQ 8]
Q–
CN-0134
EVALUATION
BOARD
08659-004
–50
A complete design support package for this circuit note can be
found at http://www.analog.com/CN0134-DesignSupport.
RF OUT
COMMON VARIATIONS
POWER SUPPLY
Figure 6. EVM Measurement Setup (Simplified Diagram)
08659-006
5.5V
It is possible to use the auxiliary outputs on the ADF4350 to
switch between two filter types where wideband operation
beyond that possible with one single filter is required. This
is shown in Figure 8. An RF double-pole, 4-throw switch
(DP4T) is used to select the differential outputs of either
Filter 1 or Filter 2.
Rev. B | Page 4 of 5
Circuit Note
CN-0134
1nF
RFOUTA+ 13
FILTER 1
9
LOIP
10
LOIN
1nF
RFOUTA– 12
for software installation and test setup. Also see the AD4350
and ADL5375 data sheets for additional details.
LEARN MORE
DP4T
SWITCH
CN0134 Design Support Package:
http://www.analog.com/CN0134-DesignSupport
RFOUTB+ 14
FILTER 2
ADF4350
ADL5375
08659-008
RFOUTB– 15
Figure 8. Application Diagram Showing Possibility of Filter Switching Using
the ADF4350 Main and Auxiliary Outputs
CIRCUIT EVALUATION AND TEST
ADIsimPLL Design Tool
ADIsimPower Design Tool
ADIsimRF Design Tool
AN-0996 Application Note. The Advantages of Using a
Quadrature Digital Upconverter (QDUC) in Point-to-Point
Microwave Transmit Systems. Analog Devices.
The CFTL-0134-EVALZ evaluation board contains the circuit
described in circuit note CN-0134, allowing for the quick setup
and evaluation of the circuit’s performance. The control
software for the CFTL-0134-EVALZ board uses the standard
ADF4350 programming software, located on the CD that
accompanies the evaluation board.
AN-1039 Application Note. Correcting Imperfections in IQ
Modulators to Improve RF Signal Fidelity. Analog Devices.
Equipment Needed
ADL5375 Data Sheet
A standard PC running Windows® XP, Windows Vista (32-bit),
or Windows 7 (32-bit) with USB port, the CFTL-0134-EVALZ
circuit evaluation board, and the ADF4350 programming
software, power supplies, I-Q signal source, such as a Rhode &
Schwarz AMIQ, and a spectrum analyzer such as the Rhode &
Schwartz FSQ8. For additional details see the evaluation guide
(CN0134-EvalGuide-RevA.pdf), which is contained in the
design support package (http://www.analog.com/CN0134DesignSupport), and the ADF4350 and ADL5375 data sheets.
ADL5375 Evaluation Board
Getting Started
See CN0134-EvalGuide-RevA.pdf for software installation and
test setup. The documentation also includes the block diagram,
the application schematic, the bill of materials, and the layout
and assembly information. Also see the AD4350 and ADL5375
data sheets for additional details.
Functional Block Diagram
See Figure 1 and Figure 6 in circuit note CN-0134 and the
CN0134-EvalGuide-RevA.pdf, Wideband TX Modulator
Solution user document in the design support package.
Data Sheets and Evaluation Boards
ADF4350 Data Sheet
ADF4350 Evaluation Board
ADP150 Data Sheet
ADP3334 Data Sheet
REVISION HISTORY
11/10—Rev. A to Rev. B
Changes to Circuit Note Title .......................................................... 1
Added Evaluation and Design Support Section ............................ 1
Changes to Circuit Description Section......................................... 2
Changes to Figure 6 .......................................................................... 4
Added Circuit Evaluation and Test Section ................................... 5
9/10—Rev. 0 to Rev. A
Changes to Circuit Note Title .......................................................... 1
Changes to Circuit Function and Benefits Section....................... 1
Changes to Circuit Description Section......................................... 2
Changes to Common Variations Section ....................................... 4
1/10—Revision 0: Initial Version
Setup and Test
See circuit note CN-0134 and the CN0134-EvalGuideRevA.pdf, Wideband TX Modulator Solution user document,
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©2010 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN08659-0-11/10(B)
Rev. B | Page 5 of 5
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