Circuit Note CN-0285

Circuit Note CN-0285
Circuit Note
CN-0285
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0285.
ADF4351
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-0285 Evaluation Board (EVAL-CN0285-EB1Z)
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 using a phase-locked loop (PLL) with a broadband,
integrated voltage controlled oscillator (VCO). Harmonic filtering
of the local oscillator (LO) from the PLL ensures excellent
quadrature accuracy, sideband suppression, and low EVM.
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
ADL5375
IBBP
1nF 1nF
FREF IN
29 REF IN
51Ω
RFOUTB+ 14
VVCO
IBBN
RFOUTB– 15
1 CLK
2 DATA
ZBIAS
ZBIAS
LOIP
RFOUTA+ 12
ADF4351
22 RSET
LOIN
RFOUTA– 13
4.7kΩ
QUADRATURE
PHASE
SPLITTER
RFOUT
VTUNE 20
1kΩ
QBBP
CPOUT 7
47nF
2.7nF
SW 5
CPGND SDGND AGND AGNDVCO
8
31
9
11 18
21
QBBN
680pF
360Ω
DGND
27
I/Q SMA INPUTS
10921-001
SPI-COMPATIBLE SERIAL BUS
3 LE
Figure 1. Direct Conversion Transmitter (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. 0
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©2013 Analog Devices, Inc. All rights reserved.
Circuit Note
10921-002
CN-0285
Figure 2. Evaluation Board for CN-0285 Direct Conversion Transmitter
Low noise, low dropout regulators (LDOs) ensure that the power
management scheme has no adverse impact on phase noise and
EVM. This combination of components represents industryleading direct conversion transmitter performance over a
frequency range of 500 MHz to 4.4 GHz
CIRCUIT DESCRIPTION
The circuit shown in Figure 1 uses the ADF4351, a fully integrated
fractional-N PLL IC, and the ADL5375 wideband transmit
modulator. The ADF4351 provides the 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 ADF4351 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 ADF4351 RF outputs to attenuate
harmonic levels 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
second harmonic (2H) and third harmonic (3H) levels of the
ADF4351 are as given in the data sheet and shown in Table 1.
To get the third harmonic below −30 dBc, approximately 20 dB of
attenuation is required.
Table 1. ADF4351 RF Output Harmonic Levels Unfiltered
Harmonic Content
Second
Third
Second
Third
Value (dBc)
−19
−13
−20
−10
Description
Fundamental VCO output
Fundamental VCO output
Divided VCO output
Divided VCO output
This circuit gives four different filter options to cover four different
bands. The filters were designed with a 100 Ω differential input
(ADF4351 RF outputs with appropriate matching) and a 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.
Table 2. ADF4351 RF Output Filter Component Values (DNI = Do Not Insert)
Frequency Range (MHz)
500 to 1300 (Filter Type A)
850 to 2450 (Filter Type B)
1250 to 2800 (Filter Type C)
2800 to 4400 (Filter Type D)
ZBIAS
27 nH||50 Ω
19 nH||(100 Ω in Position C1c)
50 Ω
3.9 nH
L1
(nH)
3.9
2.7
0Ω
0Ω
Rev. 0 | Page 2 of 5
L2
(nH)
3.9
2.7
3.6
0Ω
C1a
(pF)
DNI
3.3
DNI
DNI
C1c
(pF)
4.7
100 Ω
DNI
DNI
C2a
(pF)
DNI
4.7
2.2
DNI
C2c
(pF)
5.6
DNI
DNI
DNI
C3a
(pF)
DNI
3.3
1.5
DNI
C3c
(pF)
3.3
DNI
DNI
DNI
Circuit Note
CN-0285
The ADF4351 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 (that is, 100 Ω) in Position
C1c to minimize board space (see Filter Type B, Table 2).
Q
MAGNITUDE ERROR
(I/Q ERROR PHASE)
ERROR
VECTOR
MEASURED
SIGNAL
PHASE ERROR
(I/Q ERROR PHASE)
0
I
Figure 5. EVM Plot
3.3V
A sweep of sideband suppression vs. frequency is shown in Figure 4
for the circuit using Filter Type B (800 MHz to 2400 MHz). In
this sweep, the test conditions were the following:
120pF
120pF
0.1µF
C1a
C2a
L1
RFOUTA+ 12
L2
C1c
ZBIAS
C2c
L1
RFOUTA– 13
L2
C1a
ADF4351
C2a
C3a
•
1nF
3
LOIP
•
C3c
1nF
4
C3a
LOIN
10921-003
ZBIAS
ADL5375
Figure 3. ADF4351 RF Output Filter Schematic
As can be seen from Table 2, at frequencies lower than 1250 MHz,
a fifth-order filter is required. For 1.25 GHz to2.8 GHz, third-order
filtering is sufficient. For frequencies more than 2.8 GHz, filtering
is not required because the harmonic levels are sufficiently low
to meet the sideband suppression specifications.
–20
5dBm
FILTER B: 850MHz TO 2450MHz
SIDEBAND SUPPRESSION (dBc)
–25
–30
–35
EVM measurements are given in Table 3 comparing the results
with and without the filter. In this case, the baseband I/Q signals
were generated using 3GPP Test Model 4 using a Rohde & Schwarz
AMIQ I/Q modulation generator with differential I and Q analog
outputs. Filter Type B was also used. A block diagram of the test
setup for the EVM is shown in Figure 6. For comparative purposes,
the ADF4350 is also measured. Lower EVM due to in-band PLL
noise improvements on the ADF4351 can be seen in Table 3. Other
contributing factors to the EVM improvement are the lower
phase frequency detector (PFD) spurious levels on the ADF4351.
–45
–50
–55
–60
1200
1400
1600
1800
2000
CARRIER FREQUENCY (MHz)
2200
2400
10921-004
–65
1000
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.
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 (see Figure 5).
–40
–70
800
IDEAL SIGNAL
(REFERENCE)
10921-005
Design the filter with a cutoff approximately 1.2 times to 1.5 times
the highest frequency in the band of interest. This cutoff allows
margin in the design, because typically the cutoff is lower than
designed due to parasitics. The effect of printed circuit board
(PCB) parasitics can be simulated in an electromagnetic (EM)
simulation tool for improved accuracy.
Figure 4. Sideband Suppression for Filter Type B, 850 MHz to 2450 MHz
Rev. 0 | Page 3 of 5
CN-0285
Circuit Note
Table 3. Single-Carrier W-CDMA Composite EVM Results Comparing Filter vs. No Filter on ADF4351 RF Outputs (Measured As
Per 3GPP Specification Test Model 4)
Frequency (MHz)
ADF4350 Composite
EVM No LO Filtering
ADF4350 Composite
EVM with LO Filtering, Filter B
ADF4351 Composite
EVM with LO Filtering, Filter B
2140
3.27%
1.31%
1.02%
1800
900
1.46%
10.01%
1.13%
1.03%
0.95%
0.96%
COMMON VARIATIONS
R&S AMIQ GEN.
I+
I–
Q+
It is possible to use the auxiliary outputs on the ADF4351 to
switch between two filter types where wideband operation beyond
that possible with one single filter is required (see Figure 8).
An RF double-pole, 4-throw switch (DP4T) is used to select
the differential outputs of either Filter 1 or Filter 2.
SPECTRUM ANALYZER
[R&S FSQ 8]
Q–
CN-0285
EVALUATION
BOARD
RF OUT
1nF
RFOUTA+ 12
5.5V
FILTER 1
3
LOIP
4
LOIN
1nF
RFOUTA– 13
DP4T
SWITCH
10921-006
RFOUTB+ 14
FILTER 2
RFOUTB– 15
Figure 6. EVM Measurement Setup (Simplified Diagram)
In addition to the improvement in sideband suppression and EVM,
there is also a performance benefit to driving the ADL5375
LO inputs differentially. This benefit improves modulator output
IP2 performance by 2 dB to 5 dB, compared with single-ended
LO drive. Note that most external VCOs only come with a singleended output, so using the differential outputs on the ADF4351
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 Type B).
Figure 8. Application Diagram Showing Possibility of Filter Switching Using
the ADF4351 Main and Auxiliary Outputs
CIRCUIT EVALUATION AND TEST
The EVAL-CN0285-EB1Z evaluation board contains the circuit
described in CN-0285, allowing for the quick setup and evaluation
of the performance of the circuit. The control software for the
EVAL-CN0285-EB1Z uses the standard ADF4351 programming
software located on the CD that accompanies the evaluation board.
The following equipment is needed:
–30
SIDEBAND SUPPRESSION (dBc)
ADL5375
Equipment Needed
–20
•
–40
•
•
•
•
•
–50
–60
–4dBm
–1dBm
+2dBm
+5dBm
–70
–80
1000
1200
1400
1600
1800
2000
CARRIER FREQUENCY (MHz)
2200
2400
A standard PC running Windows® XP, Windows Vista (32bit), or Windows 7 with a USB port
The EVAL-CN0285-EB1Z circuit evaluation board
The ADF435x programming software
5.5 V power supplies
An I-Q signal source, such as the Rohde & Schwarz AMIQ
A spectrum analyzer, such as the Rohde & Schwarz FSQ8
For additional details, see the UG-521 User Guide, the ADF4351
data sheet, and the ADL5375 data sheet.
10921-007
–90
800
ADF4351
10921-008
POWER SUPPLY
Figure 7. Sideband Suppression Results for 850 MHz to 2450 MHz Filter Type B
A complete design support package for this circuit note can be
found at http://www.analog.com/CN0285-DesignSupport.
Rev. 0 | Page 4 of 5
Circuit Note
CN-0285
Getting Started
Data Sheets and Evaluation Boards
See the UG-521 User Guide for software installation and test setup.
The user guide also includes the block diagram, the application
schematic, the bill of materials, and the layout and assembly
information. In addition, see the ADF4351 data sheet and the
ADL5375 data sheet for additional details.
ADF4351 Data Sheet
Functional Block Diagram
ADP150 Data Sheet
See Figure 1 and Figure 6 in this document, and also see the
UG-521 User Guide.
ADP3334 Data Sheet
ADF4351 Evaluation Board
ADL5375 Data Sheet
ADL5375 Evaluation Board
REVISION HISTORY
Setup and Test
After setting up the equipment, use standard RF test methods to
measure the sideband suppression of the circuit.
2/13—Revision 0: Initial Version
LEARN MORE
CN0285 Design Support Package:
http://www.analog.com/CN0285-DesignSupport
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.
AN-1039 Application Note. Correcting Imperfections in IQ
Modulators to Improve RF Signal Fidelity. Analog Devices.
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©2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN10921-0-2/13(0)
Rev. 0 | Page 5 of 5
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