CN-0147: Powering a Fractional-N Voltage Controlled Oscillator (VCO) with...

CN-0147: Powering a Fractional-N Voltage Controlled Oscillator (VCO) with...
Circuit Note
CN-0147
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/CN0147.
ADF4350
Fractional-N PLL IC with Integrated VCO
ADP150
Low Noise 3.3 V LDO
Powering a Fractional-N Voltage Controlled Oscillator (VCO) with Low Noise
LDO Regulators for Reduced Phase Noise
EVALUATION AND DESIGN SUPPORT
Wideband voltage controlled oscillators (VCOs) may have
increased sensitivity to power supply noise, hence, ultralow
noise regulators are recommended for best performance.
Circuit Evaluation Boards
ADF4350 Evaluation Board (EVAL-ADF4350-EB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
The circuit shown in Figure 1 utilizes the ADF4350, a fully
integrated fractional-N PLL and VCO that can generate
frequencies from 137.5 MHz to 4400 MHz. The ADF4350 is
powered from the ultralow noise 3.3 V ADP150 regulator for
optimal LO phase noise performance.
CIRCUIT FUNCTION AND BENEFITS
This circuit uses low noise, low dropout (LDO) linear regulators
to supply power to a wideband integrated PLL and VCO.
VVCO
16
CIN
1µF
1
VIN
VOUT 5
ADP150
2
ON
GND
17
VVCO
VOUT = 3.3V
28
10
DVDD AVDD
30
26
25
4
6
32
CE PDB RF VP SDV DD MUXOUT LD
1nF 1nF
COUT
1µF
FREF IN
29 REF IN
51Ω
RFOUTB+ 14
RFOUTB– 15
1 CLK
3
EN
VVCO
2 DATA
NC 4
3.9nH
3.9nH
NC = NO CONNECT
VIN = 5.5V
CIN
1µF
1
VIN
2
GND
3
EN
VOUT 5
ADP150
ON
NC 4
VOUT = 3.3V
COUT
1µF
SPI-COMPATIBLE SERIAL BUS
3 LE
OFF
1nF
RFOUTA+ 12
ADF4350
22 RSET
RFOUTA– 13
4.7kΩ
1nF
VTUNE 20
680Ω
CPOUT 7
39nF
2700pF
SW 5
OFF
NC = NO CONNECT
CPGND SDGND AGND AGNDVCO
8
31
9
11 18
DGND
21
27
10pF
1200pF
360Ω
TEMP VCOM VREF
19
23
0.1µF 10pF
24
0.1µF 10pF
0.1µF
Figure 1. ADP150 Regulators Connected to ADF4350 (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. C
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08876-001
VIN = 5.5V
LOCK
DETECT
VDD
CN-0147
Circuit Note
Table 1. ADF4350 VCO Pushing
VCO Frequency (MHz)
2200
3300
4400
VTUNE (V)
2.5
2.5
2.5
VCO Pushing (MHz/V)
0.73
1.79
5.99
POWER SUPPLY
SPECTRUM ANALYZER
[R&S FSUP 26]
5.5V
EVAL-ADF4350EB1Z
REV B BOARD
RF OUT
PC
Figure 2. EVAL-ADF4350EB1Z Rev. B Evaluation Board
Featuring ADP150 Low Noise Regulators
08876-003
08876-002
USB
CABLE
Figure 3. ADF4350 Measurement Setup
The lower integrated rms noise of the ADP150 LDO of only
9 µV rms (10 Hz to 100 kHz) helps to minimize VCO phase
noise and reduce the impact of VCO pushing (the VCO
equivalent of power supply rejection).
Figure 2 shows a photo of the evaluation board, which uses the
ADP150 LDOs to power the ADF4350. The ADP150 represents
the industry’s lowest noise LDO in the smallest package at the
lowest cost. It is available in a 4-ball, 0.8 mm × 0.8 mm, 0.4 mm
pitch WLCSP or a convenient 5-lead TSOT package. Adding the
ADP150’s to the design, therefore, has minimal impact on
system cost and board area while providing a significant
improvement in phase noise.
CIRCUIT DESCRIPTION
The ADF4350 is a wideband PLL and VCO consisting of
three separate multiband VCOs. Each VCO covers a range of
approximately 700 MHz (with some overlap between VCOs).
Lower frequencies are generated by output dividers.
Experiments showed pushing to be at its maximum at
4.4 GHz VCO output frequency, so the comparison of
VCO performance with different regulators was made at this
frequency. Rev. A evaluation boards of the ADF4350 used the
ADP3334 LDO regulator. The integrated rms noise of this
regulator is 27 µV (integrated from 10 Hz to 100 kHz). This
compares to 9 µV for the ADP150, which is used on the
EVAL-ADF4350EB1Z, Rev B. In order to measure the impact
of the power supply noise, a narrow PLL loop bandwidth (10
kHz) was used to facilitate greater examination of VCO phase
noise. A diagram of this setup is shown in Figure 3. A more
detailed examination of the output noise density with frequency
is available from the data sheets of both the ADP3334 and
ADP150.
Figure 4 shows that the noise spectral density of the ADP3334
regulator is 150 nV/√Hz at 100 kHz offset. The same plot for
the ADP150 (Figure 5) shows 25 nV/√Hz.
VCO pushing is measured by applying a steady dc tuning voltage
to the ADF4350 VTUNE pin, varying the power supply voltage,
and measuring the frequency change. The pushing figure (P)
equals the frequency delta divided by the voltage delta, as shown
in Table 1.
The formula for calculating the degradation in phase noise due
to the power supply noise is as follows:
In a PLL system, higher VCO pushing means that power supply
noise will degrade the VCO phase noise. If VCO pushing is low,
then power supply noise will not significantly degrade phase
noise. However, for high VCO pushing, noisy power supplies
will have a measurable impact on phase noise performance.
Where L(LDO) is the noise contribution from the regulator to the
VCO phase noise (in dBc/Hz), at an offset fm; P is the VCO
pushing figure in Hz/V; Sfm is the noise spectral density at a
given frequency offset in V/√Hz; and fm is the frequency offset
at which the noise spectral density is measured in Hz.
 P × Sfm 

L(LDO) = 20 log 
 2 × fm 


Rev. C | Page 2 of 4
CN-0147
100
Table 2. Calculation and Measurement of VCO Noise
VOUT = 2.2V
IL = 1mA
Noise contribution from regulator
(nV/√Hz)
Noise contribution from regulator
(dBc/Hz)
Total calculated noise at VCO output
(dBc/Hz)
Measured VCO noise at 100 kHz offset
(dBc/Hz)
10
CL = 10µF
CNR = 10nF
CL = 10µF
CNR = 0
1
CL = 1µF
CNR = 0
0.1
CL = 1µF
CNR = 10nF
ADP3334
ADP150
150
25
−104
−119.5
−103
−109.5
−102.6
−108.5
0.01
0.001
10
100
1k
10k
100k
08876-004
VOLTAGE NOISE SPECTRAL DENSITY (µV/ Hz)
Circuit Note
1M
FREQUENCY (Hz)
Figure 4. ADP3334 Output Noise Spectrum
0.501
VOUT = 1.8V
VOUT = 2.8V
VOUT = 3.3V
0.451
0.401
NOISE (µV/ Hz)
0.351
0.301
0.251
0.201
08876-006
0.151
0.101
0.051
100
1k
10k
100k
FREQUENCY (Hz)
08876-005
Figure 6. ADF4350 Phase Noise at 4.4 GHz with ADP3334 Regulators
0.001
10
Figure 5. ADP150 output noise spectrum
The noise contribution from the supply is then rss summed
with the noise contribution of the VCO (itself measured with a
very low noise supply) to give the total noise at the VCO output
with a given regulator.
These noise performances are rss summed together to give the
expected VCO phase noise:
L(TOTAL) = L(VCO)2 + L(LDO)2
[
L (TOTAL) = 10 log (10 LVCO / 20 ) + (10 LLDO / 20 )
2
2
]
In this example, a 100 kHz noise spectral density offset is
chosen, a 6 MHz/V pushing figure is used, and −110 dBc/Hz is
taken as the VCO noise with an ideal supply.
Using a dedicated signal source analyzer (like Rohde & Schwarz
FSUP), the VCO phase noise is compared. At 100 kHz offset the
ADP3334 delivers −102.6 dBc/Hz (Figure 6), and in the same
configuration the ADP150 measures −108.5 dBc/Hz (Figure 7).
08876-007
Or expressed in dB
Figure 7. ADF4350 Phase Noise at 4.4 GHz with ADP150 Regulators
The integrated phase noise improves from 1.95° to 1.4° rms
also. The measured results correlate very closely with the
calculations and clearly show the benefit of using the ADP150
with the ADF4350.
A complete design support package for this circuit note can be
found at http://www.analog.com/CN0147-DesignSupport.
Rev. C | Page 3 of 4
CN-0147
Circuit Note
COMMON VARIATIONS
LEARN MORE
Additional regulators can be added for greater isolation
between power supplies, if desired. Also, one ADP150
regulator can be used to power the entire ADF4350 part.
However, care needs to be taken in this case to ensure the
maximum rated current of the single ADP150 regulator is not
exceeded. This is possible if the lowest output power setting
on the ADF4350 is selected.
CN0147 Design Support Package:
http://www.analog.com/CN0147-DesignSupport
ADIsimPLL Design Tool
ADIsimPower Design Tool
Basso, C., C. Fourtet, and P. Kadanka. “Get the Best from Your
Low-Dropout Regulator.” EDN, 18 Feb. 1999.
CIRCUIT EVALUATION AND TEST
Data Sheets and Evaluation Boards
This circuit note, CN-0147, uses the EVAL-ADF4350EB1Z
board 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.
ADF4350 Data Sheet
Equipment Needed
Windows® XP, Windows, Vista (32-bit), or Windows 7 (32-bit)
PC with USB Port, the EVAL-ADF4350EB1Z, the ADF4350
programming software, 5.5 V power supply, and a spectrum
analyzer such as a Rhode and Schwartz FSUP26. See this circuit
note CN-0147 and UG-109 user guide for evaluation board
EVAL-ADF4350EB1Z and the ADF4350 data sheet.
Getting Started
This circuit note, CN-0147, contains a description of the circuit,
the schematic, and a block diagram of the test setup. The ser
guide, UG-109, details the installation and use of the EVALADF4350 evaluation software. UG-109 also contains board
setup instructions and the board schematic, layout, and bill of
materials.
ADF4350 Evaluation Board
ADP150 Data Sheet
ADP3334 Data Sheet
REVISION HISTORY
6/11—Rev. B to Rev. C
Changes to Circuit Description .......................................................2
11/10—Rev. A to Rev. B
Added Evaluation and Design Support Section ............................1
Added Circuit Evaluation and Test Section ...................................4
7/10—Rev. 0 to Rev. A
Changes to Figure 1 ...........................................................................1
Changes to Figure 3 ...........................................................................2
4/10—Revision 0: Initial Version
Functional Block Diagram
This circuit note, CN-0147, contains the function block
diagram of the described test setup in Figure 3.
Setup and Test
After setting up the equipment, standard RF test methods
should be used to measure the spectral purity of the output
signal.
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©2010–2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN08876-0-6/11(C)
Rev. C | Page 4 of 4
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