Datasheet ADF4251 Datasheet (Analog Devices)

Dual Fractional-N/Integer-N

Frequency Synthesizer

ADF4251

FEATURES

3.0 GHz Fractional-N/1.2 GHz Integer-N

2.7 V to 3.3 V Power Supply
Separate V

Allows Extended Tuning Voltage to 5 V

P

Programmable Dual Modulus Prescaler

RF: 4/5, 8/9

IF: 8/9, 16/17, 32/33, 64/65
Programmable Charge Pump Currents
3-Wire Serial Interface
Digital Lock Detect
Power-Down Mode
Programmable Modulus on Fractional-N Synthesizer
Trade-Off Noise versus Spurious Performance
Software and Hardware Power-Down

APPLICATIONS
Base Stations for Mobile Radio (GSM, PCS, DCS,

CDMA, WCDMA)
Wireless Handsets (GSM, PCS, DCS, CDMA, WCDMA,

PHS)
Wireless LANs
Communications Test Equipment
CATV Equipment

FUNCTIONAL BLOCK DIAGRAM

VDD1 VDD2 VDD3 DVDDVP1 VP2R

ADF4251

REF

IN

2

DOUBLER

GENERAL DESCRIPTION

The ADF4251 is a dual fractional-N/integer-N frequency
synthesizer that can be used to implement local oscillators
(LO) in the upconversion and downconversion sections of
wireless receivers and transmitters. Both the RF and IF syn­thesizers consist of a low noise digital PFD (phase frequency
detector), a precision charge pump, and a programmable refer­ence divider. The RF synthesizer has a ⌺-⌬ based fractional
interpolator that allows programmable fractional-N division.
The IF synthesizer has programmable integer-N counters. A
complete PLL (phase-locked loop) can be implemented if the
synthesizer is used with an external loop filter and VCO (volt­age controlled oscillator).

Control of all the on-chip registers is via a simple 3-wire inter­face. The devices operate with a power supply ranging from

2.7 V to 3.3 V and can be powered down when not in use.

SET

CE

CP

RF

4-BIT R

COUNTER

PHASE

FREQUENCY

DETECTOR

REFERENCE

CHARGE

PUMP

MUXOUT

CLK

DATA

LE

FROM
REFIN

OUTPUT

MUX

24-BIT

DATA

REGISTER

DOUBLER

2

A

GND

REV. 0

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.

LOCK

DETECT

FRACTIONAL N

RF DIVIDER

INTEGER N
IF DIVIDER

PHASE

FREQUENCY

15-BIT R

COUNTER

1 A

2 D

GND

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] www.analog.com
Fax: [phone] © 2003 Analog Devices, Inc. All rights reserved.

DETECTOR

GNDCPGND

1 CP

GND

2

CHARGE

PUMP

RFINA

RF

IN

IFINB

IF

IN

CP

IF

B

A

(VDD1 = VDD2 = VDD3 = DVDD = 3 V  10%, VP1 = VP2 = 5 V  10%, GND = 0 V,

1

R

ADF4251–SPECIFICATIONS

= 2.7 k, dBm referred to 50 , TA = T

SET

MIN

to T

, unless otherwise noted.)

MAX

Parameter B Version Unit Test Conditions/Comments

RF CHARACTERISTICS

RF Input Frequency (RF

A, RFINB)

IN

RF Input Sensitivity –10/0 dBm min/max
RF Input Frequency (RF

A, RFINB)

IN

2

2

0.25/3.0 GHz min/max

0.1/3.0 GHz min/max Input Level = –8/0 dBm min/max
RF Phase Detector Frequency 30 MHz max Guaranteed by Design
Allowable Prescaler Output Frequency 375 MHz max

IF CHARACTERISTICS

IF Input Frequency (IF

IN

A, IFINB)

2

50/1200 MHz min/max
IF Input Sensitivity –10/0 dBm min/max
IF Phase Detector Frequency 55 MHz max Guaranteed by Design
Allowable Prescaler Output Frequency 150 MHz max

REFERENCE CHARACTERISTICS

Input Frequency 250 MHz max For f < 10 MHz, use dc-coupled square

REF

IN

wave (0 to V

Input Sensitivity 0.5/VDD1V p-p min/max AC-coupled. When dc-coupled, use

REF

IN

Input Current ±100 µA max

REF

IN

0 to V

DD

).

DD

max (CMOS compatible).

REFIN Input Capacitance 10 pF max

CHARGE PUMP

Sink/Source High Value 4.375 mA typ See Table V

RF I

CP

Low Value 625 µA typ

IF I

Sink/Source High Value 5 mA typ See Table IX

CP

Low Value 625 µA typ

Three-State Leakage Current 1 nA typ

I

CP

RF Sink and Source Current Matching 2 % typ 0.5 V < V

< VP – 0.5

CP

IF Sink and Source Current Matching 2 % typ
I

CP

vs. V

CP

2% typ 0.5 V < VCP < VP – 0.5
ICP vs. Temperature 2 % typ VCP = VP/2

LOGIC INPUTS

, Input High Voltage 1.35 V min

V

INH

V

, Input Low Voltage 0.6 V max

INL

I

, Input Current ±1 µA max

INH/IINL

CIN Input Capacitance 10 pF max

LOGIC OUTPUTS

, Output High Voltage VDD – 0.4 V min IOH = 0.2 mA

V

OH

VOL, Output Low Voltage 0.4 V max IOL = 0.2 mA

POWER SUPPLIES

V

DD1, VDD

DV

1, VP2V

V

P

I

DD

2, VDD3 2.7/3.3 V min/V max

DD

3

RF + IF 13 mA typ 16 mA max

VDD1

1/5.5 V min/V max

DD

RF Only 10 mA typ 13 mA max
IF Only 4 mA typ 5.5 mA max

Low Power Sleep Mode/Power-Down 10 pA typ

RF NOISE AND SPURIOUS CHARACTERISTICS

Noise Floor –141 dBc/Hz typ @ 20 MHz PFD Frequency
In-Band Phase Noise Performance

Lowest Spur Mode –90 dBc/Hz typ RF
Low Noise and Spur Mode –95 dBc/Hz typ RF
Lowest Noise Mode –103 dBc/Hz typ RF

4

@ VCO Output

= 1.8 GHz, PFD = 20 MHz

OUT

= 1.8 GHz, PFD = 20 MHz

OUT

= 1.8 GHz, PFD = 20 MHz

OUT

Spurious Signals See Typical Performance Characteristics

NOTES

1

Operating Temperature Range (B Version): –40°C to +85°C.

2

Use a square wave for frequencies less than F

3

RF = 1 GHz, RF PFD = 10 MHz, MOD = 4095, IF = 500 MHz, IF PFD = 200 kHz, REF = 10 MHz, VDD = 3 V, VP1 = 5 V, and VP2 = 3 V.

4

The in-band phase noise is measured with the EVAL-ADF4251EB2 Evaluation Board and the HP5500E Phase Noise Test System. The spectrum analyzer provides the
REFIN for the synthesizer (f

Specifications subject to change without notice.

= 10 MHz @ 0 dBm). F

REFOUT

MIN

.

= 1.74 GHz, F

OUT

= 20 MHz, N = 87, MOD = 100, Channel Spacing = 200 kHz, VDD = 3.3 V, and VP= 5 V.

REF

REV. 0–2–

ADF4251

TIMING CHARACTERISTICS

*

(VDD1 = VDD2 = VDD3 = DVDD = 3 V  10%, VP1 = VP2 = 5 V  10%, GND = 0 V, unless otherwise noted.)

Limit at

to T

T

MIN

MAX

Parameter (B Version) Unit Test Conditions/Comments

t

1

t

2

t

3

t

4

t

5

t

6

t

7

*Guaranteed by design but not production tested.

CLOCK

DATA

10 ns min LE Setup Time
10 ns min DATA to CLOCK Setup Time
10 ns min DATA to CLOCK Hold Time
25 ns min CLOCK High Duration
25 ns min CLOCK Low Duration
10 ns min CLOCK to LE Setup Time
20 ns min LE Pulsewidth

DB23

(MSB)

t

4

DB22

t

3

DB2

t

2

t

5

DB1

(CONTROL BIT C2)

DB0 (LSB)

(CONTROL BIT C1)

t

7

LE

t

1

LE

t

6

Figure 1. Timing Diagram

REV. 0

–3–

ADF4251

ABSOLUTE MAXIMUM RATINGS

1, 2

(TA = 25°C, unless otherwise noted.)

VDD1, VDD2, VDD3, DVDD to GND3 . . . . . . . . –0.3 V to +4 V

REF

, RFINA, RFINB to GND . . . . . . –0.3 V to VDD + 0.3 V

IN

1, VP2 to GND . . . . . . . . . . . . . . . . . . . . . –0.3 V to +5.8 V

V

P

V

1, VP2 to VDD1 . . . . . . . . . . . . . . . . . . . . . –3.3 V to +3.5 V

P

Digital I/O Voltage to GND . . . . . . . . –0.3 V to V

Analog I/O Voltage to GND . . . . . . . . –0.3 V to V

+ 0.3 V

DD

+ 0.3 V

DD

Operating Temperature Range

Industrial (B Version) . . . . . . . . . . . . . . . . –40°C to +85°C

Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C

Maximum Junction Temperature . . . . . . . . . . . . . . . . . 150°C

LFCSP ␪

Thermal Impedance . . . . . . . . . . . . . . . . 122°C/W

JA

Soldering Reflow Temperature

Vapor Phase (60 sec max) . . . . . . . . . . . . . . . . . . . . . 240°C

IR Reflow (20 sec max) . . . . . . . . . . . . . . . . . . . . . . . 240°C

ORDERING GUIDE

Model Temperature Range Package Option*

ADF4251BCP –40ºC to +85ºC CP-24
ADF4251BCP-REEL –40ºC to +85ºC CP-24
ADF4251BCP-REEL7 –40ºC to +85ºC CP-24

*CP = Lead Frame Chip Scale Package

NOTES

1

Stresses above those listed under Absolute Maximum Ratings may cause perma­nent damage to the device. This is a stress rating only and functional operation of
the device at these or any other conditions above those listed in the operational
sections of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.

2

This device is a high performance RF integrated circuit with an ESD rating
of <2 kW, and it is ESD sensitive. Proper precautions should be taken for handling
and assembly.

3

GND = CP

GND

1, A

GND

1, D

GND

, A

2, and CP

GND

GND

2.

PIN CONFIGURATION

1

3

2

DD

DD

22 V

9

GND

D

2

P

DD

21 V

20 V

CLK 10

DATA 11

IF

19 CP

LE 12

18 CP
17 DV
16 IFINA
15 IF

IN

14 A

GND

13 R

SET

GND

DD

B

2

2

CP

RF

CP

1 2

GND

RF

A 3

IN

RFINB 4

A

1 5

GND

MUXOUT 6

1

1

P

24 V

23 V

PIN 1
INDICATOR

ADF4251

TOP VIEW

(Not to Scale)

7

IN

CE 8

REF

CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although the
ADF4251 features proprietary ESD protection circuitry, permanent damage may occur on devices
subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended
to avoid performance degradation or loss of functionality.

REV. 0–4–

ADF4251

PIN FUNCTION DESCRIPTIONS

Mnemonic Function

CP

RF

CP

1 RF Charge Pump Ground

GND

RF

A Input to the RF Prescaler. This small signal input is normally taken from the VCO.

IN

RF

BComplementary Input to the RF Prescaler

IN

A

1Analog Ground for the RF Synthesizer

GND

MUXOUT This multiplexer output allows either the RF or IF lock detect, the scaled RF or IF, or the scaled reference fre-

REF

IN

CE Chip Enable. A Logic Low on this bit powers down the device and puts the charge pump outputs into three-state.

D

GND

CLK Serial Clock Input. This serial clock is used to clock in the serial data to the registers. The data is latched into the

DATA Serial Data Input. The serial data is loaded MSB first with the three LSBs being the control bits. This input is a

LE Load Enable, CMOS Input. When LE goes high, the data stored in the shift registers is loaded into one of the

R

SET

A

2Ground for the IF Synthesizer

GND

IF

BComplementary Input to the IF Prescaler

IN

IF

A Input to the IF Prescaler. This small signal input is normally taken from the IF VCO.

IN

DV

DD

CP

2IF Charge Pump Ground

GND

CP

IF

V

2IF Charge Pump Power Supply. Decoupling capacitors to the ground plane should be placed as close as possible

P

V

2Positive Power Supply for the IF Section. Decoupling capacitors to the ground plane should be placed as close as

DD

V

3 Positive Power Supply for the RF Digital Section. Decoupling capacitors to the ground plane should be placed as close

DD

V

1 Positive Power Supply for the RF Analog Section. Decoupling capacitors to the ground plane should be placed as close

DD

V

1 RF Charge Pump Power Supply. Decoupling capacitors to the ground plane should be placed as close as possible

P

RF Charge Pump Output. This is normally connected to a loop filter that drives the input to an external VCO.

quency to be accessed externally.

Reference Input. This is a CMOS input with a nominal threshold of VDD/2 and an equivalent input resistance of
100 kW. This input can be driven from a TTL or CMOS crystal oscillator.

A Logic High on this pin powers up the device, depending on the status of the software power-down bits.

Digital Ground for the Fractional Interpolator

shift register on the CLK rising edge. This input is a high impedance CMOS input.

high impedance CMOS input.

seven latches, the latch being selected using the control bits.

Connecting a resistor between this pin and ground sets the minimum charge pump output current. The relationship
between I

I

CP MIN

Therefore, with R

=

and R

CP

1 6875.

R

SET

is:

SET

= 2.7 kW, I

SET

= 0.625 mA.

CP MIN

Positive Power Supply for the Fractional Interpolator Section. Decoupling capacitors to the ground plane should
be placed as close as possible to this pin. DV

must have the same voltage as VDD1, VDD2, and VDD3.

DD

IF Charge Pump Output. This is normally connected to a loop filter that drives the input to an external VCO.

to this pin. This voltage should be greater than or equal to V

possible to this pin. V

as possible to this pin. V

as possible to this pin. V

2 has a value 3 V ± 10%. VDD2 must have the same voltage as VDD1, VDD3, and DVDD.

DD

3 has a value 3 V ± 10%. VDD3 must have the same voltage as VDD1, VDD2, and DVDD.

DD

1 has a value 3 V ± 10%. VDD1 must have the same voltage as VDD2, VDD3, and DVDD.

DD

DD

2.

to this pin. This voltage should be greater than or equal to VDD1.

REV. 0

–5–

ADF4251

REF

MUXOUT

CLK

DATA

IN

LE

ADF4251

FROM
REFIN

VDD1 VDD2 VDD3 DVDDVP1 VP2R

A

GND

4-BIT R

COUNTER

LOCK

DETECT

15-BIT R

COUNTER

1 A

GND

2 D

PHASE

FREQUENCY

DETECTOR

FRACTIONAL N

RF DIVIDER

INTEGER N

IF DIVIDER

PHASE

FREQUENCY

DETECTOR

GNDCPGND

1 CP

DOUBLER

OUTPUT

MUX

24-BIT

DATA

REGISTER

DOUBLER

2

2

Figure 2. Detailed Functional Block Diagram

GND

REFERENCE

CHARGE

CHARGE

2

SET

PUMP

PUMP

CE

CP

RF

RFINA

RF

IN

IFINB

IF

IN

CP

IF

B

A

REV. 0–6–

Typical Performance Characteristics–ADF4251

0

VDD = 3V, VP = 5V

= 1.875mA

I

–10

CP

PFD FREQUENCY = 10MHz

–20

CHANNEL STEP = 200kHz
LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 10Hz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–2kHz

–1kHz 1.7518GHz 1kHz 2kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

–99.19dBc/Hz

TPC 1. Phase Noise Plot, Lowest Noise Mode,

1.7518 GHz RF

, 10 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

0

VDD = 3V, VP = 5V

–10

= 1.875mA

I

CP

PFD FREQUENCY = 10MHz
CHANNEL STEP = 200kHz

–20

LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 10Hz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–2kHz

–1kHz 1.7518GHz 1kHz 2kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

–90.36dBc/Hz

TPC 2. Phase Noise Plot, Low Noise and Spur
Mode, 1.7518 GHz RF

, 10 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

0

VDD = 3V, VP = 5V

= 1.875mA

I

–10

CP

PFD FREQUENCY = 10MHz

–20

CHANNEL STEP = 200kHz
LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 1kHz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–50dBc@

100kHz

–400kHz –200kHz 1.7518GHz 200kHz 400kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

TPC 4. Spurious Plot, Lowest Noise Mode,

1.7518 GHz RF

, 10 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

0

VDD = 3V, VP = 5V

–10

= 1.875mA

I

CP

PFD FREQUENCY = 10MHz
CHANNEL STEP = 200kHz

–20

LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 1kHz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–51dBc@

–400kHz

100kHz

–200kHz 1.7518GHz 200kHz 400kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

TPC 5. Spurious Plot, Low Noise and Spur Mode,

1.7518 GHz RF

, 10 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

0

VDD = 3V, VP = 5V

–10

= 1.875mA

I

CP

PFD FREQUENCY = 10MHz
CHANNEL STEP = 200kHz

–20

LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 10Hz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–2kHz

–1kHz 1.7518GHz 1kHz 2kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

–85.86dBc/Hz

TPC 3. Phase Noise Plot, Lowest Spur Mode,

1.7518 GHz RF

, 10 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

0

VDD = 3V, VP = 5V

–10

= 1.875mA

I

CP

PFD FREQUENCY = 10MHz
CHANNEL STEP = 200kHz

–20

LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 1kHz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–400kHz

–72dBc@

100kHz

–200kHz 1.7518GHz 200kHz 400kHz

TPC 6. Spurious Plot, Lowest Spur Mode,

1.7518 GHz RF

, 10 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

TPCs 1–12 attained using EVAL-ADF4252EB1 Evaluation Board; measurements from HP8562E spectrum analyzer.

REV. 0

–7–

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

ADF4251

0

VDD = 3V, VP = 5V

= 1.875mA

I

–10

CP

PFD FREQUENCY = 20MHz
CHANNEL STEP = 200kHz

–20

LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 10Hz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–2kHz

–1kHz 1.7518GHz 1kHz 2kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

–102dBc/Hz

TPC 7. Phase Noise Plot, Lowest Noise Mode,

1.7518 GHz RF

, 20 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

0

VDD = 3V, VP = 5V

= 1.875mA

I

–10

CP

PFD FREQUENCY = 20MHz
CHANNEL STEP = 200kHz

–20

LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 10Hz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–2kHz

–1kHz 1.7518GHz 1kHz 2kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

–93.86dBc/Hz

TPC 8. Phase Noise Plot, Low Noise and Spur
Mode, 1.7518 GHz RF

, 20 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

0

VDD = 3V, VP = 5V

= 1.875mA

I

–10

CP

PFD FREQUENCY = 20MHz
CHANNEL STEP = 200kHz

–20

LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 1kHz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–53dBc@

100kHz

–400kHz –200kHz 1.7518GHz 200kHz 400kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

TPC 10. Spurious Plot, Lowest Noise Mode,

1.7518 GHz RF

, 20 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

0

VDD = 3V, VP = 5V

= 1.875mA

I

–10

CP

PFD FREQUENCY = 20MHz
CHANNEL STEP = 200kHz

–20

LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 1kHz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–63.2dBc@

–400kHz

100kHz

–200kHz 1.7518GHz 200kHz 400kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

TPC 11. Spurious Plot, Low Noise and Spur Mode,

1.7518 GHz RF

, 20 MHz PFD Frequency, 200 kHz

OUT

Channel Step Resolution

0

VDD = 3V, VP = 5V

= 1.875mA

I

–10

CP

PFD FREQUENCY = 20MHz
CHANNEL STEP = 200kHz

–20

LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 10Hz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–2kHz

–1kHz 1.7518GHz 1kHz 2kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

–89.52dBc/Hz

TPC 9. Phase Noise Plot, Lowest Spur Mode,

1.7518 GHz RF

, 20 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

0

VDD = 3V, VP = 5V

= 1.875mA

I

–10

CP

PFD FREQUENCY = 20MHz
CHANNEL STEP = 200kHz

–20

LOOP BANDWIDTH = 20kHz
FRACTION = 59/100

–30

RBW = 1kHz

–40

–50

–60

OUTPUT POWER – dB

–70

–80

–90

–100

–72.33dBc@

–400kHz

100kHz

–200kHz 1.7518GHz 200kHz 400kHz

REFERENCE
LEVEL = – 4.2dBm

FREQUENCY

TPC 12. Spurious Plot, Lowest Spur Mode,

1.7518 GHz RF

, 20 MHz PFD Frequency,

OUT

200 kHz Channel Step Resolution

REV. 0–8–

ADF4251

–70

–75

–80

–85

LOWEST SPUR MODE

–90

–95

–100

LOW NOISE AND SPUR MODE

–105

PHASE NOISE – dBc/Hz

LOWEST NOISE MODE

–110

–115

–120

1.430 1.4601.435

1.440 1.445 1.450 1.455

FREQUENCY – GHz

TPC 13. In-Band Phase Noise vs. Frequency

–10

–20

–30

–40

LOWEST NOISE MODE

–50

–60

–70

–80

SPURIOUS LEVEL – dBc

–90

–100

LOWEST SPUR MODE

–110

1.430 1.4601.435

TPC 14. 100 kHz Spur vs. Frequency

1.440 1.445 1.450 1.455
FREQUENCY – GHz

*

–20

–30

–40

–50

–60

LOWEST NOISE MODE

–70

–80

–90

SPURIOUS LEVEL – dBc

–100

–110

LOWEST SPUR MODE

–120

1.430 1.4601.435

*

TPC 16. 400 kHz Spur vs. Frequency

–20

–30

–40

–50

–60

LOWEST NOISE MODE

–70

–80

–90

SPURIOUS LEVEL – dBc

–100

–110

LOWEST SPUR MODE

–120

1.430 1.4601.435

TPC 17. 600 kHz Spur vs. Frequency

1.440 1.445 1.450 1.455
FREQUENCY – GHz

*

1.440 1.445 1.450 1.455
FREQUENCY – GHz

*

–20

–30

–40

–50

–60

LOWEST NOISE MODE

–70

–80

–90

SPURIOUS LEVEL – dBc

–100

–110

LOWEST SPUR MODE

–120

1.430 1.4601.435

1.440 1.445 1.450 1.455
FREQUENCY – GHz

TPC 15. 200 kHz Spur vs. Frequency

*

–20

–30

–40

–50

–60

–70

–80

–90

SPURIOUS LEVEL – dBc

–100

LOWEST NOISE MODE

–110

LOWEST SPUR MODE

–120

1.430 1.4601.435

1.440 1.445 1.450 1.455
FREQUENCY – GHz

TPC 18. 3 MHz Spur vs. Frequency

*TPCs 13–18: Across all fractional channel steps from f = 0/130 to f = 129/130.

RF

= 1.45 GHz, Int Reg = 55, Ref = 26 MHz, and LBW = 40 kHz. TPCs 13–24 attained using EVAL-ADF4252EB2 Evaluation Board.

OUT

REV. 0

–9–

*

ADF4251

0

–5

–10

–15

–20

AMPLITUDE – dBm

–25

–30

–35

PRESCALER = 4/5

061

TPC 19. RF Input Sensitivity

0

VDD = 3V
VP2 = 3V

5

10

15

20

25

IF INPUT POWER – dBm

30

35

40

–0.4 1.60.1

TPC 20. IF Input Sensitivity

PRESCALER = 8/9

2345

FREQUENCY –GHz

0.6 1.1

IF INPUT FREQUENCY – GHz

–120

VDD = 3V

= 3V

V

P

–130

–140

–150

–160

PHASE NOISE – dB/Hz

–170

–180

10k 10M100k

PHASE DETECTOR FREQUENCY – Hz

1M

TPC 22. Phase Noise (Referred to CP Output) vs.
PFD Frequency, IF Side

6

4

2

VDD = 3V
VP1 = 5.5V

0

– mA

CP

I

–2

–4

–6

00.5

1.5

1.0 2.0 3.0

2.5
VCP – V

4.03.5 4.5 5.5

5.0

TPC 23. RF Charge Pump Output Characteristics

–120

VDD = 3V

= 5V

V

P

–130

–140

–150

–160

PHASE NOISE – dB/Hz

–170

–180

10k

100k

PHASE DETECTOR FREQUENCY – Hz

1M

10M

TPC 21. Phase Noise (Referred to CP Output) vs.
PFD Frequency, RF Side

6

4

2

V

= 3V

DD

VP2 = 3V

0

– mA

CP

I

–2

–4

–6

0

0.5

1.0 2.0 3.0

1.5

VCP – V

2.5

TPC 24. IF Charge Pump Output Characteristics

REV. 0–10–

ADF4251

CIRCUIT DESCRIPTION
Reference Input Section

The reference input stage is shown in Figure 3. SW1 and SW2
are normally closed switches. SW3 is normally open. When
power-down is initiated, SW3 is closed and SW1 and SW2 are
opened. This ensures that there is no loading of the REF

pin

IN

on power-down.

POWER-DOWN

CONTROL

100k

NC

NO

SW2

SW3

BUFFER

XOEB

TO R
COUNTER

REF

OUT

REF

IN

NC

SW1

NC = NORMALLY CLOSED
NO = NORMALLY OPEN

Figure 3. Reference Input Stage

RF and IF Input Stage

The RF input stage is shown in Figure 4. The IF input stage is
the same. It is followed by a two-stage limiting amplifier to
generate the CML clock levels needed for the N counter.

BIAS

GENERATOR

A

RF

IN

B

RF

IN

1.6V

V

1

DD

2k2k

A

GND

Figure 4. RF Input Stage

RF INT Divider

The RF INT CMOS counter allows a division ratio in the PLL
feedback counter. Division ratios from 31 to 255 are allowed.

INT, FRAC, MOD, and R Relationship

The INT, FRAC, and MOD values, in conjunction with the
RF R counter, make it possible to generate output frequencies
that are spaced by fractions of the RF phase frequency detector
(PFD). The equation for the RF VCO frequency (RF

RF F INT

=¥ +

OUT PFD

where RF

is the output frequency of external voltage controlled

OUT

Ê
Á

Ë

FRAC

MOD

ˆ
˜

¯

OUT

) is

(1)

oscillator (VCO).

1

D

+

FREF

=¥

PFD IN

()

R

(2)

REFIN = the reference input frequency, D = RF REFIN Doubler

Bit, R = the preset divide ratio of the binary 4-bit program­mable reference counter (1 to 15), INT = the preset divide ratio of
the binary 8-bit counter (31 to 255), MOD = the preset modulus
ratio of binary 12-bit programmable FRAC counter (2 to 4095),
and FRAC = the preset fractional ratio of the binary 12-bit
programmable FRAC counter (0 to MOD).

FROM RF

INPUT STAGE

RF N DIVIDER

N-COUNTER

INT

REG

N = INT + FRAC/MOD

THIRD-ORDER

FRACTIONAL

INTERPOLATOR

MOD

REG

FRAC

VALUE

TO PFD

Figure 5. N Counter

RF R Counter

The 4-bit RF R counter allows the input reference frequency
(REF

) to be divided down to produce the reference clock to

IN

the RF PFD. Division ratios from 1 to 15 are allowed.

IF R Counter

The 15-bit IF R counter allows the input reference frequency
(REF

) to be divided down to produce the reference clock to

IN

the IF PFD. Division ratios from 1 to 32767 are allowed.

IF Prescaler (P/P + 1)

The dual modulus IF prescaler (P/P + 1), along with the IF A
and B counters, enables the large division ratio, N, to be realized
(N = PB + A). Operating at CML levels, it takes the clock from
the IF input stage and divides it down to a manageable frequency
for the CMOS IF A and CMOS IF B counters.

IF A and B Counters

The IF A CMOS and IF B CMOS counters combine with the
dual modulus IF prescaler to allow a wide ranging division ratio
in the PLL feedback counter. The counters are guaranteed to
work when the prescaler output is 150 MHz or less.

Pulse Swallow Function

The IF A and IF B counters, in conjunction with the dual modulus
IF prescaler, make it possible to generate output frequencies
that are spaced only by the reference frequency divided by R.
See the Device Programming after Initial Power-Up section for
examples. The equation for the IF VCO (IF

IF P B A F

=¥

()

[]

OUT PFD

where IF

= the output frequency of the external voltage controlled

OUT

+

¥

) frequency is

OUT

(3)

oscillator (VCO), P = the preset modulus of the IF dual modulus
prescaler, B = the preset divide ratio of the binary 12-bit counter
(3 to 4095), and A = the preset divide ratio of the binary 6-bit
swallow counter (0 to 63). F

is obtained using Equation 2.

PFD

REV. 0

–11–

ADF4251

Phase Frequency Detector (PFD) and Charge Pump

The PFD takes inputs from the R counter and N counter and
produces an output proportional to the phase and frequency
difference between them. Figure 6 is a simplified schematic. The
PFD includes a delay element that controls the width of the
antibacklash pulse. This pulse ensures that there is no dead zone
in the PFD transfer function and minimizes phase noise and
reference spurs.

U1

HI

D1

UP

Q1

+IN

–IN

CLR1

CHARGE

PUMP

DELAY

ELEMENT

CLR2

HI

D2

DOWN

Q2

U2

U3

CP

Figure 6. PFD Simplified Schematic

MUXOUT and Lock Detect

The output multiplexer on the ADF4251 allows the user to
access various internal points on the chip. The state of MUXOUT
is controlled by M4, M3, M2, and M1 in the Master Register.
Table VI shows the full truth table. Figure 7 shows the MUXOUT
section in block diagram format.

DV

IF ANALOG LOCK DETECT

LOGIC LOW

IF R DIVIDER OUTPUT

IF N DIVIDER OUTPUT

RF ANALOG LOCK DETECT

IF/RF ANALOG LOCK DETECT

IF DIGITAL LOCK DETECT

LOGIC HIGH

RF R DIVIDER OUTPUT

RF N DIVIDER OUTPUT

THREE STATE OUTPUT

RF DIGITAL LOCK DETECT

RF/IF DIGITAL LOCK DETECT

LOGIC HIGH

LOGIC LOW

MUX CONTROL

DD

MUXOUT

D

GND

Figure 7. MUXOUT Circuit

Lock Detect

MUXOUT can be programmed for two types of lock detect: digital
and analog. Digital is active high. The N-channel open-drain
analog lock detect should be operated with an external pull-up
resistor of 10 kW nominal. When lock has been detected, this
output will be high with narrow low going pulses.

Hardware Power-Down/Chip Enable

In addition to the software power-down methods described on
pages 21 and 22, the ADF4251 also has a hardware power­down feature. This is accessed via the Chip Enable (CE) pin.
When this pin is Logic High, the device is in normal operation.
Bringing the CE pin Logic Low will power down the device.
When this happens, the following events occur:

1. All active dc current paths are removed.

2. The RF and IF counters are forced to their load
state conditions.

3. The RF and IF charge pumps are forced into three-state mode.

4. The digital lock detect circuitry is reset.

5. The RF

6. The REF

and IFIN inputs are debiased.

IN

input buffer circuitry is disabled.

IN

7. The serial interface input register remains active and capable
of loading and latching data.

Bringing the CE pin back up again to Logic High will reinstate
normal operation, depending on the software power-down settings.

Input Shift Register

Data is clocked in on each rising edge of CLK. The data is
clocked in MSB first. Data is transferred from the input register
to one of seven latches on the rising edge of LE. The destination
latch is determined by the state of the three control bits (C2, C1,
and C0) in the shift register. These are the three LSBs: DB2,
DB1, and DB0, as shown in Figure 1. The truth table for these
bits is shown in Table I. Table II shows a summary of how the
registers are programmed.

Table I. Control Bit Truth Table

C2 C1 C0 Data Latch

000 RF N Divider Reg
001 RF R Divider Reg
010 RF Control Reg
011 Master Reg
100 IF N Divider Reg
101 IF R Divider Reg
110 IF Control Reg

REV. 0–12–

RF N DIVIDER REG

ADF4251

Table II. Register Summary

RESERVED

DB23 DB22

N8

P1

8-BIT RF INTEGER VALUE (INT)

DB21

DB20 DB19 DB18 DB17 DB16 DB15 DB14 DB13 DB12 DB11 DB10

N6

N7

N4N5

RF R DIVIDER REG

IN

RF REF

DOUBLER

PRES CALER

DB20 DB19 DB18 DB17 DB16 DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

P2

P3

N2

4-BIT RF R COUNTER

R4

R2

R1R3

RF CONTROL REG

SPUR

SETTING 3

NOISE AND

DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

M11M12

RESERVED

12-BIT RF FRACTIONAL VALUE (FRAC)

DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

F8

F9F10F11F12N1N3

12-BIT INTERPOLATOR MODULUS VALUE (MOD)

M8

M9M10

RF CP

CURRENT

SPUR

SETTING

SETTING 2

NOISE AND

MASTER REG

DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

M7

MUXOUT

M6

RF PD

RESERVED

POLARITY

M4M5

SPUR

SETTING 1

NOISE AND

RESERVED

RF

POWER-

DOWN

POWER-

DOWN

M2M3

RF CP

STATE

THREE-

STATE

CP THREE-

F1F2F3F4F5F6F7

M1

RF

COUNTER

RESET

COUNTER

C3 (0)

C3 (0)

RESET

C3 (0)

C3 (0)

CONTROL

BITS

C2 (0)

CONTROL

BITS

C2 (0) C1 (1)

CONTROL

BITS

C2 (1) C1 (0)P4P5P6N1P8 0CP1CP2N2T1T2T3N3

CONTROL

BITS

C2 (1) C1 (1)P9P10P11M1M2M3M4

C1 (0)

IF N DIVIDER REG

IF PD

6-BIT IF A COUNTER12-BIT IF B COUNTER

POLARITY

IF LDP

IF POWER-

IF PRESCALER

IF CP GAIN

DB20 DB19 DB18 DB17 DB16 DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

DB21

DB22DB23

P14P15 C3 (1)

IF R DIVIDER REG

IN

IF REF

DOUBLER

DB18 DB17 DB16 DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

IF CONTROL REG

RF PHASE

RESYNC

DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

RESERVED

15-BIT IF R COUNTER

IF CP CURRENT

SETTING

RESYNC

RF PHASE

CONTROL

BITS

C2 (0) C1 (0)A1A2A3A4A5B1B2B3B4B5B6B7B8B9B10B11B12P13 A6

CONTROL

BITS

C2 (0) C1 (1)R1R2R3R4R5R6R7R8R9R10R11R12R13R15P16

C3 (1)R14

RESET

COUNTER

C3 (1)

CONTROL

BITS

C2 (1) C1 (0)P17P18P19P20P21CP1CP2CP3PR1T7T8PR2PR3

IF

IF CP

DOWN

STATE

THREE-

REV. 0

–13–

ADF4251

Table III. RF N Divider Register Map

8-BIT RF INTEGER VALUE (INT)

RESERVED

DB21

DB23 DB22

N8

P1

P1 RESERVED

0RESERVED

N8 N7 N6 N5 N4 N3 N2 N1
00011111 31
00100000 32
00100001 33
00100010 34

........ .

........ .

........ .

11111101 253

11111110 254

11111111 255

DB20 DB19 DB18 DB17 DB16 DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

N7

N6

N4N5

N2

F12 F11 F10 F3 F2 F1 FRACTIONAL VALUE (FRAC)

000.......... 0 0 0 0

000.......... 0 0 1 1

000.......... 0 1 0 2

000.......... 0 1 1 3

............. . . . .

............. . . . .

............. . . . .

111.......... 1 0 0 4092

111.......... 1 0 1 4093

111.......... 1 1 0 4094

111.......... 1 1 1 4095

12-BIT RF FRACTIONAL VALUE (FRAC)

F8

F9F10F11F12N1N3

RF INTEGER

VALU E (INT)*

C3 (0)

CONT ROL

BITS

C2 (0)

C1 (0)F1F2F3F4F5F6F7

*WHEN P = 8/9, N

MIN

= 91

REV. 0–14–

Table IV. RF R Divider Register Map

ADF4251

IN

RF REF

DOUBLER

DOUBLER

4-BIT RF R COUNTER

R2

M12 M11 M10 M3 M2 M1 VALUE (MOD) DIVIDE RATIO

IN

R4 R3 R2 R1 DIVIDE RATIO
00011
00102
00113

.....

.....

.....

110113

111014

111115

000.......... 0102

000.......... 0113

000.......... 1004

............. . . . .

............. . . . .

............. . . . .

111.......... 1004092

111.......... 1014093

111.......... 1104094

111.......... 1114095

PRE-

SCALER

DB20 DB19 DB18 DB17 DB16 DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

P3

P2 RF REF

0DISABLED
1ENABLED

P3 RF PRESCALER

04/5
18/9

12-BIT INTERPOLATOR MODULUS VALUE (MOD)

M9M10M11M12R1R3R4P2

M8

RF R COUNTER

M1M2M3M4M5M6M7

INTERPOLATOR MODULUS

C3 (0)

CONT ROL

BITS

C2 (0) C1 (1)

REV. 0

–15–

ADF4251

Table V. RF Control Register Map

SPUR

NOISE AND

DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

N3 N2 N1 NOISE AND SPUR
SETTING
000LOWEST SPUR
001LOW NOISE AND SPUR
111LOWEST NOISE

RESERVED

SETTING 3

THESE BITS SHOULD

EACH BE SET TO 0 FOR
NORMAL OPERATION

CP2 CP1 1.5k 2.7k 5.6k

001.125 0.625 0.301

013.375 1.875 0.904

105.625 3.125 1.506

117.7875 4.375 2.109

SPUR

SETTING 2

NOISE AND

N2T1T2T3N3

RF CP

CURRENT

SETTING

CP2

I

(mA)

CP

RF PD

RESERVED

POLARITY

SPUR

SETTING 1

NOISE AND

RF CP

DOWN

RF POWER-

P6 RF POWER-DOWN
0DISABLED
1ENABLED

RESET

C3 (0)

RESET

CONT ROL

BITS

C2 (1) C1 (0)P4P5P6N1P8 0CP1

RF

STAT E

THREE-

COUNTER

P4 RF COUNTER

0DISABLED
1ENABLED

P5 RF CP THREE-STATE
0DISABLED
1THREE-STATE

P8 RF PD POLARITY
0NEGATIVE
1POSITIVE

REV. 0–16–

Table VI. Master Register Map

ADF4251

MUXOUT

DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

MUXOUT

M4

0 0 0 0 LOGIC LOW
0 0 0 1 IF ANALOG LOCK DETECT
0 0 1 0 IF R DIVIDER OUTPUT
0 0 1 1 IF N DIVIDER OUTPUT
0 1 0 0 RF ANALOG LOCK DETECT
0 1 0 1 RF/IF ANALOG LOCK DETECT
0 1 1 0 IF DIGITAL LOCK DETECT
0 1 1 1 LOGIC HIGH
1 0 0 0 RF R DIVIDER OUTPUT
1 0 0 1 RF N DIVIDER OUTPUT
1 0 1 0 THREE-STATE OUTPUT
1 0 1 1 LOGIC LOW
1 1 0 0 RF DIGITAL LOCK DETECT
1 1 0 1 RF/IF DIGITAL LOCK DETECT
1 1 1 0 LOGIC HIGH
1 1 1 1 LOGIC LOW

M2M3

M1

CONT ROL

STAT E

DOWN

POWE R-

RESERVED

P10

0
1

P11

POWER-DOWN

DISABLED

0

ENABLED

1

RESERVED

P12

0 THIS BIT SHOULD BE SET TO “0” FOR

NORMAL OPERATION.

RESET

COUNTER

CP THREE-

P9

0
1

CP THREE-STATE

DISABLED
THREE-STATE

BITS

C2 (1) C1 (1)P9P10P11M1M2M3M4

C3 (0)

COUNTER RESET

DISABLED
ENABLED

REV. 0

–17–

ADF4251

Table VII. IF N Divider Register Map

IF

PRESCALER*

GAIN

IF CP

DB21

DB22DB23

P14

P15

P14 P13 PRESCALER VALUE
00 8/9
01 16/17
10 32/33
11 64/65

P15 IF CP GAIN

0DISABLED
1ENABLED

DB19 DB18 DB17 DB16 DB15 DB14 DB13 DB12 DB11

DB20

B12 B11 B10 B3 B2 B1 B COUNTER DIVIDE RATIO

000.......... 0113

000.......... 1004

............. ....

............. ....

............. ....

111.......... 1004092

111.......... 1014093

111.......... 1104094

111.......... 1114095

DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

A6 A5 .......... A2 A1 DIVIDE RATIO

00.......... 0 0 0

00.......... 0 1 1

00.......... 1 0 2

00.......... 1 1 3

............ . . .

............ . . .

............ . . .

11.......... 0 0 60

11.......... 0 1 61

11.......... 1 0 62

11.......... 1 1 63

6-BIT IF A COUNTER*12-BIT IF B COUNTER*

A1A2A3A4A5B1B2B3B4B5B6B7B8B9B10B11B12P13 A6

C3 (1)

CONTROL

BITS

C2 (0) C1 (0)

A COUNTER

*N = BP + A, P IS PRESCALER VALUE. B MUST BE GREATER THAN OR EQUAL TO A FOR CONTIGUOUS VALUES OF N, N

IS (P2 – P) .

MIN

REV. 0–18–

Table VIII. IF R Divider Register Map

ADF4251

IN

IF REF

DOUBLER

DB18 DB17 DB16 DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

R14

R14 R13 R12 .......... R3 R2 R1 DIVIDE RATIO

000.......... 0011

000.......... 0102

000.......... 0113

000.......... 1004

............. ....

............. ....

............. ....

111.......... 10016380

111.......... 10116381

111.......... 11016382

111.......... 11116383

P16 IF REF

0DISABLED
1ENABLED

IN

DOUBLER

R15
0
0
0
0
.
.
.
32764

32765

32766

32767

15-BIT IF R COUNTER

R8

R9R10R11R12R13R15P16

C3 (1)

CONT ROL

BITS

C2 (0) C1 (1)R1R2R3R4R5R6R7

REV. 0

–19–

ADF4251

Table IX. IF Control Register Map

RF PHASE

RESYNC

DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

PR3 PR2 PR1 RF PHASE RESYNC
000DISABLED
111ENABLED

IF CP3 IF CP2 IF CP1 1.5k 2.7k 5.6k
000 1.1250.625 0.301
001 2.251.25 0.602
010 3.3751.875 0.904
011 4.52.51.205
100 5.6253.125 1.506
101 6.753.75 1.808
110 7.78754.375 2.109
111 95.02.411

RESERVED

THESE BITS
SHOULD BE SET
TO 0 FOR NORMAL
OPERATION

RESYNC

RF PHASE

PR1T7T8PR2PR3

IF CP CURRENT

SETTING

CP3

I

(mA)

CP

IF PD

POLARITY

P20 IF LDP
0 3
1 5

P21 IF PD POLARITY
0 NEGATIVE
1 POSITIVE

DOWN

IF LDP

IF POWER-

P19 IF POWER-DOWN
0 DISABLED
1 ENABLED

IF

IF CP

STAT E

RESET

THREE-

COUNTER

P17 IF COUNTER RESET
0 DISABLED
1 ENABLED

P18 IF CP THREE-STATE
0 DISABLED
1 THREE-STATE

C3 (1)

CONT ROL

BITS

C2 (1) C1 (0)P17P18P19P20P21CP1CP2

REV. 0–20–

ADF4251

RF N DIVIDER REGISTER
(Address R0)

With R0[2, 1, 0] set to [0, 0, 0], the on-chip RF N Divider register
will be programmed. Table III shows the input data format for
programming this register.

8-Bit RF INT Value

These eight bits control what is loaded as the INT value. This is
used to determine the overall feedback division factor. It is used in
Equation 1.

12-Bit RF FRAC Value

These 12 bits control what is loaded as the FRAC value into the
fractional interpolator. This is part of what determines the overall
feedback division factor. It is used in Equation 1. The FRAC
value must be less than or equal to the value loaded into the
MOD register.

RF R DIVIDER REGISTER
(Address R1)

With R1[2, 1, 0] set to [0, 0, 1], the on-chip RF R Divider register
will be programmed. Table IV shows the input data format for
programming this register.

RF Prescaler (P/P + 1)

The RF dual-modulus prescaler (P/P +1), along with the INT,
FRAC, and MOD counters, determine the overall division ratio
from the RF

to the PFD input. Operating at CML levels, it

IN

takes the clock from the RF input stage and divides it down to
amanageable frequency for the CMOS counters. It is based on
a synchronous 4/5 core (see Table IV).

RF REFIN Doubler

Setting this bit to 0 feeds the REFIN signal directly to the 4-bit
RF R counter, disabling the doubler. Setting this bit to 1 multiplies
the REF
bit RF R counter. When the doubler is disabled, the REF

frequency by a factor of 2 before feeding into the 4-

IN

IN

falling edge is the active edge at the PFD input to the fractional-N
synthesizer. When the doubler is enabled, both the rising and
falling edges of REF

become active edges at the PFD input.

IN

When the doubler is enabled and lowest spur mode is chosen,
the in-band phase noise performance is sensitive to the REF

IN

duty cycle. The phase noise degradation can be as much as 5 dB
for REF
noise is insensitive to REF

duty cycles outside a 45% to 55% range. The phase

IN

duty cycle in the lowest noise mode

IN

and in low noise and spur mode. The phase noise is insensitive
to REF

duty cycle when the doubler is disabled.

IN

4-Bit RF R Counter

The 4-bit RF R counter allows the input reference frequency

) to be divided down to produce the reference clock to

(REF

IN

the phase frequency detector (PFD). Division ratios from 1 to
15 are allowed.

12-Bit Interpolator Modulus

This programmable register sets the fractional modulus. This is
the ratio of the PFD frequency to the channel step resolution on
the RF output.

RF CONTROL REGISTER
(Address R2)

With R2[2, 1, 0] set to [0, 1, 0], the on-chip RF control register
will be programmed. Table V shows the input data format for
programming this register. Upon initialization, DB15–DB11
should all be set to 0.

Noise and Spur Setting

The noise and spur setting (R2[15, 11, 06]) is a feature that
allows the user to optimize his or her design either for improved
spurious performance or for improved phase noise performance.
When set to [0, 0, 0], the lowest spurs setting is chosen. Here,
dither is enabled. This randomizes the fractional quantization
noise so that it looks more like white noise than spurious noise.
This means that the part is optimized for improved spurious
performance. This operation would normally be used when the
PLL closed-loop bandwidth is wide
A wide loop filter does not attenuate the spurs to a level that a

2

narrow

loop bandwidth would. When this bit is set to [0, 0, 1],

1

for fast-locking applications.

the low noise and spur setting is enabled. Here, dither is disabled.
This optimizes the synthesizer to operate with improved noise
performance. However, the spurious performance is degraded in
this mode compared to lowest spurs setting. To improve noise
performance even further, another option is available that reduces
the phase noise. This is the lowest noise setting [1, 1, 1]. As well
as disabling the dither, it also ensures the charge pump is oper­ating in an optimum region for noise performance. This setting is
extremely useful where a narrow loop filter bandwidth is available.
The synthesizer ensures extremely low noise and the filter attenu­ates the spurs. The Typical Performance Characteristics (TPCs)
give the user an idea of the trade-off in a typical WCDMA setup
for the different noise and spur settings.

RF Counter Reset

DB3 is the RF counter reset bit for the ADF4251. When this is
1, the RF synthesizer counters are held in reset. For normal
operation, this bit should be 0.

RF Charge Pump Three-State

This bit puts the charge pump into three-state mode when pro­grammed to a 1. It should be set to 0 for normal operation.

RF Power-Down

DB5 on the ADF4251 provides the programmable power-down
mode. Setting this bit to a 1 will perform a power-down on both
the RF and IF sections. Setting this bit to 0 will return the RF
and IF sections to normal operation. While in software power­down, the part will retain all information in its registers. Only
when supplies are removed will the register contents be lost.

When a power-down is activated, the following events occur:

1. All active RF dc current paths are removed.

2. The RF synthesizer counters are forced to their load state
conditions.

3. The RF charge pump is forced into three-state mode.

4. The RF digital lock detect circuitry is reset.

5. The RF

input is debiased.

IN

6. The input register remains active and capable of loading and
latching data.

NOTES

1

Wide loop bandwidth is seen as a loop bandwidth greater than 1/10th of the
RF

channel step resolution (F

OUT

2

Narrow loop bandwidth is seen as a loop bandwidth less than 1/10th of the
RF

channel step resolution (F

OUT

RES

RES

).

).

REV. 0

–21–

ADF4251

RF Phase Detector Polarity

DB7 in the ADF4251 sets the RF phase detector polarity.
When the VCO characteristics are positive, this should be set to 1.
When they are negative, it should be set to 0.

RF Charge Pump Current Setting

DB9 and DB10 set the RF charge pump current setting. This
should be set to whatever charge pump current the loop filter
has been designed with (see Table V).

RF Test Modes

These bits should be set to 0, 0, 0 for normal operation.

MASTER REGISTER
(Address R3)

With R3[2, 1, 0] set to 0, 1, 1, the on-chip master register will be
programmed. Table VI shows the input data format for program­ming the Master Register.

RF and IF Counter Reset

DB3 is the counter reset bit for the ADF4251. When this is 1,
both the RF and IF R, INT, and MOD counters are held in reset.
For normal operation, this bit should be 0. Upon powering up, the
DB3 bit needs to be disabled, the INT counter resumes counting
in “close” alignment with the R counter. (The maximum error is
one prescaler cycle).

Charge Pump Three-State

This bit puts both the RF and IF charge pump into three-state
mode when programmed to a 1. It should be set to 0 for normal
operation.

Power-Down

R3[3] on the ADF4251 provides the programmable power-down
mode. Setting this bit to a 1 will perform a power-down on both
the RF and IF sections. Setting this bit to 0 will return the RF
and IF sections to normal operation. While in software power­down, the part will retain all information in its registers. Only
when supplies are removed will the register contents be lost.

When a power-down is activated, the following events occur:

1. All active dc current paths are removed.

2. The RF and IF counters are forced to their load state conditions.

3. The RF and IF charge pumps are forced into three-state mode.

4. The digital lock detect circuitry is reset.

5. The RF

input and IFIN input are debiased.

IN

6. The oscillator input buffer circuitry is disabled.

7. The input register remains active and capable of loading and
latching data.

MUXOUT Control

The on-chip multiplexer is controlled by R3[10–7] on the
ADF4251. Table VI shows the truth table.

If the user updates the RF control register or the IF control
register, the MUXOUT contents will be lost. To retrieve the
MUXOUT signal, the user must write to the Master Register.

IF N DIVIDER REGISTER
(Address R4)

With R4[2, 1, 0] set to [1, 0, 0], the on-chip IF N divider register
will be programmed. Table VII shows the input data format for
programming this register.

IF CP Gain

When set to 1, this bit changes the IF charge pump current
setting to its maximum value. When the bit is set to 0, the charge
pump current reverts back to its previous state.

IF Prescaler

The dual-modulus prescaler (P/P + 1), along with the IF A and
IF B counters, determine the overall division ratio, N, to be real­ized (N = PB + A) from the IF

to the IF PFD input. Operating

IN

at CML levels, it takes the clock from the IF input stage and divides
it down to a manageable frequency for the CMOS counters. It
is based on a synchronous 4/5 core. See Equation 2 and Table VII.

IF B and A Counter

The IF A and IF B counters, in conjunction with the dual modu­lus prescaler, make it possible to generate output frequencies
that are spaced only by the reference frequency (REF
by R. The equation for the IF

VCO frequency is given in

OUT

), divided

IN

Equation 2.

IF R DIVIDER REGISTER
(Address R5)

With R5[2, 1, 0] set to 1, 0, 1, the on-chip IF R divider register
will be programmed. Table VIII shows the input data format for
programming this register.

IF REFIN Doubler

Setting this bit to 0 feeds the REFIN signal directly to the 15-bit
IF R counter. Setting this bit to 1 multiplies the REF

IN

frequency by a factor of 2 before feeding into the 15-bit IF R
counter.

15-Bit IF R Counter

The 15-bit IF R counter allows the input reference frequency
(REF

) to be divided down to produce the reference clock to

IN

the IF phase frequency detector (PFD). Division ratios from
1 to 32767 are allowed.

IF CONTROL REGISTER
(Address R6)

With R6[2, 1, 0] set to 1, 1, 0, the on-chip IF control register
will be programmed. Table IX shows the input data format for
programming this register. Upon initialization, DB15–DB11
should all be set to 0.

IF Counter Reset

DB3 is the IF counter reset bit for the ADF4251. When this is
1, the IF synthesizer counters are held in reset. For normal
operation, this bit should be 0.

IF Charge Pump Three-State

This bit puts the IF charge pump into three-state mode when pro­grammed to a 1. It should be set to 0 for normal operation.

IF Power-Down

DB5 on the ADF4251 provides the programmable power-down
mode. Setting this bit to a 1 will perform a power-down on the IF
section. Setting this bit to 0 will return the section to normal
operation. While in software power-down, the part will retain all
information in its registers. Only when supplies are removed will
the register contents be lost.

REV. 0–22–

ADF4251

When a power-down is activated, the following events occur:

1. All active IF dc current paths are removed.

2. The IF synthesizer counters are forced to their load state
conditions.

3. The IF charge pump is forced into three-state mode.

4. The IF digital lock detect circuitry is reset.

5. The IF

input is debiased.

IN

6. The input register remains active and capable of loading and
latching data.

IF Phase Detector Polarity

DB7 in the ADF4251 sets the IF phase detector polarity. When
the VCO characteristics are positive, this should be set to 1.
When they are negative, it should be set to 0.

IF Charge Pump Current Setting

DB8, DB9, and DB10 set the IF charge pump current setting.
This should be set to whatever charge pump current the loop
filter has been designed with (see Table VII).

IF Test Modes

These bits should be set to 0, 0 for normal operation.

RF Phase Resync

Setting the Phase Resync Bits [15, 14, 11] to 1, 1, 1 enables
the phase resync feature. With a fractional modulus of M, a
fractional-N PLL can settle with any one of (2 ⫻ ␲)/M valid
phase offsets with respect to the reference input. This is different
to integer-N (where the RF output always settles to the same
static phase offset with respect to the input reference, which is
zero ideally), but does not matter in most applications where all
that is required is consistent frequency lock.

For applications where a consistent phase relationship between
the output and reference is required (i.e., digital beamforming),
the ADF4251 fractional-N synthesizer can be used with the phase
resync feature enabled. This ensures that if the user programs
the PLL to jump from frequency (and Phase) A to frequency
(and Phase) B and back again to frequency A, the PLL will return
to the original phase (Phase A).

When enabled, it will activate every time the user programs
register R0 or R1 to set a new output frequency. However, if a
cycle slip occurs in the settling transient after the phase re-resync
operation, the phase resync will be lost. This can be avoided by
delaying the resync activation until the locking transient is close
to its final frequency. In the IF R Divider register, Bits R5[17–3]
are used to set a time interval from when the new channel is pro­grammed to the time the resync is activated. Although the time
interval resolution available from the 15-bit IF R register is one

clock cycle, IF R should be programmed to be a value that

REF

IN

is an integer multiple of the programmed MOD value to set a
time interval that is at least as long as the RF PLL loop’s lock time.

For example, if REF
output steps (F

= 26 MHz, MOD = 130 to give 200 kHz

IN

), and the RF loop has a settling time of 150 µs,

RES

then IF R should be programmed to 3900, as:

26 150 3900 MHz s¥=m

Note that if it is required to use the IF synthesizer with phase
resync enabled on the RF synth, the IF synth must operate with
a PFD frequency of 26 MHz/3900. In an application where the
IF synth is not required, the user should ensure that registers R4

and R6 are not programmed so that the rest of the IF circuitry
remains in power-down.

DEVICE PROGRAMMING AFTER INITIAL POWER-UP

After initially applying power to the supply pins, there are three
ways to operate the device.

RF and IF Synthesizers Operational

All registers must be written to when powering up both the RF
and IF synthesizer.

RF Synthesizer Operational, IF Power-Down

It is necessary to write to registers R3, R2, R1, and R0 only
when powering up the RF synthesizer only. The IF side will
remain in power-down until registers R6, R5, R4, and R3 are
written to.

IF Synthesizer Operational, RF Power-Down

It is necessary to write only to registers R6, R5, R4, and R3 when
powering up the IF synthesizer only. The RF side will remain in
power-down until registers R3, R2, R1, and R0 are written to.

RF Synthesizer: An Example

The RF synthesizer should be programmed as follows:

RF INT

OUT PFD

where RF

Ê

=+

Á
Ë

= the RF frequency output, INT = the integer division

OUT

FRAC

MOD

ˆ

¥

F

˜
¯

(4)

factor, FRAC = the fractionality, and MOD = the modulus.

Ê

F REF

=¥

Á

PFD IN

Ë

ˆ

1

D

+

˜

R

¯

(5)

where REFIN = the reference frequency input, D = the RF
REF

Doubler Bit, and R = the RF reference division factor.

IN

For example, in a GSM 1800 system where 1.8 GHz RF frequency
output (RF
(REF

IN

) is required, a 13 MHz reference frequency input

OUT

) is available and a 200 kHz channel resolution (F

RES

) is

required on the RF output.

REF

=

MHz

kHz

IN

F

RES

65

MOD

13

MOD

==

200

So, from Equation 5:

13

F MHz

=¥+=

PFD

18 13

. GHz MHz INT +

MHz

=¥

10

1

Ê
Á

Ë

13

FRAC

65

ˆ
˜

¯

where INT = 138 and FRAC = 30.

IF Synthesizer: An Example

The IF synthesizer should be programmed as follows:

IF P B A F

=¥

()

[]

OUT PFD

where IF

= the output frequency of external voltage controlled

OUT

+

¥

(6)

oscillator (VCO), P = the IF prescaler, B = the B counter value,
and A = the A counter value.

Equation 5 applies in this example as well.

REV. 0

–23–

ADF4251

For example, in a GSM1800 system, where 540 MHz IF fre­quency output (IF
input (REF
(F

RES

IF REF

IN

) is required on the IF output. The prescaler is set to 16/17.

doubler is disabled.

IN

) is required, a 13 MHz reference frequency

OUT

) is available and a 200 kHz channel resolution

By Equation 5:

10

200 13

kHz MHz=¥

+

R

if R = 65.

By Equation 6:

540 200 16 MHz kHz=¥¥

()

[]

+

BA

if B = 168 and A = 12.

Modulus

The choice of modulus (MOD) depends on the reference signal
(REF

) available and the channel resolution (F

IN

) required at

RES

the RF output. For example, a GSM system with 13 MHz

would set the modulus to 65. This means that the RF

REF

IN

output resolution (F

) is the 200 kHz (13 MHz/65) necessary

RES

for GSM.

Reference Doubler and Reference Divider

There is a reference doubler on-chip. This allows the input
reference signal to be doubled. This is useful for increasing the
PFD comparison frequency. Making the PFD frequency higher
improves the noise performance of the system. Doubling the
PFD frequency will usually result in an improvement in noise
performance of 3 dB. It is important to note that the PFD can­not be operated above 30 MHz. This is due to a limitation in
the speed of the ⌺-⌬ circuit of the N divider.

12-Bit Programmable Modulus

Unlike most other fractional-N PLLs, the ADF4251 allows the
user to program the modulus over a 12-bit range. This means
that the user can set up the part in many different configurations
for his or her application, when combined with the reference
doubler and the 4-bit R counter.

Take for example an application that requires 1.75 GHz RF and
200 kHz channel step resolution. The system has a 13 MHz
reference signal.

One possible setup is feeding the 13 MHz directly to the PFD
and programming the modulus to divide by 65. This would result
in the required 200 kHz resolution.

Another possible setup is using the reference doubler to create
26 MHz from the 13 MHz input signal. This 26 MHz is then
fed into the PFD. The modulus is now programmed to divide by

130. This also results in 200 kHz resolution. This would offer
superior phase noise performance over the previous setup.

The programmable modulus is also very useful for multistandard
applications. If a dual-mode phone requires PDC and GSM1800
standards, the programmable modulus is of huge benefit. PDC
requires 25 kHz channel step resolution, whereas GSM1800
requires 200 kHz channel step resolution. A 13 MHz reference
signal could be fed directly to the PFD. The modulus would be
programmed to 520 when in PDC mode (13 MHz /520 = 25 kHz).

The modulus would be reprogrammed to 65 for GSM1800
operation (13 MHz/65 = 200 kHz). It is important that the PFD
frequency remains constant (13 MHz). This allows the user to
design one loop filter that can be used in both setups without
running into stability issues. It is the ratio of the RF frequency
to the PFD frequency that affects the loop design. Keeping this
relationship constant and instead changing the modulus factor,
results in a stable filter.

Spurious Optimization and Fastlock

As mentioned in the Noise and Spur Setting section, the part can
be optimized for spurious performance. However, in fast-lock­ing applications, the loop bandwidth needs to be wide.
Therefore, the filter does not provide much attenuation of the
spurious. The programmable charge pump can be used to get
around this issue. The filter is designed for a narrow loop band­width so that steady-state spurious specifications are met. This is
designed using the lowest charge pump current setting. To
implement fastlock during a frequency jump, the charge pump
current is set to the maximum setting for the duration of the jump.
This has the effect of widening the loop bandwidth, which im­proves lock time. When the PLL has locked to the new frequency,
the charge pump is again programmed to the lowest charge pump
current setting. This will narrow the loop bandwidth to its origi­nal cutoff frequency to allow for better attenuation of the
spurious than the wide loop bandwidth.

Spurious Signals—Predicting Where They Will Appear

Just as in integer-N PLLs, spurs will appear at PFD frequency
offsets on either side of the carrier (and multiples of the PFD
frequency). In a fractional-N PLL, spurs will also appear at
frequencies equal to the RF

OUT

channel step resolution (F

RES

).
The ADF4251 uses a high order fractional interpolator engine.
This results in spurious signals also appearing at frequencies equal
to 1/2 of the channel step resolution. For example, examine the
GSM-1800 setup with a 26 MHz PFD and 200 kHz resolution.
Spurs will appear at ±26 MHz from the RF carrier (at an extremely
low level due to filtering). Also, there will be spurs at ±200 kHz from
the RF carrier. Due to the fractional interpolator architecture
used in the ADF4251, spurs will also appear at ±100 kHz from
the RF carrier. Harmonics of all spurs mentioned will also appear.
With lowest spur setting enabled, the spurs will be attenuated
into the noise floor.

Prescaler

The prescaler limits the INT value. With P = 4/5, N
With P = 8/9, N

MIN

= 91.

MIN

= 31.

The prescaler can also influence the phase noise performance.
If INT < 91, a prescaler of 4/5 should be used. For applications
where INT > 91, P = 8/9 should be used for optimum noise
performance.

Filter Design: ADIsimPLL

A filter design and analysis program is available to help users
implement their PLL design. Visit www.analog.com/pll for a
free download of the ADIsimPLL software. The software
designs, simulates, and analyzes the entire PLL frequency
domain and time domain response. Various passive and active
filter architectures are allowed.

REV. 0–24–

IF Side Not In Use

ADuC812

ADF4251

SCLK

SDATA

LE

CE

MUXOUT
(LOCK DETECT)

SCLOCK

MOSI

I/O PORTS

If the IF side is not being used, the following pinout is recom­mended for the IF side:

Pin No. Mnemonic Description

14 A

15 IF

16 IF

2 Short to all other ground pins.

GND

BLeave open circuit. (This is biased

IN

up to V

ALeave open circuit. (This is biased

IN

up to V

/2 internally.)

DD

/2 internally.)

DD

19 CP_IF Leave open circuit. (This is internally

three-stated until power-up.)

20 V

2 Short to VP1. (VP1 is the RF CP

P

supply.)

21 V

2 Short to VDD1 (VDD1 is the RF V

DD

DD

supply.)

INTERFACING

The ADF4251 has a simple SPI compatible serial interface for
writing to the device. SCLK, SDATA, and LE control the data
transfer. When LE (Latch Enable) goes high, the 24 bits that have
been clocked into the input register on each rising edge of SCLK
will be transferred to the appropriate latch. See Figure 1 for the
Timing Diagram and Table I for the Control Bit Truth Table.

The maximum allowable serial clock rate is 20 MHz. This means
that the maximum update rate possible for the device is 833 kHz
or one update every 1.2 µs. This is certainly more than adequate
for systems that will have typical lock times in hundreds of
microseconds.

ADF4251

Figure 8. ADuC812 to ADF4251 Interface

ADuC812 Interface

Figure 8 shows the interface between the ADF4251 and the
ADuC812 microconverter. Since the ADuC812 is based on
an 8051 core, this interface can be used with any 8051-based
microcontroller. The microconverter is set up for SPI Master
mode with CPHA = 0. To initiate the operation, the I/O port
driving LE is brought low. Each latch of the ADF4251 needs
(at most) a 24-bit word. This is accomplished by writing three
8-bit bytes from the microconverter to the device. When the third
byte has been written, the LE input should be brought high to
complete the transfer.

I/O port lines on the ADuC812 are also used to control power­down (CE input) and to detect lock (MUXOUT configured as
lock detect and polled by the port input).

When operating in the mode described, the maximum SCLOCK
rate of the ADuC812 is 4 MHz. This means that the maximum
rate at which the output frequency can be changed will be 166 kHz.

100pF

18

REV. 0

V

P

POWER-DOWN CONTROL

SV

DD

IF

OUT

V



18



18

100pF

CC

IF VCO

GND

51



IF LOOP

FILTER

REF

IN

100pF

V

DD

100pF

V

DD

IF CP

REFIN

IFINA

IFINB

DVDDV

ADF4251

GND

CP

P

RF CP

RFINA

RFINB

GNDDGND

A

CE

R

SET

2.7k

100pF

100pF

Figure 9. Power-Down Circuit

RF LOOP

FILTER

51



DECOUPLING CAPACITORS AND INTERFACE SIGNALS
HAVE BEEN OMITTED FROM THE DIAGRAM IN THE
INTERESTS OF GREATER CLARITY.

ADG702

IN

D

GND

V

CC

RF VCO

GND

100pF

100pF

18

RF

18

18

OUT

–25–

ADF4251

SCLK

DT

TFS

I/O FLAGS

ADSP-21xx

Figure 10. ADSP-21xx to ADF4251 Interface

ADSP-2181 Interface

Figure 10 shows the interface between the ADF4251 and the
ADSP-21xx digital signal processor. Each latch of the ADF4251
needs (at most) a 24-bit word. The easiest way to accomplish this
using the ADSP-21xx family is to use the autobuffered transmit
mode of operation with alternate framing. This provides a means
for transmitting an entire block of serial data before an interrupt is
generated. Set up the word length for eight bits and use three memory
locations for each 24-bit word. To program each 24-bit latch, store
the three 8-bit bytes, enable the autobuffered mode, and then write
to the transmit register of the DSP. This last operation initiates the
autobuffer transfer.

SCLK

SDATA

LE

CE

MUXOUT
(LOCK DETECT)

ADF4251

Powerdown Circuit

The attached circuit in Figure 10 shows how to shut down the
ADF4251 and the accompanying RF and IF VCO. The
ADG701 switch goes closed circuit when a Logic High is applied
to the IN input. The low cost switch is available in SOT-23 and
SOIC packages.

PCB DESIGN GUIDELINES FOR CHIP-SCALE PACKAGE

The leads on the chip-scale package (CP-24) are rectangular. The
printed circuit board pad for these should be 0.1 mm longer than
the package land length and 0.05 mm wider than the package land
width. The land should be centered on the pad. This will ensure
that the solder joint size is maximized.

The bottom of the chip-scale package has a central thermal pad.
The thermal pad on the printed circuit board should be at least
as large as this exposed pad. On the printed circuit board, there
should be a clearance of at least 0.25 mm between the thermal
pad and the inner edges of the pad pattern. This will ensure that
shorting is avoided.

Thermal vias may be used on the printed circuit board thermal pad
to improve thermal performance of the package. If vias are used,
they should be incorporated in the thermal pad at 1.2 mm pitch
grid. The via diameter should be between 0.3 mm and 0.33 mm,
and the via barrel should be plated with 1 oz copper to plug the via.

IF

C15
100pF

R12
18

R13

18

R14
18

R15

51

C17

100pF

R45

0

R46

0

C46
22F

OUT

J6

C16

100pF

5V

3V

C45
10pF

C43

R27

T14

6.3V

C5

22F

C6

10pF

V

DD

R1
20

VDD’

R43

1

2

DD

DD

V

V

VP2

CP

IF

ADF4251BCP

IFINA

0

U1

3

DD

V

VP1

CP

CP

GND

RFINA

RF

MUXOUT

A

GND

D

GND

A

GND

CP

GND

6.3V

C7

22F

C8

10pF

DD

DV

RF

C23

10nF

1

B

IN

C44

100pF

1

2

2

6.3V

C9

22F

C10

10pF

R17

13k

C20
82pF

C14
1nF

V

P

R44
0

R16

7.5k

C19

2.2nF

DATA

CLK

C18
270pF

LE

100pF

2.7k

VVCO

R48
0

6.3V

C3

22F

C4

10pF

14

VCC

10

RF

OUT

VCO2

VCO190–540T VCO190–1730T

REF

IN

T13

J5

R47

0

4

B+

3

O/P

Y3

V

C13
1nF

GND

IN

R11

51

2

2

C24

100nF

R19

270

R27

10k

V

T16

P

6.3V

C11

22F

C12

10pF

R20

470

3.3nF

R28

10k

R29

10k

D4

C25

2

V

DD

VVCO

R49

0

6.3V

C29

22F

RF

C26

100pF

J7

C27

100pF

R22

18

R21

18

R23

18

R24
51

C28

100pF

OUT

C30

10pF

14

VCC

V

IN

VCO1

10

RF

OUT

Figure 11. Typical PLL Circuit Schematic

REV. 0–26–

OUTLINE DIMENSIONS

24-Lead Frame Chip Scale Package [LFCSP]

4 mm  4 mm Body

(CP-24)

Dimensions shown in millimeters

ADF4251

PIN 1

INDICATOR

1.00

0. 90

0.85

0.25
REF

12 MAX

SEATING
PLANE

4.0

BSC SQ

0.30

0.23

0.18

3.75

BSC SQ

0.20 REF

TOP

VIEW

1.00 MAX

0.65 NOM

COMPLIANT TO JEDEC STANDARDS MO-220-VGGD-2

0.60 MAX

0.05 MAX

0.02 NOM

COPLANARITY

0.08

0.50
BSC

0.50

0.40

0.30

0.60 MAX

19

18

BOTTOM

13

12

VIEW

2.50
REF

0.25
PIN 1

MIN

INDICATOR

1

24

2.25

6

7

1.70

0.75

SQ

REV. 0

–27–

C03269–0–10/03(0)

–28–