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GSM RF OUTPUT POWER (dBm)

100

EVM(%

RMS)

NOISEFLOORAT6MHz

OFFSET(

dBc/100kHz)

1

3

4

5

6 2 0

2

106

104

100

98

96

102

8 4 2 4 6

55682 TA02

EVM

NOISE

GSM/EDGE Optimized,High Linearity Direct

Quadrature Modulator

The LT5568-2 is a direct I/Q modulator designed for highperformance wireless applications, including wirelessinfrastructure. It allows direct modulation of an RF signalusing differential baseband I and Q signals. It supportsGSM, EDGE, CDMA, CDMA2000 and other systems thatoperate in the 850MHz to 965MHz band. It may be config-ured as an image reject upconverting mixer, by applying90 phase-shifted signals to the I and Q inputs. The I/Qbaseband inputs consist of voltage-to-current convertersthat in turn drive double-balanced mixers. The outputs ofthese mixers are summed and applied to an on-chip RFtransformer, which converts the differential mixer signalsto a 50 single-ended output. The four balanced I and Qbaseband input ports are intended for DC coupling from asource with a common mode voltage level of about 0.5V.The LO path consists of an LO buffer with single-endedinput, and precision quadrature generators that producethe LO drive for the mixers. The supply voltage range is4.5V to 5.25V.

Infrastructure Tx for GSM/Cellular Bands Image Reject Up-Converters for Cellular Bands Low-Noise Variable Phase-Shifter for 700MHz to

1050MHz Local Oscillator Signals RFID Reader

Optimized Image Rejection for 850MHz to 965MHz High OIP3: +22.9dBm at 900MHz Low Output Noise Floor at 5MHz Offset:

No RF: 159.4dBm/HzPOUT = 4dBm: 153dBm/Hz

Integrated LO Buffer and LO Quadrature PhaseGenerator

50 AC-Coupled Single-Ended LO and RF Ports 50 DC Interface to Baseband Inputs Low Carrier Leakage: 43dBm at 900MHz High Image Rejection: 52dBc at 900MHz 16-Lead 4mm 4mm QFN Package

850MHz to 965MHz Direct Conversion Transmitter Application

APPLICATIO SU

FEATURES DESCRIPTIOU

TYPICAL APPLICATIOU

GSM EVM and Noisevs RF Output Power at 900MHz

90

0

LT5568-2

BASEBANDGENERATOR

PA

VCO/SYNTHESIZER

RF = 850MHzTO 965MHz

100nFx 2

EN

5V

V-I

V-I

I-CHANNEL

Q-CHANNELBALUN

VCC

55682 TA01

I-DAC

Q-DAC

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.All other trademarks are the property of their respective owners.

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Supply Voltage .........................................................5.5VCommon Mode Level of BBPI, BBMI and

BBPQ, BBMQ .......................................................2.5VOperating Ambient Temperature

(Note 2) ............................................... 40C to 85CStorage Temperature Range ................... 65C to 125CVoltage on Any Pin

Not to Exceed...................... 500mV to VCC + 500mV

CAUTION: This part is sensitive to ESD. It is veryimportant that proper ESD precautions be observedwhen handling the LT5568-2.

(Note 1)

ABSOLUTE AXI U RATI GSW WW U

VCC = 5V, EN = High, TA = 25C, fLO = 900MHz, fRF = 902MHz, PLO = 0dBm.BBPI, BBMI, BBPQ, BBMQ inputs 0.54VDC, Baseband Input Frequency = 2MHz, I&Q 90 shifted (upper side-band selection).PRF, OUT = 10dBm, unless otherwise noted. (Note 3)

ELECTRICAL CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

RF Output (RF)

fRF RF Frequency RangeRF Frequency Range

3dB Bandwidth1dB Bandwidth

0.6 to 1.10.7 to 1

GHzGHz

S22, ON RF Output Return Loss EN = High (Note 6) 16 dB

S22, OFF RF Output Return Loss EN = Low (Note 6) 18 dB

NFloor RF Output Noise Floor No Input Signal (Note 8)POUT = 4dBm (Note 9)

POUT = 4dBm (Note 10)

159.4153

152.6

dBm/HzdBm/Hz

dBm/Hz

GP Conversion Power Gain POUT/PIN, I&Q 9 6.8 3 dB

GV Conversion Voltage Gain 20 Log (VOUT, 50/VIN, DIFF, I or Q) 6.8 dB

POUT Absolute Output Power 1VP-P DIFF CW Signal, I and Q 2.8 dBm

G3LO vs LO 3 LO Conversion Gain Difference (Note 17) 23 dB

OP1dB Output 1dB Compression (Note 7) 8.6 dBm

OIP2 Output 2nd Order Intercept (Notes 13, 14) 59 dBm

OIP3 Output 3rd Order Intercept (Notes 13, 15) 22.9 dBm

PIN CONFIGURATION

16 15 14 13

5 6 7 8

TOP VIEW

9

10

11

12

4

3

2

1EN

GND

LO

GND

GND

RF

GND

GND

BBMI

GND

BBPI

VCC

BBMQ

GND

BBPQ

VCC

17

UF PACKAGE16-LEAD (4mm 4mm) PLASTIC QFN

TJMAX = 125C, JA = 37C/WEXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE

LT5568-2EUF#PBF LT5568-2EUF#TRPBF 55682 16-Lead (4mm 4mm) Plastic QFN 40C to 85C

Consult LTC Marketing for parts specified with wider operating temperature ranges.Consult LTC Marketing for information on non-standard lead based finish parts.

For more information on lead free part marking, go to:http://www.linear.com/leadfree/For more information on tape and reel specifications, go to:http://www.linear.com/tapeandreel/
http://www.linear.com/leadfree/http://www.linear.com/leadfree/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/tapeandreel/http://www.linear.com/leadfree/

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VCC = 5V, EN = High, TA = 25C, fLO = 900MHz, fRF = 902MHz, PLO = 0dBm.BBPI, BBMI, BBPQ, BBMQ inputs 0.54VDC, Baseband Input Frequency = 2MHz, I&Q 90 shifted (upper side-band selection).PRF, OUT = 10dBm, unless otherwise noted. (Note 3)

Note 1: Stresses beyond those listed under Absolute Maximum Ratingsmay cause permanent damage to the device. Exposure to any AbsoluteMaximum Rating condition for extended periods may affect devicereliability and lifetime.

Note 2: Specifications over the 40C to 85C temperature range are assured

by design, characterization and correlation with statistical process controls.Note 3: Tests are performed as shown in the configuration of Figure 7.

Note 4: On each of the four baseband inputs BBPI, BBMI, BBPQ and BBMQ.

Note 5: V(BBPI) V(BBMI) = 1VDC, V(BBPQ) V(BBMQ) = 1VDC.

Note 6: Maximum value within 850MHz to 965MHz.

Note 7: An external coupling capacitor is used in the RF output line.

Note 8: At 20MHz offset from the LO signal frequency.

Note 9: At 20MHz offset from the CW signal frequency.

Note 10: At 5MHz offset from the CW signal frequency.

Note 11: RF power is within 10% of final value.

Note 12: RF power is at least 30dB lower than in the ON state.

Note 13: Baseband is driven by 2MHz and 2.1MHz tones. Drive level is setin such a way that the two resulting RF tones are 10dBm each.

Note 14: IM2 measured at LO frequency + 4.1MHz.

Note 15: IM3 measured at LO frequency + 1.9MHz and LO frequency + 2.2MHz.

Note 16: Amplitude average of the characterization data set without imageor LO feedthrough nulling (unadjusted).

Note 17: The difference in conversion gain between the spurious signal atf = 3 LO BB versus the conversion gain at the desired signal at f = LO +BB for BB = 2MHz and LO = 900MHz.

Note 18: The input voltage corresponding to the output P1dB.

ELECTRICAL CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

IR Image Rejection fBB = 100kHz (Note 16) 52 dBc

LOFT Carrier Leakage(LO Feedthrough)

EN = High, PLO = 0dBm (Note 16)EN = Low, PLO = 0dBm (Note 16)

4365

dBmdBm

LO Input (LO)

fLO LO Frequency Range 0.6 to 1.1 GHz

PLO LO Input Power 10 0 5 dBm

S11, ON LO Input Return Loss EN = High (Note 6) 15 dB

S11, OFF LO Input Return Loss EN = Low (Note 6) 2.5 dB

NFLO LO Input Referred Noise Figure (Note 5) at 900MHz 14.7 dB

GLO LO to RF Small Signal Gain (Note 5) at 900MHz 14.7 dB

IIP3LO LO Input 3rd Order Intercept (Note 5) at 900MHz 3 dBm

Baseband Inputs (BBPI, BBMI, BBPQ, BBMQ)

BWBB Baseband Bandwidth 3dB Bandwidth 380 MHz

VCMBB DC Common Mode Voltage (Note 4) 0.54 V

RIN, SE Single-Ended Input Resistance (Note 4) 47

PLO2BB Carrier Feedthrough on BB POUT = 0 (Note 4) 38 dBm

IP1dB Input 1dB Compression Point Differential Peak-to-Peak (Notes 7, 18) 4.3 VP-P, DIFF

Power Supply (VCC)

VCC Supply Voltage 4.5 5 5.25 V

ICC, ON Supply Current EN = High 80 110 145 mA

ICC, OFF Supply Current, Sleep Mode EN = 0V 100 A

tON Turn-On Time EN = Low to High (Note 11) 0.3 s

tOFF Turn-Off Time EN = High to Low (Note 12) 1.4 s

Enable (EN), Low = Off, High = On

Enable Input High VoltageInput High Current

EN = HighEN = 5V

1.0245

VA

Sleep Input Low VoltageInput Low Current

EN = LowEN = 0V 0.01

0.5 VA

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SUPPLY VOLTAGE (V)4.5

SUPPLYCURRENT(mA)

120

110

100

90

55682 G01

5 5.5

85C

25C

40C

LO FREQUENCY (MHz)

14

RFOUTPUTPOWER(dBm)

6

10

12

8

4

2

0

55682 G02

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

550 650 750 850 950 1050 1150 1250LO FREQUENCY (MHz)

18

VOLTAGEGAIN(dB)

10

14

16

12

8

6

4

55682 G03

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

550 650 750 850 950 1050 1150 1250

LO FREQUENCY (MHz)

12

OIP3(dBm)

20

16

14

18

22

24

26

55682 G04

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

550 650 750 850 950 1050 1150 1250

fBB, 1 = 2MHzfBB, 2 = 2.1MHz

LO FREQUENCY (MHz)

55045

OIP2(dBm) 60

55

50

65

70

650 750 850 950

55682 G05

1050 1150 1250

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

fIM2 = fBB, 1 + fBB, 2 + fLOfBB, 1 = 2MHzfBB, 2 = 2.1MHz

LO FREQUENCY (MHz)

5502

OP1dB(dBm)

6

4

8

10

650 750 850 950

55682 G06

1050 1150 1250

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

LO FREQUENCY (MHz)

55046

LOFEEDTHROUGH(dBm)

42

44

40

38

650 750 850 950

55682 G07

1050 1150 1250

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

2 LO FREQUENCY (GHz)

1.165

P(2LO)(dBm)

55

60

50

45

1.3 1.5 1.7 1.9

55682 G08

2.1 2.3 2.5

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

3 LO FREQUENCY (GHz)

1.6570

65

P(3LO)(dBm)

55

60

50

45

1.95 2.25 2.55 2.85

55682 G09

3.15 3.45 3.75

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

VCC = 5V, EN = High, TA = 25C, fLO = 900MHz,PLO = 0dBm. BBPI, BBMI, BBPQ, BBMQ inputs 0.54VDC, Baseband Input Frequency fBB = 2MHz, I&Q 90 shifted. fRF = fBB + fLO (uppersideband selection). PRF, OUT = 10dBm (10dBm/tone for 2-tone measurements), unless otherwise noted. (Note 3)

Supply Current vs Supply Voltage

RF Output Power vs LO Frequencyat 1VP-P Differential Baseband Drive

Voltage Gain vs LO Frequency

Output IP3 vs LO Frequency

Output IP2 vs LO Frequency

Output 1dB Compressionvs LO Frequency

TYPICAL PERFOR A CE CHARACTERISTICSUW

LO Feedthrough to RF Outputvs LO Frequency

2 LO Leakage to RF Outputvs 2 LO Frequency

3 LO Leakage to RF Outputvs 3 LO Frequency

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LO INPUT POWER (dBm)

2055

IM

AGEREJECTION(dBc)

45

50

40

35

16 12 8 4

55682 G14

0 4 8

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

PRF = 10dBmfBB = 100kHz

RF FREQUENCY (MHz)550

163

163NOISEFLOOR(dBm/Hz)

160

161

159

158

650 750 850 950

55682 G10

1050 1150 1250

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

fLO = 900MHz(FIXED)NO RF

LO FREQUENCY (MHz)550

55

IMAGEREJECTION(dBc)

40

50

45

35

30

650 750 850 950

55682 G11

1050 1150 1250

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

fBB = 100kHz

RF FREQUENCY (MHz)550

40

S11(dB)

20

30

10

0

650 750 850 950

55682 G12

1050 1150 1250

LO PORT, EN = LOW

RF PORT,EN = LOW

RF PORT,EN = HIGH,PLO = 0dBm

RF PORT, EN = HIGH, No LO

LO PORT,EN = HIGH,PLO = 10dBm

LO PORT, EN = HIGH,PLO= 0dBm


16

14

VOLTAGEGAIN(dB)

10

12

8

6

4

55682 G15

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

20 16 12 8 4 0 4 8



Noise Floor vs RF Frequency

Image Rejection vs LO Frequency

LO and RF Port Return Lossvs RF Frequency

Voltage Gain vs LO Power

Output IP3 vs LO Power

Image Rejection vs LO Input PowerLO Feedthrough to RF Outputvs LO Input Power

I AND Q BASEBAND VOLTAGE (VPP, DIFF)

080

HD2(dBc),HD3(dBc)

RFCWOUTPUTPOWER(dBm)

40

60

20

10

1 2 3 4

55682 G18

50

70

30

60

20

40

0

10

30

50

10

5

HD2 = MAX POWER AT fLO + 2 fBB OR fLO 2 fBBHD3 = MAX POWER AT fLO + 3 fBB OR fLO 3 fBB

RF

HD24.5V

5.5V

5V

HD3

5V4.5V

RF CW Output Power, HD2 andHD3 vs CW Baseband Voltageand Supply Voltage


50

48LOFEEDTHROUGH(dBm)

44

46

42

40

38

55682 G13

5.5V, 25C4.5V, 25C5V, 85C5V, 25C

5V, 40C

20 16 12 8 4 0 4 8


13

15

0IP3(dBm)

19

17

21

23

25

55682 G16

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

20 16 12 8 4 0 4 8

fBB, 1 = 2MHzfBB, 2 = 2.1MHz

I AND Q BASEBAND VOLTAGE (VPP, DIFF)

080

HD2(dBc),HD3(

dBc)

RFCWO

UTPUTPOWER(dBm)

40

60

20

10

1 2 3 4

55682 G17

50

70

30

60

20

40

0

10

30

50

10

5

HD2 = MAX POWER AT fLO + 2 fBB OR fLO 2 fBBHD3 = MAX POWER AT fLO + 3 fBB OR fLO 3 fBB

40C

HD3

85C

RF

40C 85C

HD2

25C

40C

25C

85C

25C

RF CW Output Power, HD2 andHD3 vs CW Baseband Voltageand Temperature

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I AND Q BASEBAND VOLTAGE (VP-P,DIFF)0

55

IMAGEREJECTION(dBc)

50

45

1 1.5 2 2.5

55682 G20

0.5 3

5.5V, 25C4.5V, 25C

5V, 85C5V, 25C5V, 40C

fBB = 100kHz

I AND Q BASEBAND VOLTAGE (VPP, DIFF, EACH TONE)0.1

80

PRF,T

ONE(dBm),IM2(dBc),IM3(dBc)

40

60

20

10

1

55682G21

50

70

30

0

10

10

IM2 = POWER AT fLO + 4.1MHzIM3 = MAX POWER AT fLO + 1.9MHz OR fLO + 2.2MHz

RF

IM3

IM2

85C40C

85C

40C

40C

25C

85C

25C

fBBI = 2MHz, 2.1MHz, 0fBBQ = 2MHz, 2.1MHz, 90

25C

I AND Q BASEBAND VOLTAGE (VPP, DIFF, EACH TONE)

0.180

PRF,T

ONE(dBm

),IM2(dBc),IM3(dBc)

40

60

20

10

1

55682G22

50

70

30

0

10

10

IM2 = POWER AT fLO + 4.1MHzIM3 = MAX POWER AT fLO + 1.9MHz OR fLO + 2.2MHz

IM3

IM2

fBBI = 2MHz, 2.1MHz, 0fBBQ = 2MHz, 2.1MHz, 90

4.5V

4.5V

5V, 5.5V

5V

5V, 5.5V

4.5V

5.5V

RF

Image Rejection

vs CW Baseband Voltage

RF Two-Tone Power (Each Tone),IM2 and IM3 vs Baseband Voltage

and Temperature

RF Two-Tone Power (Each Tone),IM2 and IM3 vs Baseband Voltageand Supply Voltage



GAIN (dB)

90

PERCENTAGE(%)

5

10

15

20

25

7.5

55682 G23

8.5 6.578 6 5.5

40C25C85C

NOISE FLOOR (dBm/Hz)

160.4

PER

CENTAGE(%)

159.6

55682 G24

160 159.2 158.8

40C25C85C

0

5

10

15

20

35

30

25

LO LEAKAGE (dBm)

< 540

PERCENTAGE(%)

10

20

30

35

5

15

25

40

46

55682 G25

50 42 38 3034

40C25C85C

IMAGE REJECTION (dBc)

< 700

PERCENTAGE(%)

10

20

5

15

25

62

55682 G26

66 58 54 4650

40C25C85C

Noise Floor Distribution

LO Leakage Distribution Image Rejection Distribution

Gain Distribution

I AND Q BASEBAND VOLTAGE (VP-P,DIFF)0

44

LOFEEDTHROUGH(dBm)

40

42

38

36

1 32 4

55682 G19

5

5.5V, 25C4.5V, 25C5V, 85C5V, 25C5V, 40C

LO Feedthrough to RF Output

vs CW Baseband Voltage

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EN (Pin 1): Enable Input. When the enable pin voltage ishigher than 1V, the IC is turned on. When the input voltageis less than 0.5V, the IC is turned off.

GND (Pins 2, 4, 6, 9, 10, 12, 15): Ground. Pins 6, 9, 15and 17 (exposed pad) are connected to each other inter-nally. Pins 2 and 4 are connected to each other internallyand function as the ground return for the LO signal. Pins10 and 12 are connected to each other internally andfunction as the ground return for the on-chip RF balun.For best RF performance, pins 2, 4, 6, 9, 10, 12, 15 andthe Exposed Pad 17 should be connected to the printedcircuit board ground plane.

LO (Pin 3): LO Input. The LO input is an AC-coupled single-

ended input with approximately 50 input impedance atRF frequencies. Externally applied DC voltage should bewithin the range 0.5V to VCC + 0.5V in order to avoidturning on ESD protection diodes.

BBPQ, BBMQ (Pins 7, 5): Baseband Inputs for the Q-chan-nel, each 50 input impedance. Internally biased at about0.54V. Applied voltage must stay below 2.5V.

VCC (Pins 8, 13): Power Supply. Pins 8 and 13 are con-nected to each other internally. It is recommended to use0.1F capacitors for decoupling to ground on each ofthese pins.

RF (Pin 11): RF Output. The RF output is an AC-coupledsingle-ended output with approximately 50 output im-pedance at RF frequencies. Externally applied DC voltageshould be within the range 0.5V to VCC + 0.5V in orderto avoid turning on ESD protection diodes.

BBPI, BBMI (Pins 14, 16): Baseband Inputs for theI-channel, each with 50 input impedance. Internally biasedat about 0.54V. Applied voltage must stay below 2.5V.

Exposed Pad (Pin 17): Ground. This pin must be solderedto the printed circuit board ground plane.

PI FU CTIO SUUU

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The LT5568-2 consists of I and Q input differential volt-age-to-current converters, I and Q up-conversion mixers,an RF output balun, an LO quadrature phase generatorand LO buffers.

Figure 1. Simplified Circuit Schematic of the LT5568-2(Only I-Half is Drawn)

External I and Q baseband signals are applied to the dif-ferential baseband input pins, BBPI, BBMI, and BBPQ,BBMQ. These voltage signals are converted to currents andtranslated to RF frequency by means of double-balancedup-converting mixers. The mixer outputs are combinedin an RF output balun, which also transforms the output

impedance to 50. The center frequency of the resultingRF signal is equal to the LO signal frequency. The LO inputdrives a phase shifter which splits the LO signal into in-phase and quadrature LO signals. These LO signals are thenapplied to on-chip buffers which drive the up-conversionmixers. Both the LO input and RF output are single-ended,50-matched and AC coupled.

Baseband Interface

The baseband inputs (BBPI, BBMI), (BBPQ, BBMQ) presenta differential input impedance of about 100. At each of thefour baseband inputs, a first-order lowpass filter using 25

APPLICATIO S I FOR ATIOW U UU

BLOCK DIAGRAW

90

0

LT5568-2

V-I

V-I

BALUN

VCC

RF

LO55682 BD

11

EN1

396

GND

42

5

7

16

14

8 13

BBPI

BBMI

BBPQ

BBMQ

1715

GND

1210

RF

VCC = 5V

BBPI

BBMI

C

GND

LOMI LOPI

R4

FROMQ

55682 F01

BALUN

CM

VREF = 540mV

R3

R1B23

R1A25

12pF

R2A25

R2B23

12pF

LT5568-2

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Figure 3. LT5568-2 GSM Baseband Interface with 3rd Order Lowpass Filter and Ground Referenced DAC (Only I-Channel is Shown)

Figure 2. DC Voltage Levels for a Generator Programmed at0.27VDC for a 50 Load and the LT5568-2 as a Load

The baseband inputs should be driven differentially; other-wise, the even-order distortion products will degrade theoverall linearity severely. Typically, a DAC will be the signal

source for the LT5568-2. Reconstruction filters shouldbe placed between the DAC output and the LT5568-2sbaseband inputs. In Figure 3, a typical baseband interfaceschematic for GSM is drawn. It shows a ground referencedDAC output interface followed by a 3rd order active OpAmpRC lowpass filter with a 400kHz cutoff frequency (3dB).The DAC in this example sources a current from 0mA to20mA, with a voltage compliance range of at least 0V to1V. This interface is DC coupled, which allows adjust-ment of the DACs differential output current to minimizethe LO feedthrough. The voltage swing at the LT5568-2

baseband inputs is about 2VP-P,DIFF, which results in a1.2dBm GSM RF output power at 900MHz with noise floorof 154.3dBm/Hz at 6MHz offset (= 104.3dBm/100kHz).The RMS EVM is about 0.6%. The LT1819, which housestwo LT1818s, can be used instead of U2 and U3. The totalcurrent in the positive supply is about 157mA and thecurrent in the negative supply is about 40mA.

and 12pF to ground is incorporated (see Figure 1), whichlimits the baseband bandwidth to approximately 330MHz(1dB point). The common mode voltage is about 0.54V

and is approximately constant over temperature.

It is important that the applied common mode voltage levelof the I and Q inputs is about 0.54V in order to properlybias the LT5568-2. Some I/Q test generators allow settingthe common mode voltage independently. In this case, thecommon mode voltage of those generators must be setto 0.27V to match the LT5568-2 internal bias, because forDC signals, there is no 6dB source-load voltage division(see Figure 2).

55682 F02

4850

LT5568-2GENERATOR

0.54VDC0.54VDC

0.54VDC+

+

50

50

GENERATOR

0.54VDC

0.27VDC+

RF =1.2dBm,GSM

VCC

= 4.5 TO 5.25V

VCC

VSS

C

GND

GND

LOMILOPI

FROM

U1

Q

R433

16mA

55682 F03

LT5568-2

CM

VREF = 540mV

R333

R553.6

R145

2

3

BBPI

BBMI

R245

DAC

0mA to 20mA

0mA to 20mA

C31nF

BALUN

R7200

R9249

R653.6

R8200

R10249

+

+

C11.2nF

GNDR13499

R11249

C41nF

4

6

7

U2LT1818

0.54V

0.54V

0.54V

0.54V

0.54V

VCC

VSS

VSS = 2V to 5.25V

3

2

C21.2nF

C510nF

C610nF

R12249

4

6

7

U3LT1818

R1450

R1550

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LO Section

The internal LO input amplifier performs single-ended to

differential conversion of the LO input signal. Figure 4shows the equivalent circuit schematic of the LO input.

Table 1. LO Port Input Impedance vs Frequency for EN = Highand PLO = 0dBm

Frequency Input Impedance S11

MHz Mag Angle500 47.5 + j12.1 0.126 95.0

600 59.4 + j8.4 0.115 37.8

700 66.2 j1.14 0.140 3.41

800 67.2 j13.4 0.185 31.7

900 61.1 j23.9 0.232 53.2

1000 53.3 j26.8 0.252 68.7

1100 48.2 j26.1 0.258 79.4

1200 42.0 j27.4 0.297 90.0

If the part is in shutdown mode, the input impedance ofthe LO port will be different. The LO input impedance for

EN = Low is given in Table 2.Table 2. LO Port Input Impedance vs Frequency for EN = Low andPLO = 0dBm

Frequency Input Impedance S11MHz Mag Angle

500 33.6 + j41.3 0.477 85.4

600 59.8 + j69.1 0.539 49.8

700 140 + j89.8 0.606 19.6

800 225 j62.6 0.659 6.8

900 92.9 j128 0.704 29.6

1000 39.8 j95.9 0.735 45.5

1100 22.8 j72.7 0.755 65.6

1200 16.0 j57.3 0.763 79.7

RF Section

After up-conversion, the RF outputs of the I and Q mixers arecombined. An on-chip balun performs internal differentialto single-ended output conversion, while transforming theoutput signal impedance to 50. Table 3 shows the RFport output impedance vs frequency.

Table 3. RF Port Output Impedance vs Frequency for EN = Highand PLO = 0dBm


500 22.0 + j5.7 0.395 164.2

600 28.2 + j12.5 0.317 141.3

700 38.8 + j14.8 0.206 117.5

800 49.4 + j7.2 0.072 90.6

900 49.3 j5.1 0.051 94.7

1000 42.5 j11.1 0.143 117.0

1100 36.7 j11.7 0.202 130.7

1200 33.0 j10.3 0.238 141.6

LOINPUT

20pF

51

5568 F04

VCC

The internal, differential LO signal is then split into in-phaseand quadrature (90 phase shifted) signals that drive LObuffer sections. These buffers drive the double balanced Iand Q mixers. The phase relationship between the LO inputand the internal in-phase LO and quadrature LO signalsis fixed, and is independent of start-up conditions. Theinternal phase shifters are designed to deliver accuratequadrature signals. For LO frequencies significantly be-low 650MHz or above 1.25GHz, however, the quadratureaccuracy will diminish, causing the image rejection todegrade. The LO pin input impedance is about 50, and

the recommended LO input power is 0dBm. For lowerLO input power, the gain, OIP2, OIP3 and noise floor atPRF = 4dBm will degrade, especially for PLO below 2dBmand at TA = 85C. For high LO input power (e.g., +5dBm),the image rejection will degrade with no improvement inlinearity or gain. Harmonics present on the LO signal candegrade the image rejection because they can introduce asmall excess phase shift in the internal phase splitter. Forthe second (at 1.8GHz) and third harmonics (at 2.7GHz) at20dBc, the resulting signal at the image frequency is about61dBc or lower, corresponding to an excess phase shift

of much less than 1 degree. For the second and third LOharmonics at 10dBc, the introduced signal at the imagefrequency is about 51dBc. Higher harmonics than the thirdwill have less impact. The LO return loss typically will bebetter than 11dB over the 700MHz to 1.05GHz range. Table1 shows the LO port input impedance vs frequency.

Figure 4. Equivalent Circuit Schematic of the LO Input

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The RF output S22 with no LO power applied is given inTable 4.

Table 4. RF Port Output Impedance vs Frequency for EN = Highand No LO Power Applied


500 22.7 + j5.6 0.381 164.0

600 29.7 + j11.6 0.290 142.0

700 40.5 + j11.6 0.164 121.9

800 47.3 + j2.2 0.037 139.6

900 44.1 j6.7 0.094 126.9

1000 38.2 j9.8 0.171 133.9

1100 34.0 j9.4 0.218 143.1

1200 31.5 j7.8 0.245 151.6

For EN = Low the S22 is given in Table 5.

Table 5. RF Port Output Impedance vs Frequency for EN = Low


500 21.2 + j5.4 0.409 164.9

600 26.6 + j12.5 0.340 142.5

700 36.6 + j16.6 0.241 118.1

800 49.2 + j11.6 0.116 87.4

900 52.9 j2.0 0.034 33.1

1000 46.4 j11.2 0.121 101.1

1100 39.3 j13.2 0.188 120.6

1200 34.4 j12.1 0.231 133.8

Note that an ESD diode is connected internally from theRF output to ground (see Figure 5). For strong outputRF signal levels (higher than 3dBm), this ESD diode can

degrade the linearity performance if the 50 terminationimpedance is connected directly to ground. To prevent this,a coupling capacitor can be inserted in the RF output line.This is strongly recommended during a 1dB compressionmeasurement.

Enable Interface

Figure 6 shows a simplified schematic of the EN pininterface. The voltage necessary to turn on the LT5568-2is 1V. To disable (shut down) the chip, the enable voltage

must be below 0.5V. If the EN pin is not connected, thechip is disabled. This EN = Low condition is assured bythe 75k on-chip pull-down resistor. It is important thatthe voltage at the EN pin does not exceed VCC by morethan 0.5V. If this should occur, the supply current couldbe sourced through the EN pin ESD protection diodes,which are not designed to carry the full supply current,and damage may result.

Figure 5. Equivalent Circuit Schematic of the RF Output Figure 6. EN Pin Interface

75k

55682 F06

VCC

25kEN

21pF

1pF7nH 51

55682 F05

VCC

RFOUTPUT

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Evaluation Board

Figure 7 shows the evaluation board schematic. A good

ground connection is required for the exposed pad. If thisis not done properly, the RF performance will degrade. Ad-ditionally, the exposed pad provides heat sinking for the partand minimizes the possibility of the chip overheating.

R1 (optional) limits the EN pin current in the event thatthe EN pin is pulled high while the VCC inputs are low. InFigures 8 and 9 the silk screens and the PCB board layout

are shown.

Figure 8. Component Side of Evaluation Board

Figure 9. Bottom Side of Evaluation Board


Figure 7. Evaluation Circuit Schematic

BBPIBBMI

J1

16 15 14 13

VCC

VCC EN

9

10

11

12

4

3

2

1

5 6 7 8

55682 F07

17

BBMQ

BBPQ

BOARD NUMBER: DC1178A

C1100nF

J6

RFOUT

J3

LOIN

J4

GND

J5

C2100nF

J2

BBMI

LT5568-2

BBPI VCC

BBMQ GND

GND

BBPQ VCC

GND

GND

RF

GND

GND

LO

GND

EN

GND

100R1

55682 F09

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RF OUTPUT POWER PER CARRIER (dBm)

30

ACPR,

AltCPR(dBc)

NOISEFLOORAT30MHz

OFFSET(dBm/Hz)

70

60

10

55682 F11

80

9025 20 15 5

50

145

135

155

165

125

DOWNLINK TEST MODEL 64 DPCH

1-CH. NOISE

3-CH. NOISE

1-CH. AltCPR

1-CH.ACPR

3-CH. ACPR

3-CH. AltCPR

RF FREQUENCY (MHz)

896.25

POW

ERIN30kHzBW

(dBm)

70

50

30

902.25

55682 F12

90

110

80

60

40

100

120

130897.75 899.25 900.75 903.75

SPECTRUM ANALYSER NOISE FLOOR

UNCORRECTED

SPECTRUM

CORRECTEDSPECTRUM

DOWNLINK TESTMODEL 64 DPCH

RF FREQUENCY (MHz)

894

POW

ERIN30kHzBW

(dBm)

70

50

30

902 904

55682 F13

90

110

80

60

40

100

120

130896 898 900 906

CORRECTEDSPECTRUM

UNCORRECTEDSPECTRUM

DOWNLINKTEST

MODEL 64DPCH

SPECTRUM ANALYSERNOISE FLOOR


Application Measurements

The LT5568-2 is recommended for base-station applica-

tions using various modulation formats. Figure 10 shows atypical application. Figure 11 shows the ACPR performancefor CDMA2000 using 1- and 3-carrier modulation. Figures12 and 13 illustrate the 1- and 3-carrier CDMA2000 RFspectrum. To calculate ACPR, a correction is made for thespectrum analyzer noise floor. If the output power is high,the ACPR will be limited by the linearity performance of thepart. If the output power is low, the ACPR will be limitedby the noise performance of the part. In the middle, anoptimum ACPR is observed.

Because of the LT5568-2s very high dynamic range, thetest equipment can limit the accuracy of the ACPR mea-surement. See Application Note 99. Consult the factory

for advice on the ACPR measurement, if needed.

The ACPR performance is sensitive to the amplitude matchof the BBPI and BBMI (or BBPQ and BBMQ) inputs. Thisis because a difference in AC current amplitude will giverise to a difference in amplitude between the even-orderharmonic products generated in the internal V-I converter.As a result, they will not cancel out entirely. Therefore, itis important to keep the currents in those pins exactly

Figure 10. 850MHz to 965MHz DirectConversion Transmitter Application

90

0

LT5568-2

BASEBANDGENERATOR

PA

VCO/SYNTHESIZER

EN

2, 4, 6, 9, 10, 12, 15, 17

5V

V-I

V-I

I-CHANNEL

Q-CHANNELBALUN

14

16

1

7

5

8, 13VCC

11

3 55682 F10

I-DAC

Q-DAC

RF = 850MHzTO 965MHz

100nFx2

Figure 12. 1-Carrier CDMA2000 Spectrum Figure 13. 3-Carrier CDMA2000 Spectrum

Figure 11. ACPR, AltCPR and NoiseCDMA2000 Modulation

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TEMPERATURE (C)

40 2090

LOFEEDTHRO

UGH(dBm),IR(dBc)

70

50

0 40 80

55682 F14

80

60

20 60

EN = High

VCC = 5V

fBBI = 2MHz, 0fBBQ = 2MHz, 90

fLO = 900MHzfRF = fBB + fLOPLO = 0dBm

IMAGE REJECTION

LO FEED-THROUGH

CALIBRATED WITH PRF = 10dBm

I AND Q BASEBAND VOLTAGE (VP-P, DIFF)

0

PRF(dBm),L

OFT(dBm),IR(dBc)

30

10

10

455682F15

50

70

40

20

0

60

80

901 2 3 5

fBBI = 2MHz, 0fBBQ = 2MHz, 90

VCC = 5VEN = High

fLO = 900MHzfRF = fBB + fLOPLO = 0dBm

40C

40C

40C

85C

85C

85CLOFT

IR

PRF

25C

25C

25C


the same (but of opposite sign). The current will enterthe LT5568-2s common-base stage, and will flow to themixer upper switches. This can be seen in Figure 1 where

the internal circuit of the LT5568-2 is drawn.

After calibration when the temperature changes, the LOfeedthrough and the image rejection performance will

change. This is illustrated in Figure 14. The LO feedthroughand image rejection can also change as a function of thebaseband drive level, as is depicted in Figure 15. In Figure

16 the GSM EVM and noise performance vs RF outputpower is drawn.

Figure 15. LO Feedthrough and Image Rejectionvs Baseband Drive Voltage after Calibration at 25C

Figure 14. LO Feedthrough and Image Rejectionvs Temperature after Calibration at 25C

Figure 16. GSM EVM and Noise vs RF Output Power at 900MHz

GSM RF OUTPUT POWER (dBm)

100

EVM

(%RMS)

NOISEFLOORAT6MHz

OFFSET(dBc/100kHz)

1

3

4

5

6 2 0

2

106

104

100

98

96

102

8 4 2 4 6

55682 F16

EVM

NOISE

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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However,

no responsibility is assumed for its use. Linear Technology Corporation makes no representation thatthe interconnection of its circuits as described herein will not infringe on existing patent rights.

PACKAGE DESCRIPTIOU

UF Package16-Lead Plastic QFN (4mm 4mm)

(Reference LTC DWG # 05-08-1692)

4.00 0.10(4 SIDES)

NOTE:1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGC)2. DRAWING NOT TO SCALE3. ALL DIMENSIONS ARE IN MILLIMETERS4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE5. EXPOSED PAD SHALL BE SOLDER PLATED6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION

ON THE TOP AND BOTTOM OF PACKAGE

PIN 1TOP MARK(NOTE 6)

0.55 0.20

1615

1

2

BOTTOM VIEWEXPOSED PAD

2.15 0.10(4-SIDES)

0.75 0.05 R = 0.115TYP

0.30 0.05

0.65 BSC

0.200 REF

0.00 0.05

(UF16) QFN 10-04

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

0.72 0.05

0.30 0.050.65 BSC

2.15 0.05(4 SIDES)2.90 0.05

4.35 0.05

PACKAGE OUTLINE

PIN 1 NOTCH R = 0.20 TYPOR 0.35 45 CHAMFER

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Linear Technology CorporationLT 0307 PRINTED IN USA

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