ADF4151 datasheet(24/28 Pages) AD

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ADF4151

Data Sheet

Rev. B | Page 24 of 28

APPLICATIONS INFORMATION

DIRECT CONVERSION MODULATOR

Direct conversion architectures are increasingly being used to

implement base station transmitters. Figure 29 shows how Analog

Devices, Inc., parts can be used to implement such a system.

The circuit block diagram shows the AD9788 TxDAC® being

used with the ADL5375. The use of dual integrated DACs, such

as the AD9788 with its specified ±0.02 dB and ±0.004 dB gain

and offset matching characteristics, ensures minimum error

contribution (over temperature) from this portion of the

signal chain. The signal for the I channel of the quadrature

modulator is taken from the OUT1 differential outputs of the

AD9788, and the OUT2 differential outputs provide the signal

for the Q channel of the quadrature modulator ADL5375.

The local oscillator (LO) is implemented using the ADF4151.

The low-pass filter was designed using ADIsimPLL™ for a channel

spacing of 200 kHz and a closed-loop bandwidth of 35 kHz.

The LO ports of the ADL5375 can be driven from the VCO

output. To ensure that all three RF ports (VCO output, RFIN and

LOIP) are connected to 50 Ω impedance, the matching network

of three 18 Ω resistors must be placed as in Figure 29. AC

coupling of the RF signal is implemented by the capacitors

connected in serial with the 18 Ω resistors . It is possible, as

well, to use a balun to convert from a single-ended LO input to

the differential LO inputs for the ADL5375.

If the I and Q inputs are driven in quadrature by 2 V p-p

signals, the resulting output power from the modulator is

approximately 2 dBm.

AD9788

TxDAC

REFIO

FSADJ

OUT2_N

OUT1_P

OUT1_N

OUT2_P

2kΩ

LOW-PASS

FILTER

LOW-PASS

FILTER

2700pF

1200pF

39nF

680

Ω

360

Ω

IBBP

IBBN

QBBP

QBBN

LOIP

LOIN

ADF4151

CPGND

AGND

SDGND

1nF

4.7kΩ

RSET

DATA

CLK

REFIN

FREFIN

CPOUT

AVDD2 AVDD2 CE

MUXOUT

VCC

VCOOUT

VCO

VTUNE

AVDD1

VDD

LOCK

DETECT

51Ω

DVDD

SDVDD VP

ADL5375

RFOUT

QUADRATURE

PHASE

SPLITTER

DSOP

RFIN–

RFIN+

1nF

100pF

VVCO

18Ω

100pF

18Ω

MODULATED

DIGITAL

DATA

AGND

DGND

Figure 29. Direct Conversion Modulator

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