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Data Sheet

ADF4351

Rev. 0 | Page 25 of 28

APPLICATIONS INFORMATION

DIRECT CONVERSION MODULATOR

Direct conversion architectures are increasingly being used

to implement base station transmitters. Figure 34 shows how

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

system.

Figure 34 shows the AD9788 TxDAC® used with the ADL5375.

The use of dual integrated DACs, such as the AD9788 with its

specified ±2% FSR and ±0.001% FSR gain and offset character-

istics, ensures minimum error contribution (over temperature)

from this portion of the signal chain.

The local oscillator (LO) is implemented using the ADF4351. 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 differentially from

the complementary RFOUTA± outputs of the ADF4351. This setup

provides better performance than a single-ended LO driver and

eliminates the use of a balun to convert from a single-ended LO

input to the more desirable differential LO input for the ADL5375.

The typical rms phase noise (100 Hz to 5 MHz) of the LO in this

configuration is 0.61° rms.

The ADL5375 accepts LO drive levels from −6 dBm to +6 dBm.

The optimum LO power can be software programmed on the

ADF4351, which allows levels from −4 dBm to +5 dBm from

each output.

The RF output is designed to drive a 50 Ω load, but it must be

ac-coupled, as shown in Figure 34. If the I and Q inputs are

driven in quadrature by 2 V p-p signals, the resulting output

power from the ADL5375 modulator is approximately 2 dBm.

AD9788

TxDAC

MODULATED

DIGITAL

DATA

OUT2_N

OUT1_P

OUT1_N

OUT2_P

LOW-PASS

FILTER

LOW-PASS

FILTER

2700pF

1200pF

39nF

680

Ω

360

Ω

IBBP

IBBN

QBBP

QBBN

LOIP

LOIN

ADF4351

VVCO

CPGND

AGND

DGND

RFOUTB–

RFOUTB+

CPOUT

1nF

4.7k

Ω

RSET

DATA

CLK

REFIN

fREFIN

VTUNE

DVDD AVDD CE

MUXOUT

VDD

LOCK

DETECT

Ω

AGNDVCO

PDBRF

SDGND

TEMP VCOM

VREF

SDVDD

10pF

0.1µF 10pF

0.1µF

ADL5375

RFOUT

QUADRATURE

PHASE

SPLITTER

DSOP

RFOUTA–

RFOUTA+

1nF

3.9nH

VVCO

LPF

Figure 34. Direct Conversion Modulator

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