Datasheet

ADF4350
Rev. A | Page 25 of 32
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.
The circuit block diagram shows the AD9761 TxDAC® being
used with the ADL5375. The use of dual integrated DACs, such
as the AD9788 with its specified ±0.02 dB and ±0.001 dB gain
and offset matching characteristics, ensures minimum error
contribution (over temperature) from this portion of the
signal chain.
The local oscillator (LO) is implemented using the ADF4350.
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 RF
OUT
A and RF
OUT
B outputs of the ADF4350.
This gives 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. At LO frequencies below 3 GHz some harmonic
filtering may be necessary to ensure best single sideband
performance.
The ADL5375 accepts LO drive levels from −6 dBm to +7 dBm.
The optimum LO power can be software programmed on the
ADF4350, which allows levels from −4 dBm to +5 dBm from
each output. For more details on this circuit, consult CN-0134.
The RF output is designed to drive a 50 Ω load, but 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 modulator is approximately 2 dBm.
07325-021
2700pF 1200pF
39nF
680
360
SPI-COMPATIBLE SERIAL BUS
ADF4350
V
VCO
V
VCO
CP
GND
AGND
DGND
RF
OUT
B–
RF
OUT
B+
CP
OUT
1nF1nF
4.7k
R
SET
LE
DATA
CLK
REF
IN
FREF
IN
V
TUNE
DV
DD
AV
DD
CE
MUXOUT
10
28
16
29
1
2
3
22
8 31 9 11 18 21 27
V
DD
LOCK
DETECT
51
A
GNDVCO
V
COM
14
15
19 23 24
25
30
LD
17
20
7
PDB
RF
26
SD
GND
TEMP
V
REF
6 32
SDV
DD
V
P
5
SW
10pF
0.1µF
10pF
0.1µF
10pF
0.1µF
4
AD9761
TxDAC
REFIO
FSADJ
MODULATED
DIGITAL
DATA
QOUTB
IOUTA
IOUTB
QOUTA
2k
LOW-PASS
FILTER
LOW-PASS
FILTER
IBBP
IBBN
QBBP
QBBN
LOIP
LOIN
51 51
51 51
ADL537 5
RFO
QUADRATURE
PHASE
SPLITTER
DSOP
RF
OUT
A–
RF
OUT
A+
13
12
V
VCO
3.9nH 3.9nH
1nF
1nF
LPF
LPF
Figure 34. Direct Conversion Modulator