Datasheet
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 RF
OUT
A± 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.
09800-034
AD9788
TxDAC
MODULATED
DIGITAL
DATA
OUT2_N
OUT1_P
OUT1_N
OUT2_P
LOW-PASS
FILTER
LOW-PASS
FILTER
2700pF 1200pF
39nF
680Ω
360Ω
IBBP
IBBN
QBBP
QBBN
LOIP
LOIN
SPI-COMPATIBLE SERIAL BUS
ADF4351
V
VCO
V
VCO
CP
GND AGND DGND
RF
OUT
B–
RF
OUT
B+
CP
OUT
1nF1nF
4.7kΩ
R
SET
LE
DATA
CLK
REF
IN
f
REFIN
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
51Ω
51Ω 51Ω
51Ω 51Ω
A
GNDVCO
14
15
19 23 24
25
30
LD
17
20
7
PDB
RF
26
SD
GND TEMP
V
COM
V
REF
6 32
SDV
DD
V
P
5
SW
10pF
0.1µF
10pF
0.1µF
10pF
0.1µF
4
ADL5375
RFOUT
QUADRATURE
PHASE
SPLITTER
DSOP
RF
OUT
A–
RF
OUT
A+
13
12
1nF
1nF
3.9nH 3.9nH
V
VCO
LPF
LPF
Figure 34. Direct Conversion Modulator