
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.
The use of dual integrated DACs, such as t
he 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 th
e ADF4351. The
low-pass filter was designed usin
g 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 th
e 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 load, but it must be
ac-coupled, as shown i
n 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
680
360
IBBP
IBBN
QBBP
QBBN
LOIP
LOIN
S
P
I-
CO
M
P
AT
IBL
E
S
E
RI
AL
BUS
ADF4351
VVCO
CPGND
AGND
DGND
RFOUTB–
RFOUTB+
CPOUT
1nF
4.7k
RSET
LE
DATA
CLK
REFIN
fREFIN
VTUNE
DVDD AVDD CE
MUXOUT
10
28
16
29
1
2
3
22
8
31
9
11
18
21
27
VDD
LOCK
DETECT
51
51
51
51
51
AGNDVCO
14
15
19
23
24
25
30
LD
17
20
7
PDBRF
26
SDGND
TEMP VCOM
VREF
6
32
SDVDD
VP
5
SW
10pF
0.1F 10pF
0.1F
4
ADL5375
RFOUT
QUADRATURE
PHASE
SPLITTER
DSOP
RFOUTA–
RFOUTA+
13
12
1nF
3.9nH
VVCO
LPF
Figure 34. Direct Conversion Modulator