LT5568-2EUF-PBF LINER [Linear Technology], LT5568-2EUF-PBF Datasheet - Page 9

• Manufacturer Part Number: LT5568-2EUF-PBF
• Description: GSM/EDGE Optimized, High Linearity Direct Quadrature Modulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
and 12pF to ground is incorporated (see Figure 1), which
limits the baseband bandwidth to approximately 330MHz
(–1dB point). The common mode voltage is about 0.54V
and is approximately constant over temperature.
It is important that the applied common mode voltage level
of the I and Q inputs is about 0.54V in order to properly
bias the LT5568-2. Some I/Q test generators allow setting
the common mode voltage independently. In this case, the
common mode voltage of those generators must be set
to 0.27V to match the LT5568-2 internal bias, because for
DC signals, there is no –6dB source-load voltage division
(see Figure 2).
+
–
Figure 3. LT5568-2 GSM Baseband Interface with 3rd Order Lowpass Filter and Ground Referenced DAC (Only I-Channel is Shown)
Figure 2. DC Voltage Levels for a Generator Programmed at
0.27V
GENERATOR
0.54V
50Ω
DAC
DC
DC
for a 50Ω Load and the LT5568-2 as a Load
0mA to 20mA
0mA to 20mA
0.27V
V
50Ω
DC
SS
= –2V to –5.25V
U
200Ω
200Ω
U
R7
R8
+
–
53.6Ω
53.6Ω
GENERATOR
R5
R6
0.54V
50Ω
249Ω
249Ω
R10
R9
DC
W
0.54V
C1
1.2nF
C2
1.2nF
1nF
1nF
C3
C4
GND
DC
3
2
2
3
55682 F02
+
–
–
+
LT5568-2
LT1818
LT1818
0.54V
U2
U3
U
7
4
7
4
48Ω
V
V
V
V
DC
CC
SS
CC
SS
+
–
0.54V
6
6
0.54V
0.54V
R11
249Ω
R13
499Ω
R12
249Ω
50Ω
50Ω
R14
R15
The baseband inputs should be driven differentially; other-
wise, the even-order distortion products will degrade the
overall linearity severely. Typically, a DAC will be the signal
source for the LT5568-2. Reconstruction fi lters should
be placed between the DAC output and the LT5568-2’s
baseband inputs. In Figure 3, a typical baseband interface
schematic for GSM is drawn. It shows a ground referenced
DAC output interface followed by a 3rd order active OpAmp
RC lowpass fi lter with a 400kHz cutoff frequency (–3dB).
The DAC in this example sources a current from 0mA to
20mA, with a voltage compliance range of at least 0V to
1V. This interface is DC coupled, which allows adjust-
ment of the DAC’s differential output current to minimize
the LO feedthrough. The voltage swing at the LT5568-2
baseband inputs is about 2V
1.2dBm GSM RF output power at 900MHz with noise fl oor
of –154.3dBm/Hz at 6MHz offset (= –104.3dBm/100kHz).
The RMS EVM is about 0.6%. The LT1819, which houses
two LT1818s, can be used instead of U2 and U3. The total
current in the positive supply is about 157mA and the
current in the negative supply is about 40mA.
0.54V
C5
10nF
C6
10nF
GND
RF =
1.2dBm,
GSM
BBMI
0.54V
BBPI
C
45Ω
R1
BALUN
R3
33Ω
V
REF
LOMI
GND
P-P,DIFF
CM
= 540mV
V
CC
= 4.5 TO 5.25V
, which results in a
LT5568-2
R4
33Ω
16mA
45Ω
R2
LT5568-2
FROM
LOPI
55682 F03
U1
Q
55682f
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