LT5568-2EUF-PBF LINER [Linear Technology], LT5568-2EUF-PBF Datasheet - Page 10

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

APPLICATIO S I FOR ATIO
LT5568-2
LO Section
The internal LO input amplifi er performs single-ended to
differential conversion of the LO input signal. Figure 4
shows the equivalent circuit schematic of the LO input.
The internal, differential LO signal is then split into in-phase
and quadrature (90° phase shifted) signals that drive LO
buffer sections. These buffers drive the double balanced I
and Q mixers. The phase relationship between the LO input
and the internal in-phase LO and quadrature LO signals
is fi xed, and is independent of start-up conditions. The
internal phase shifters are designed to deliver accurate
quadrature signals. For LO frequencies signifi cantly be-
low 650MHz or above 1.25GHz, however, the quadrature
accuracy will diminish, causing the image rejection to
degrade. The LO pin input impedance is about 50Ω, and
the recommended LO input power is 0dBm. For lower
LO input power, the gain, OIP2, OIP3 and noise fl oor at
P
and at T
the image rejection will degrade with no improvement in
linearity or gain. Harmonics present on the LO signal can
degrade the image rejection because they can introduce a
small excess phase shift in the internal phase splitter. For
the second (at 1.8GHz) and third harmonics (at 2.7GHz) at
–20dBc, the resulting signal at the image frequency is about
–61dBc or lower, corresponding to an excess phase shift
of much less than 1 degree. For the second and third LO
harmonics at –10dBc, the introduced signal at the image
frequency is about –51dBc. Higher harmonics than the third
will have less impact. The LO return loss typically will be
better than 11dB over the 700MHz to 1.05GHz range. Table
1 shows the LO port input impedance vs frequency.
10
RF
= 4dBm will degrade, especially for P
Figure 4. Equivalent Circuit Schematic of the LO Input
A
= 85°C. For high LO input power (e.g., +5dBm),
INPUT
U
LO
V
CC
5568 F04
U
20pF
W
51Ω
LO
below –2dBm
U
Table 1. LO Port Input Impedance vs Frequency for EN = High
and P
If the part is in shutdown mode, the input impedance of
the LO port will be different. The LO input impedance for
EN = Low is given in Table 2.
Table 2. LO Port Input Impedance vs Frequency for EN = Low and
P
RF Section
After up-conversion, the RF outputs of the I and Q mixers are
combined. An on-chip balun performs internal differential
to single-ended output conversion, while transforming the
output signal impedance to 50Ω. Table 3 shows the RF
port output impedance vs frequency.
Table 3. RF Port Output Impedance vs Frequency for EN = High
and P
LO
Frequency
Frequency
Frequency
= 0dBm
1000
1100
1200
1000
1100
1200
1000
1100
1200
MHz
MHz
MHz
500
600
700
800
900
500
600
700
800
900
500
600
700
800
900
LO
LO
= 0dBm
= 0dBm
Input Impedance
Input Impedance
Input Impedance
47.5 + j12.1
66.2 – j1.14
67.2 – j13.4
61.1 – j23.9
53.3 – j26.8
48.2 – j26.1
42.0 – j27.4
33.6 + j41.3
59.8 + j69.1
39.8 – j95.9
22.8 – j72.7
16.0 – j57.3
28.2 + j12.5
38.8 + j14.8
42.5 – j11.1
36.7 – j11.7
33.0 – j10.3
140 + j89.8
225 – j62.6
92.9 – j128
59.4 + j8.4
22.0 + j5.7
49.4 + j7.2
49.3 – j5.1
Ω
Ω
Ω
0.126
0.115
0.140
0.185
0.232
0.252
0.258
0.297
0.477
0.539
0.606
0.659
0.704
0.735
0.755
0.763
0.395
0.317
0.206
0.072
0.051
0.143
0.202
0.238
Mag
Mag
Mag
S
S
S
11
11
22
–117.0
–130.7
–141.6
Angle
Angle
Angle
–3.41
–31.7
–53.2
–68.7
–79.4
–90.0
–29.6
–45.5
–65.6
–79.7
164.2
141.3
117.5
–94.7
95.0
37.8
85.4
49.8
19.6
–6.8
90.6
55682f