lt5571 Linear Technology Corporation, lt5571 Datasheet - Page 10

• Manufacturer Part Number: lt5571
• Description: 620mhz - 1100mhz High Linearity Direct Quadrature Modulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT5571
full 0V to 1V swing on each baseband input (2V
This maximum RF output level is limited by the 0.5V
maximum baseband swing possible for a 0.5V
mon-mode voltage level (assuming no negative supply
bias voltage is available).
It is possible to bias the LT5571 to a common mode
voltage level other than 0.5V. Table 1 shows the typical
performance for different common mode voltages.
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 split into in-phase and
quadrature (90° phase shifted) signals to 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
phase shifters are designed to deliver accurate quadrature
signals for an LO frequency near 900MHz. For frequen-
cies signifi cantly below 750MHz or above 1100MHz, the
quadrature accuracy will diminish, causing the image
rejection to degrade. The LO pin input impedance is about
Table 1. Typical Performance Characteristics vs V
10
V
CM
0.25
0.1
0.2
0.3
0.4
0.5
0.6
Figure 4. Equivalent Circuit Schematic of the LO Input
(V)
I
CC
108.1
55.3
65.3
70.3
75.7
86.4
97.1
(mA)
INPUT
LO
V
CC
G
V
–4.5
–3.9
–3.7
–3.6
–3.5
–3.6
–3.7
20pF
(dB)
Z
5571 F04
IN
≈ 60Ω
OP1dB (dBm)
–1.5
2.0
3.4
4.5
6.3
7.9
8.4
CM
for f
DC
P-P,DIFF
LO
OIP2 (dBm)
com-
= 900MHz, P
PEAK
53.4
51.7
51.9
52.1
53.1
53.0
53.7
).
50Ω, and the recommended LO input power window is
–2dBm to 2dBm. For P
OIP2, OIP3, dynamic-range (in dBc/Hz) and image rejection
will degrade, especially at T
Harmonics present on the LO signal can degrade the image
rejection, because they 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 level, the in-
troduced signal at the image frequency is about –61dBc
or lower, corresponding to an excess phase shift much
less than 1 degree. For the second and third harmonics at
–10dBc, still 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 750MHz to 1GHz range. Table 2 shows the
LO port input impedance vs frequency.
Table 2. LO Port Input Impedance vs Frequency for EN = High
and P
The return loss S
lower frequencies by adding a shunt capacitor. The input
impedance of the LO port is different if the part is in
shut-down mode. The LO input impedance for EN = Low
is given in Table 3.
OIP3 (dBm)
LO
FREQUENCY
= 0dBm
11.2
13.3
15.6
18.7
20.6
22.1
LO
9.2
(MHz)
1000
1100
1200
500
600
700
800
900
= 0dBm
NFloor (dBm/Hz)
11
INPUT IMPEDANCE
–163.6
–161.8
–161.2
–160.5
–159.6
–158.7
–157.9
on the LO port can be improved at
47.2 + j11.7
66.1 – j12.2
60.7 – j22.5
53.3 – j25.1
48.4 – j25.1
42.7 – j26.4
58.4 + j8.3
65.0 – j0.6
LO
(Ω)
< –2dBm input power, the gain,
A
= 85°C.
LOFT (dBm)
–53.6
–50.3
–49.0
–47.7
–45.3
–43.1
–41.2
0.123
0.108
0.131
0.173
0.221
0.239
0.248
0.285
Mag
S
11
IR (dBc)
37.0
40.4
43.5
43.9
45.1
45.4
45.6
Angle
–31
–53
–69
–79
–89
97
40
–2
5571f