LT5571_15 Datasheet, PDF (10/16 Pages) Linear Technology – 620MHz – 1100MHz High Linearity Direct Quadrature Modulator

LT5571
APPLICATIONS INFORMATION
Table 1. Typical Performance Characteristics vs V_CM for f_LO = 900MHz, P_LO = 0dBm
V_CM (V)
0.1
I_CC (mA)
55.3
G_V (dB)
–4.5
OP1dB (dBm) OIP2 (dBm) OIP3 (dBm)
–1.5
53.4
9.2
0.2
65.3
–3.9
2.0
51.7
11.2
0.25
70.3
–3.7
3.4
51.9
13.3
0.3
75.7
–3.6
4.5
52.1
15.6
0.4
86.4
–3.5
6.3
53.1
18.7
0.5
97.1
–3.6
7.9
53.0
20.6
0.6
108.1
–3.7
8.4
53.7
22.1
NFloor (dBm/Hz)
–163.6
–161.8
–161.2
–160.5
–159.6
–158.7
–157.9
LOFT (dBm)
–53.6
–50.3
–49.0
–47.7
–45.3
–43.1
–41.2
IR (dBc)
37.0
40.4
43.5
43.9
45.1
45.4
45.6
full 0V to 1V swing on each baseband input (2V_P-P,DIFF).
This maximum RF output level is limited by the 0.5V_PEAK
maximum baseband swing possible for a 0.5V_DC com-
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 amplifier 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 fixed, 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 significantly below 750MHz or above 1100MHz, the
quadrature accuracy will diminish, causing the image
rejection to degrade. The LO pin input impedance is about
V_CC
LO
20pF
INPUT
Z_IN ≈ 60Ω
5571 F04
Figure 4. Equivalent Circuit Schematic of the LO Input
50Ω, and the recommended LO input power window is
–2dBm to 2dBm. For P_LO < –2dBm input power, the gain,
OIP2, OIP3, dynamic-range (in dBc/Hz) and image rejection
will degrade, especially at T_A = 85°C.
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_LO = 0dBm
FREQUENCY INPUT IMPEDANCE
S₁₁
(MHz)
(Ω)
Mag
Angle
500
47.2 + j11.7
0.123
97
600
58.4 + j8.3
0.108
40
700
65.0 – j0.6
0.131
–2
800
66.1 – j12.2
0.173
–31
900
60.7 – j22.5
0.221
–53
1000
53.3 – j25.1
0.239
–69
1100
48.4 – j25.1
0.248
–79
1200
42.7 – j26.4
0.285
–89
The return loss S₁₁ on the LO port can be improved at
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.
5571f
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