LT5528 Datasheet, PDF (10/16 Pages) Linear Technology – 1.5GHz to 2.4GHz High Linearity Direct Quadrature Modulator

LT5528
APPLICATIO S I FOR ATIO
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.
V_CC
20pF
LO
INPUT
5528 F04
Z_IN ≈ 57Ω
Figure 4. 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 fixed, and is independent of start-up condi-
tions. The phase shifters are designed to deliver accurate
quadrature signals for an LO frequency near 2GHz. For
frequencies significantly below 1.8GHz or above 2.4GHz,
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 dynamic-
range will degrade, especially below –5dBm and at T_A =
85°C. For high LO input power (e.g. 5dBm), the LO feed-
through will increase 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
4GHz) and third harmonics (at 6GHz) at –20dBc level, the
introduced signal at the image frequency is about –56dBc
or lower, corresponding to an excess phase shift much
below 1 degree. For the second and third harmonics at
–10dBc, the introduced signal at the image frequency is
about –47dBc. Higher harmonics than the third will have
less impact. The LO return loss typically will be better than
17dB over the 1.7GHz to 2.3GHz range. Table 1 shows the
LO port input impedance vs. frequency.
10
Table 1. LO Port Input Impedance vs Frequency for EN = High
Frequency
MHz
1000
Input Impedance
Ω
49.9 + j18.5
S₁₁
Mag
Angle
0.182
80
1400
68.1 + j8.8
0.171
22
1600
71.0 + j2.0
0.175
4.8
1800
70.0 – j8.6
0.182
–6.6
2000
62.0 – j12.8
0.156
–40
2200
53.8 – j13.6
0.135
–66
2400
47.3 – j12.4
0.128
–95
2600
41.1 – j12.0
0.161
–119
If the part is in shut-down 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
Frequency
MHz
1000
1400
1600
1800
2000
2200
2400
2600
Input Impedance
Ω
46.6 + j47.6
136 + j44.5
157 – j24.5
114 – j70.6
70.7 – j72.1
45.3 – j59.0
31.2 – j45.2
22.8 – j34.2
S₁₁
Mag
Angle
0.443
67.8
0.507
13.8
0.526
–6.2
0.533
–24.6
0.533
–43.2
0.528
–62.8
0.527
–83.5
0.543
–103
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 = 0dBm
Frequency Output Impedance
MHz
Ω
S₂₂
Mag
Angle
1000
23.1 + j7.9
0.382
158
1400
34.4 + j20.7
0.298
113
1600
45.8 + j22.3
0.231
87.6
1800
54.5 + j12.4
0.125
63.2
2000
48.7 + j1.7
0.022
127
2200
39.1 + j1.0
0.123
174
2400
32.9 + j4.4
0.213
163
2600
29.7 + j7.4
0.269
155
5528f