LT5572_15 Datasheet, PDF (11/16 Pages) Linear Technology – 1.5GHz to 2.5GHz High Linearity Direct Quadrature Modulator

LT5572
APPLICATIO S I FOR ATIO
Table 3. LO Port Input Impedance vs Frequency for EN = Low
and P_LO = 0dBm
FREQUENCY INPUT IMPEDANCE
S₁₁
(MHz)
(Ω)
Mag
Angle
1000
51.2+j45.6
0.409
64
1400
133-j11.8
0.456
–4.5
1600
97.8-j65.8
0.502
–30
1800
58.6-j67.8
0.534
–51
2000
39.0-j55.6
0.540
–69
2200
29.6-j43.2
0.527
–87
2400
23.7-j30.8
0.506
–108
2600
19.7-j20.5
0.503
–130
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 4 shows the RF
port output impedance vs frequency.
Table 4. RF Port Output Impedance vs Frequency for EN = High
and P_LO = 0dBm
FREQUENCY OUTPUT IMPEDANCE
S₂₂
(MHz)
(Ω)
Mag
Angle
1000
20.7+j9.9
0.434
153
1400
32.2+j20.3
0.319
117
1600
44.9+j21.8
0.230
90
1800
56.4+j12.2
0.129
56
2000
52.6+j0.5
0.025
10
2200
43.0+j0.5
0.075
176
2400
36.8+j5.6
0.164
153
2600
32.9+j11.0
0.243
140
The RF output S₂₂ with no LO power applied is given in
Table 5.
Table 5. RF Port Output Impedance vs Frequency for EN = High
and No LO Power Applied
FREQUENCY
(MHz)
OUTPUT IMPEDANCE
(Ω)
S₂₂
Mag
Angle
1000
21.2+j10.1
0.424
153
1400
35.3+j18.4
0.270
117
1600
46.1+j14.1
0.150
97
1800
47.4+j5.0
0.057
114
2000
42.0+j3.0
0.093
157
2200
37.5+j6.8
0.162
147
2400
34.8+j11.8
0.224
134
2600
32.8+j16.1
0.279
126
For EN = Low the S₂₂ is given in Table 6.
Table 6. RF Port Output Impedance vs Frequency for EN = Low
FREQUENCY
(MHz)
OUTPUT IMPEDANCE
(Ω)
S₂₂
Mag
Angle
1000
20.3+j9.7
0.440
154
1400
30.6+j20.2
0.338
120
1600
41.8+j23.6
0.264
95
1800
55.6+j18.5
0.181
63
2000
58.3+j49.1
0.089
28
2200
48.8-j0.1
0.012
-172
2400
40.4+j3.1
0.112
160
2600
34.7+j8.3
0.205
146
To improve S₂₂ for lower frequencies, a shunt capacitor
can be added to the RF output. At higher frequencies, a
shunt inductor can improve the S₂₂. Figure 5 shows the
equivalent circuit schematic of the RF output.
Note that an ESD diode is connected internally from
the RF output to ground. For strong output RF signal
levels (higher than 3dBm) this ESD diode can degrade
the linearity performance if the 50Ω termination imped-
ance is connected directly to ground. To prevent this, a
coupling capacitor can be inserted in the RF output line.
This is strongly recommended for 1dB compression
measurements.
V_CC
52.59
20pF
2.1pF 3nH
RF
OUTPUT
5572 F05
Figure 5. Equivalent Circuit Schematic of the RF Output
Enable Interface
Figure 6 shows a simplified schematic of the EN pin
interface. The voltage necessary to turn on the LT5572
is 1V. To disable (shut down) the chip, the enable voltage
must be below 0.5V. If the EN pin is not connected, the
chip is disabled. This EN = Low condition is guaranteed
by the 75k on-chip pull-down resistor. It is important
that the voltage at the EN pin does not exceed V_CC by
more than 0.5V. If this should occur, the full-chip supply
5572f
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