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ADL5593 Datasheet(PDF) 4 Page - Analog Devices

Part No. ADL5593
Description  Correcting Imperfections in IQ Modulators to Improve RF Signal Fidelity
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ADL5593 Datasheet(HTML) 4 Page - Analog Devices

   
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AN-1039
Application Note
Rev. 0 | Page 4 of 8
Table 1. IQ Modulator Selection Table Showing Uncompensated Gain and Phase Imbalance
PART
NUMBER
FREQUENCY
(MHz)
IQ 3dB
BANDWIDTH
(MHz)
CARRIER
SUPPRESS
(dBm)
SIDEBAND
SUPPRESSION
(dBc)
GAIN
IMBALANCE
(dB)
PHASE
IMBALANCE
(°)
NOISE
FLOOR
(dBm/Hz)
P1dB
(dBm)
OUTPUT
IP3 (dBm)
SUPPLY
VOLTAGE
(V)
SUPPLY
CURRENT
(mA)
SPECS @
(MHz)
AD8345
AD8346
AD8349
ADL5370
ADL5371
ADL5372
ADL5373
ADL5374
ADL5375
ADL5385
ADL5386
ADL5590
ADL5591
140 TO 1000
800 TO 2500
700 TO 2700
300 TO 1000
500 TO 1500
1500 TO 2500
2300 TO 3000
3000 TO 4000
400 TO 6000
50 TO 2200
50 TO 2200
869 TO 960
1805 TO 1990
80
70
160
500
500
500
500
500
750
700
700
250
250
–42
–42
–45
–50
–50
–45
–32
–33
–46
–46
–38
–44
–39
–42
–36
–35
–41
–55
–45
–57
–50
–52
–50
–46
–50
–48
0.200
0.200
0.100
0.030
0.100
0.090
0.010
0.015
–0.050
–0.030
0.050
0.050
0.050
0.50
1.00
1.90
0.76
–0.03
0.21
0.10
0.25
–0.29
–0.39
–0.50
0.20
0.30
–155.0
–147.0
–155.0
–160.0
–158.6
–158.0
–157.0
–160.0
–160.0
–159.0
–160.0
–157.0
–157.0
2.5
–3.0
7.6
11.0
14.4
14.2
13.8
12.0
9.4
11.0
11.1
16.0
16.0
N/A
N/A
21.0
24.0
27.0
27.0
26.0
22.8
27.0
26.0
25.0
29.0
30.0
2.70 TO 5.50
2.70 TO 5.50
4.75 TO 5.50
4.75 TO 5.25
4.75 TO 5.25
4.75 TO 5.25
4.75 TO 5.25
4.75 TO 5.25
4.75 TO 5.25
4.75 TO 5.50
4.75 TO 5.50
4.75 TO 5.50
4.75 TO 5.50
65
45
135
205
175
165
174
173
200
215
230
170
170
800
1900
900
450
900
1900
2500
3500
900
350
350
940
1960
CORRECTING MODULATOR IMPERFECTIONS
Note that in Figure 1, in addition to the direct conversion signal
chain, an optional loop-back or transmit observation receiver
has also been incorporated into the radio. The primary function
of this receiver is to analyze the adjacent channel power ratio
(ACPR) of the transmitter that is primarily caused by distortion
in the high power amplifier (HPA). By continually observing
the ACPR of the transmitter, digital predistortion of the
baseband signal can be employed to partially correct HPA
nonlinearities while allowing the HPA to operate closer to its
compression point.
The presence of a loop-back receiver can be opportunistically
used to also correct for modulator imperfections.
A detailed discussion of the IQ modulator correction tech-
niques used when a loop-back receiver is present is beyond the
scope of this application note. However, the general procedure
involves using the loop-back receiver to demodulate the I and
Q bit streams. The demodulated constellation is then examined
for evidence of IQ gain imbalance, imperfect quadrature, and/
or LO leakage. Once these imperfections have been identified,
the I and Q data streams can be preprocessed so that the IQ
modulator imperfections cancel out. For example, if the
demodulated constellation from the loop-back receiver shows
a rectangular constellation with the width of I being larger than
the height of Q (see Figure 2B), gain adjust registers in the DAC
can be used to either decrease the size of the I data stream or
increase the size of the Q data stream. Likewise, the phase
adjustment registers of the DAC can be used to slightly skew
the I and Q channels so that the imperfect quadrature of the
IQ modulator’s phase splitter is compensated.
FACTORY CALIBRATION
If a wireless transmitter does not use digital predistortion, it
would be difficult to justify the cost of a loop-back receiver
purely for the sake of the IQ modulator. In such situations,
the two options that remain are:
Do not perform any correction of the IQ modulator’s
imperfections.
Complete a one-time factory calibration and store the
correction coefficients in nonvolatile memory.
In recent years, the performance of IQ modulators has
improved to such a degree that it is now feasible (depending
on the modulation scheme) to design a transmitter without any
need to provide correction for imperfections. For example, the
ADL5375 from Analog Devices has gain and quadrature imbal-
ances of 0.05 dB and 0.29°, respectively, at 900 MHz, with little
or no degradation over temperature. As a result, in many appli-
cations, it may be adequate to dispense with any correction
algorithms. Table 1 shows the performance of this and other
members of the Analog Devices IQ modulator family.


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