CN-0283 Datasheet, PDF (5/6 Pages) Analog Devices – Providing Fixed Power Gain at the Output of an IQ Modulator

Circuit Note
CIRCUIT EVALUATION AND TEST
The circuit was implemented using the ADL5375 evaluation board
(ADL5375-05-EVALZ) that includes the ADL5320 driver amplifier.
This board can be configured to provide the IQ modulator output
signal, or the composite modulator and amplifier signal. The
default configuration for this board is the modulator and amplifier
composite output with the amplifier tuned for operation in the
1800 MHz to 2200 MHz range. As already noted, the ADL5320
data sheet provides the values and placement locations for tuning
capacitors that support other frequencies.
Equipment Needed
The following equipment is needed:
• The ADL5375 evaluation board (ADL5375-05-EVALZ)
• Two RF signal generators: Agilent 8648C or equivalent
operating at 25 MHz and 26 MHz
• A RF signal generator: Agilent 8648C or equivalent
operating at approximately 2 GHz
• A RF spectrum analyzer: Rohde & Schwarz FSIQ, Rohde &
Schwarz FSQ, Agilent PSA, or equivalent
• A ZFSC-2-2-S+ 180° power splitter/combiner, Mini-Circuits
• A ZMSCQ-2-50+ 90° power splitter, Mini-Circuits
• Two ADT2-1T 1:2 baluns, Mini-Circuits
• Four ZFBT-6GW-FT+ bias tees, Mini-Circuits
CN-0283
Setup and Test
Figure 7 shows the test setup that was used for the IP3 testing
and for the power sweep testing. The signals from two RF signal
generators running at 25 MHz and 26 MHz are passively combined
using a 180° phase splitter/combiner that provides good input-to-
input isolation. The 2-tone signal is then applied to a 90° phase
splitter that is specified to operate from 25 MHz to 50 MHz. These
phase splitter outputs are then applied to two 1:2 transformers to
create differential output signals (the 0° output of the phase splitter
should go towards the IP and IN inputs of the IQ modulator). The
differential signals are applied to four bias tees that bias the signals
to 0.5 V. The network is terminated by two 100 Ω resistors (pads for
these resistors are provided on the ADL5375 evaluation board).
The local oscillator (LO) for the ADL5375 is provided by a third
signal generator, generating 0 dBm. The final output frequency
is equal to the difference between the input RF signal frequencies
and the LO frequency. Therefore, if the 2-tone signals are at
25 MHz and 26 MHz, and the LO is at 2150 MHz, the output
spectrum appears at 2124 MHz and 2125 MHz.
The circuit can also be implemented using the AD9122 dual
DAC evaluation board (AD9122-M5375-EBZ) that includes the
ADL5375 IQ modulator. In this case, connect the output of the
ADL5375 IQ modulator to a standalone ADL5320 evaluation
board (ADL5320-EVALZ). The advantage of this approach is
that the DAC generates appropriately biased differential signals
without the need for bias tees, phase splitters, and transformers.
RF SIG GEN 1
+8 dBm @ 25MHz
RF SIG GEN 2
+8 dBm @ 26MHz
+0.5V
+5V
ZFSC-2-2-S+
180 POWER
SPLITTER/COMBINER
ADT2-1T
1:2
BALUN
ZMSCQ-2-50
90 POWER
SPLITTER
ADT2-1T
1:2
BALUN
ZFBT-6GW-FT+
BIAS TEE
ZFBT-6GW-FT+
BIAS TEE
ZFBT-6GW-FT+
BIAS TEE
ZFBT-6GW-FT+
BIAS TEE
IBBP
VPOS GND
R7
100Ω
AMP_OUT
IBBN
ADL5375-05
EVALUATION BOARD
(ADL5375-05-EVALZ)
QBBP
R12
100Ω
QBBN
LOIP
RF SPECTRUM
ANALYZER
RF
IN
RF SIG GEN 2
0 dBm @ 2150MHz
Figure 7. Measurement Setup for IP3 Testing and Power Sweep
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