CN-0283 Datasheet, PDF (2/6 Pages) Analog Devices – Providing Fixed Power Gain at the Output of an IQ Modulator

CN-0283
CIRCUIT DESCRIPTION
The ADL5375 is a general-purpose, high performance IQ
modulator. It operates at output frequencies from 400 MHz
to 6 GHz. Because of its low noise and wide input baseband
bandwidth (3 dB) of 750 MHz, it can be driven by signals with
a wide variety of modulations and bandwidths. These input
signals can be centered at dc or at a complex IF.
The LO interface to the ADL5375 is a 1XLO type, that is, the
output frequency and LO frequency is equal (when the base-
band signal is centered at dc). Circuit Note CN-0134 describes
how the ADL5375 can be driven by the ADF4350.
System Level Calculations and RF Amplifier Choice
In the 1 GHz to 2 GHz frequency range, the ADL5375 has an
output compression point (OP1dB) and a third-order compression
point (OIP3) of approximately 10 dBm and 25 dBm, respectively.
In choosing an RF amplifier to provide gain after the IQ modulator,
it is important to choose a device whose input P1dB and input
IP3 are equal or a little bit higher than these numbers. Choosing a
device with lower specifications results in degraded performance
for the cascade while choosing a device whose input P1dB and
input IP3 are significantly higher than those of the ADL5375,
has little benefit and is likely to needlessly increase the overall
supply current of the signal chain.
Circuit Note
The ADL5320 is a driver amplifier (RF amplifier that requires
external tuning components) that is specified for operation
from 400 MHz to 2700 MHz. It consumes 104 mA when
operating from a 5 V supply (operation down to 3.3 V is
possible with reduced power consumption and performance).
Table 1 shows the output-referred IP3 (OIP3) and P1dB (OP1dB)
of the ADL5375 IQ modulator along with the input-referred
specifications of the ADL5320 driver amplifier at 1900 MHz. In
both cases, there is approximately a 3 dB difference between the
output-referred specifications of the IQ modulator and the input-
referred specifications of the amplifier.
Table 1. IP3 and P1dB Specifications for the ADL5375 IQ
Modulator and the ADL5320 Driver Amplifier at 1900 MHz
ADL5375
Parameter (Output Referred)
ADL5320
(Input Referred)
IP3
24.2 dBm
28.3 dBm
P1dB
10 dBm
13 dBm
Figure 2 shows the simulated cascaded performance of the IQ
modulator and drive amplifier at 2140 MHz. This simulation was
done using the ADIsimRF Design Tool. It is notable that the
12.3 dB difference between the OIP3 of the modulator (24.2 dBm)
and the composite OIP3 (36.5 dBm) is just slightly less than the
gain of the ADL5320 driver amplifier, 13.7 dB. This indicates that
the driver amplifier has only a very slight effect on the overall OIP3.
Figure 2. ADIsimRF Design Tool Screenshot Showing Cascaded Performance of ADL5375 and ADL5320
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