CN-0283

Circuit Note

Rev. 0 | Page 2 of 6

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.

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

Parameter

ADL5375

(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