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ADL5374ACPZ-WP

3000 MHz to 4000 MHz Quadrature Modulator

Analog Devices 

THEORY OF OPERATION

CIRCUIT DESCRIPTION

Overview

The ADL5374 can be divided into five circuit blocks: the LO

interface, the baseband voltage-to-current (V-to-I) converter,

the mixers, the differential-to-single-ended (D-to-S) stage, and

the bias circuit. A detailed block diagram of the device is shown

in Figure 24.

LOIP

LOIN

PHASE

SPLITTER

IBBP

IBBN

QBBP

QBBN

Σ

OUT

Figure 24. Block Diagram

The LO interface generates two LO signals in quadrature. These

signals are used to drive the mixers. The I and Q baseband input

signals are converted to currents by the V-to-I stages, which

then drive the two mixers. The outputs of these mixers combine

to feed the output balun, which provides a single-ended output.

The bias cell generates reference currents for the V-to-I stage.

LO Interface

The LO interface consists of a polyphase quadrature splitter

followed by a limiting amplifier. The LO input impedance is set

by the polyphase. For optimal performance, the LO should be

driven differentially. Each quadrature LO signal then passes

through a limiting amplifier that provides the mixer with a

limited drive signal.

ADL5374

V-to-I Converter

The differential baseband inputs (QBBP, QBBN, IBBN, and

IBBP) consist of the bases of PNP transistors, which present a

high impedance. The voltages applied to these pins drive the

V-to-I stage that converts baseband voltages into currents. The

differential output currents of the V-to-I stages feed each of their

respective Gilbert-cell mixers. The dc common-mode voltage at

the baseband inputs sets the currents in the two mixer cores.

Varying the baseband common-mode voltage influences the

current in the mixer and affects overall modulator performance.

The recommended dc voltage for the baseband common-mode

voltage is 500 mV dc.

Mixers

The ADL5374 has two double-balanced mixers: one for the

in-phase channel (I-channel) and one for the quadrature

channel (Q-channel). Both mixers are based on the Gilbert cell

design of four crossconnected transistors. The output currents

from the two mixers sum together into a load. The signal

developed across this load is used to drive the D-to-S stage.

D-to-S Stage

The output D-to-S stage consists of an on-chip balun that

converts the differential signal to a single-ended signal. The

balun presents high impedance to the output (VOUT). Therefore,

a matching network may be needed at the output for optimal

power transfer.

Bias Circuit

An on-chip band gap reference circuit is used to generate a

proportional-to-absolute temperature (PTAT) reference current

for the V-to-I stage.

Rev. 0 | Page 11 of 20

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