ADL5372

Rev. 0 | Page 11 of 24

THEORY OF OPERATION

CIRCUIT DESCRIPTION

Overview

The ADL5372 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.

PHASE

SPLITTER

OUT

LOIP

LOIN

IBBP

IBBN

QBBP

QBBN

Σ

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. The LO can be driven either single-ended or

differentially. When driven single-ended, the LOIN pin should

be ac grounded via a capacitor. Each quadrature LO signal then

passes through a limiting amplifier that provides the mixer with

a limited drive signal.

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 ADL5372 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 cross-connected 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). Hence, 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.