ADL5370

Rev. 0 | Page 10 of 20

THEORY OF OPERATION

CIRCUIT DESCRIPTION

Overview

The ADL5370 can be divided into five circuit blocks: the local

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

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

amplifier, 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 differential-to-single-ended amplifier, which

provides a 50 Ω output interface. The bias cell generates

reference currents for the V-to-I stage and the D-to-S amplifier.

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, 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 varies 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 ADL5370 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 amplifier.

D-to-S Amplifier

The output D-to-S amplifier consists of a totem pole output

stage. The 50 Ω output impedance is established by an on-chip

resistor. The D-to-S output is internally dc-biased and should be

ac-coupled at its output (VOUT).

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 and a temperature independent current for

the D-to-S output stage.