adl5372 Analog Devices, Inc., adl5372 Datasheet - Page 11

• Manufacturer Part Number: adl5372
• Description: 1500 Mhz To 2500 Mhz Quadrature Modulator
• Manufacturer: Analog Devices, Inc.

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.
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.
QBBP
QBBN
LOIN
IBBP
IBBN
LOIP
Figure 24. Block Diagram
SPLITTER
PHASE
Σ
OUT
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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.
ADL5372