ADF4112 AD [Analog Devices], ADF4112 Datasheet - Page 10

• Manufacturer Part Number: ADF4112
• Description: 300 MHz to 1000 MHz Quadrature Modulator
• Manufacturer: AD [Analog Devices]

ADL5370
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.
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.
QBBN
QBBP
IBBN
LOIN
IBBP
LOIP
Figure 24. Block Diagram
SPLITTER
PHASE
Σ
OUT
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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.