adl5385 Analog Devices, Inc., adl5385 Datasheet - Page 12

• Manufacturer Part Number: adl5385
• Description: 50 Mhz To 2200 Mhz Quadrature Modulator
• Manufacturer: Analog Devices, Inc.

ADL5385
CIRCUIT DESCRIPTION
OVERVIEW
The ADL5385 can be divided into five sections: 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 26.
The LO interface generates two LO signals at 90° of phase
difference to drive two mixers in quadrature. Baseband signals
are converted into currents by the V-to-I converters that feed
into the two mixers. The outputs of the mixers are combined in
the differential-to-single-ended amplifier, which provides a 50 Ω
output interface. Reference currents to each section are
generated by the bias circuit. A detailed description of each
section follows.
LO INTERFACE
The LO interface consists of a buffer amplifier followed by a
pair of frequency dividers that generate two carriers at half the
input frequency and in quadrature with each other. Each carrier
is then amplified and amplitude-limited to drive the double-
balanced mixers.
QBBP
QBBN
IBBN
IBBP
LOIP
LOIN
ENBL
BIAS
Figure 26. ADL5385 Block Diagram
QUADRATURE
DIVIDE-BY-2
SPLITTER
PHASE
TEMPERATURE
SENSOR
TEMP
VOUT
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V-TO-I CONVERTER
The differential baseband input voltages that are applied to the
baseband input pins are fed to a pair of common-emitter,
voltage-to-current converters. The output currents then
modulate the two half-frequency LO carriers in the mixer stage.
MIXERS
The ADL5385 has two double-balanced mixers: one for the in-
phase channel (I channel) and one for the quadrature channel
(Q channel). These mixers are based on the Gilbert cell design
of four cross-connected transistors. The output currents from
the two mixers are summed together in the resistor-inductor
(RL) loads in the D-to-S amplifier.
D-TO-S AMPLIFIER
The output D-to-S amplifier consists of two emitter followers
driving a totem-pole output stage. Output impedance is
established by the emitter resistors in the output transistors.
The output of this stage connects to the output (VOUT) pin.
BIAS CIRCUIT
A band gap reference circuit generates the proportional-to-
absolute-temperature (PTAT) as well as temperature-independ-
ent reference currents used by different sections. The band-gap
circuit is turned on by a logic HIGH at the ENBL pin, which in
turn powers up the whole device. A PTAT voltage output is
available at the TEMP pin, which can be used for temperature
monitoring as well as for temperature compensation purposes.