ADL5356
Rev. 0 | Page 17 of 24
CIRCUIT DESCRIPTION
The ADL5356 consists of two primary components: the radio
frequency (RF) subsystem and the local oscillator (LO) subsystem.
The combination of design, process, and packaging technology
allows the functions of these subsystems to be integrated into
a single die using mature packaging and interconnection
technologies to provide a high performance, low cost design
with excellent electrical, mechanical, and thermal properties.
In addition, the need for external components is minimized,
optimizing cost and size.
The resulting balanced RF signal is applied to a passive mixer that
commutates the RF input with the output of the LO subsystem.
The passive mixer is essentially a balanced, low loss switch that
adds minimum noise to the frequency translation. The only
noise contribution from the mixer is due to the resistive loss of
the switches, which is in the order of a few ohms.
Because the mixer is inherently broadband and bidirectional, it
is necessary to properly terminate all the idler (M × N product)
frequencies generated by the mixing process. Terminating the
mixer avoids the generation of unwanted intermodulation
products and reduces the level of unwanted signals at the input
of the IF amplifier, where high peak signal levels can compromise
the compression and intermodulation performance of the system.
This termination is accomplished by the addition of a sum network
between the IF amplifier and the mixer and in the feedback
elements in the IF amplifier.
The RF subsystem consists of integrated, low loss RF baluns,
passive MOSFET mixers, sum termination networks, and IF
amplifiers. The LO subsystem consists of an SPDT-terminated FET
switch and two multistage limiting LO amplifiers. The purpose of
the LO subsystem is to provide a large, fixed amplitude, balanced
signal to drive the mixer independent of the level of the LO input.
A block diagram of the device is shown in Figure 52.
The IF amplifier is a balanced feedback design that simultaneously
provides the desired gain, noise figure, and input impedance that
is required to achieve the overall performance. The balanced open-
collector output of the IF amplifier, with impedance modified
by the feedback within the amplifier, permits the output to be
connected directly to a high impedance filter, differential amplifier,
or an analog-to-digital input while providing optimum second-
order intermodulation suppression. The differential output
impedance of the IF amplifier is approximately 200 . If
operation in a 50 system is desired, the output can be
transformed to 50 by using a 4:1 transformer.
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ADL5356
07883-001
VGS0
VGS1
VGS2
LOSW
PWDN
VPOS
COMM
LOI2
LOI1
MNIN
MNCT
COMM
DVIN
VPOS
COMM
VPOS
COMM
DVCT
V
P
O
S
D
V
G
M
C
O
M
M
D
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O
P
D
V
O
N
D
V
L
E
V
P
O
S
D
V
L
G
N
C
M
N
O
N
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O
M
M
M
N
G
M
V
P
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S
M
N
O
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M
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L
E
V
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M
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L
G
N
C
The intermodulation performance of the design is generally limited
by the IF amplifier. The IP3 performance can be optimized by
adjusting the IF current with an external resistor.
Figure 41,
Figure 43, and Figure 44 illustrate how various IF and LO bias
resistors affect the performance with a 5 V supply. Additionally,
dc current can be saved by increasing either or both resistors. It
is permissible to reduce the dc supply voltage to as low as 3.3 V,
further reducing the dissipated power of the part. (No performance
enhancement is obtained by reducing the value of these resistors,
and excessive dc power dissipation may result.)
Figure 52. Simplified Schematic
RF SUBSYSTEM
The single-ended, 50 RF input is internally transformed to a
balanced signal using a low loss (<1 dB) unbalanced-to-balanced
(balun) transformer. This transformer is made possible by an
extremely low loss metal stack, which provides both excellent
balance and dc isolation for the RF port. Although the port can be
dc connected, it is recommended that a blocking capacitor be used
to avoid running excessive dc current through the part. The RF
balun can easily support an RF input frequency range of 1200 MHz
to 2500 MHz.
