ADL5812-EVALZ AD [Analog Devices], ADL5812-EVALZ Datasheet - Page 20

• Manufacturer Part Number: ADL5812-EVALZ
• Description: Dual High IP3, 700 MHz to 2800 MHz, Double Balanced, Passive Mixer, IF Amplifier, and Wideband LO Amplifier
• Manufacturer: AD [Analog Devices]

ADL5812
CIRCUIT DESCRIPTION
The
subsystem and the 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 device with excellent electrical, mechanical, and
thermal properties. The wideband frequency response and
flexible frequency programming simplifies the receiver design,
saves on-board space, and minimizes the need for external
components.
The RF subsystem consists of an integrated, tunable, low loss RF
balun; a double balanced, passive MOSFET mixer; a tunable sum
termination network; and an IF amplifier.
The LO subsystem consists of a multistage limiting LO amplifier.
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 59.
RF SUBSYSTEM
The single-ended, 50 Ω RF input is internally transformed to a
balanced signal using a tunable, low loss, 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
700 MHz to 2800 MHz. This balun is tuned over the frequency
range by SPI controlled switched capacitor networks at the
input and output of the RF balun.
RFCT1
RFCT2
ADL5812
RF1
RF2
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consists of two primary components: the RF
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Figure 59. Simplified Schematic
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BIAS
GEN
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INTERFACE
ADL5812
SERIAL
PORT
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V1LO1
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LOIP
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Rev. 0 | Page 20 of 28
The resulting balanced RF signal is applied to a passive mixer
that commutates the RF input in accordance 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 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 programmable
low-pass filter network between the IF amplifier and the mixer
and in the feedback elements in the IF amplifier.
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 an impedance
modified by the feedback within the amplifier, permits the
output to be connected directly to a high impedance filter, a
differential amplifier, or an analog-to-digital converter (ADC)
input while providing optimum second-order intermodulation
suppression. The differential output impedance of the IF amplifier
is approximately 200 Ω. If operation in a 50 Ω system is desired,
the output can be transformed to 50 Ω by using a 4:1 transformer
or an LC impedance matching network.
The intermodulation performance of the design is generally limited
by the IF amplifier. The IP3 performance can be optimized by
adjusting the low-pass filter between the mixer and the IF amplifier.
Further optimization can be made by adjusting the IF current
with an external resistor. Figure 42 and Figure 43 illustrate how
various IF resistors affect the performance with a 5 V supply.
Additionally, dc current can be saved by increasing the IF resistor.
It is permissible to reduce the IF amplifier’s dc supply voltage
to as low as 3.3 V, further reducing the dissipated power of the
part. (Note that no performance enhancement is obtained by
reducing the value of these resistors, and excessive dc power
dissipation may result.)
Because the mixer is bidirectional, the tuning of the RF and IF
ports is linked, and it is possible for the user to optimize gain,
noise figure, IP3, and impedance match via the SPI. This feature
permits high performance operation and is achieved entirely
using SPI control. Additionally, the performance of the mixer
can be improved by setting the optimum gate voltage on the
passive mixer, which is also controlled by the SPI to enable
optimum performance of the part. See the Applications
Information section for examples of this tuning.