ADL5811ACPZ-R7 datasheet(20/28 Pages) AD

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ADL5811

Rev. 0 | Page 20 of 28

CIRCUIT DESCRIPTION

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.

The ADL5811 consists of two primary components: the RF

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.

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 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 58.

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.

BIAS

GEN

SERIAL

PORT

INTERFACE

ADL5811

RFCT

RFIN

LOIP

LOIN

DATA

CLK

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.)

Figure 58. Block Diagram

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.

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.

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