ADL5812ACPZ-R7 datasheet(20/28 Pages) AD

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ADL5812

Rev. 0 | Page 20 of 28

CIRCUIT DESCRIPTION

The ADL5812 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.

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 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.

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.

RF1

SERIAL

PORT

INTERFACE

BIAS

GEN

ADL5812

RFCT1

RFCT2

RF2

V1LO1

LOIP

LOIN

DATA

CLK

V2LO1

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 59. Simplified Schematic

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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