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 Ω. If

operation in a 50 Ω system is desired, the output can be

transformed to 50 Ω by using a 4:1 transformer.

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ADL5356

VGS0

VGS1

VGS2

LOSW

PWDN

VPOS

COMM

LOI2

LOI1

MNIN

MNCT

COMM

DVIN

VPOS

COMM

VPOS

COMM

DVCT

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 Ω 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.