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Data Sheet

ADL5369

Rev. A | Page 17 of 23

CIRCUIT DESCRIPTION

The ADL5369 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, low cost design with excellent electrical,

mechanical, and thermal properties. In addition, the need for

external components is minimized, optimizing cost and size.

The RF subsystem consists of an integrated, low loss RF balun,

passive metal-oxide semiconductor field-effect transistor

(MOSFET) mixer, sum termination network, and IF amplifier.

The LO subsystem consists of a single pole, double throw (SPDT)-

terminated FET switch and a three-stage 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 44.

BIAS

GENERATOR

VPMX

RFIN

RFCT

COMM

LOI2

VPSW

VGS1

VGS0

LOI1

VCMI

IFOP

IFON

PWDN

COMM

VLO3

LGM3

VLO2

LOSW

NIC

ADL5369

NIC = NOT INTERNALLY CONNECTED.

Figure 44. 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, using a blocking capacitor is recommended to

avoid running excessive dc current through the device. The RF

balun can easily support an RF input frequency range of 300 MHz

to 1100 MHz.

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 IF

output. This termination is accomplished by the addition of a

sum network between the IF output and the mixer.

Additionally, dc current can be saved by reducing the dc supply

voltage to as low as 3.3 V, further reducing the dissipated power

of the device. Note that no performance enhancement is obtained

by reducing the value of the resistors; reducing the value of the

resistors may result in excessive dc power dissipation.

LO SUBSYSTEM

The LO amplifier provides a large signal level to the mixer to

obtain optimum intermodulation performance. The resulting

amplifier provides extremely high performance centered on an

operating frequency of 700 MHz. The best operation is achieved

with high-side LO injection for RF signals in the 300 MHz to

1100 MHz range. Operation outside these ranges is permissible,

and conversion loss is extremely wideband, easily spanning 300

MHz to 1100 MHz, but intermodulation is optimal over the

aforementioned ranges.

The ADL5369 has two LO inputs permitting multiple synthesizers

to be rapidly switched with extremely short switching times

(<40 ns) for frequency agile applications. The two inputs are

applied to a high isolation SPDT switch that provides a constant

input impedance, regardless of whether the port is selected, to

avoid pulling the LO sources. This multiple section switch also

ensures high isolation to the off input, minimizing any leakage

from the unwanted LO input that may result in undesired IF

responses.

The single-ended LO input is converted to a fixed amplitude

differential signal using a multistage, limiting LO amplifier.

This results in consistent performance over a range of LO input

power. Optimum performance is achieved from −6 dBm to

+10 dBm, but the circuit continues to function at considerably

lower levels of LO input power.

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