ADL5592

Rev. 0 | Page 11 of

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THEORY OF OPERATION

Figure 16 shows a simplified schematic of the ADL5592.

RFOUT

LOIP

LOOP EN

LOOP OUT

SPI PORT*

ADL5592

RFIN

ATTENUATOR

CONTROL

CIRCUITRY

*COMPRISES THE DATA, CLK, AND LE PINS.

Figure 16. Simplified Schematic

INPUT SWITCH

The high performance single-pole, double throw (SPDT) GaAs

pHEMT switch is connected to the RF input pin of the ADL5592

to switch the input signal between the VGA and the mixer. To

diminish the impact of the switch on the performance of the VGA

and the mixer, this SPDT switch exhibits low insertion loss and

high isolation in the operating frequency range. The switch-state

control signal is provided by a Si CMOS control circuit.

DIGITAL ATTENUATOR

The digital attenuator consists of five attenuation blocks—1 dB,

2 dB, 4 dB, 8 dB, and 16 dB—each separately controlled by a Si

CMOS control circuit. Each attenuation block consists of field

effect transistor (FET) switches and resistors that form either a

pi- or a T- shaped attenuator. By controlling the states of the

FET switches through the Si CMOS control lines, each attenuation

block can be set to be in the pass state (0 dB) or the attenuation

state (n dB). The various combinations of the five blocks

provide the attenuation states from 0 dB to 31 dB, in 1 dB

increments.

SPI INTERFACE

The ADL5592 includes a SPI-compatible, 3-wire serial interface.

The Si CMOS interface internally level-shifts the SPI signals,

which are used to program a 10-bit shift register and to control

the loading of a 10-bit parallel latch. The outputs of the latch are

fed into drivers, which convert the logic-level outputs of the

latches to signals appropriate for driving the attenuators.

FIXED-GAIN AMPLIFIER

The output of the input attenuator (ATTN 1) is connected to a

fixed-gain amplifier that drives the output attenuator (ATTN 2).

Because the passive attenuators are linear and contribute minimal

noise, the fixed-gain amplifier is the major source of nonlinear

distortion and noise. This results in a constant OIP3 and noise

figure throughout the different attenuation stages. The fixed-gain

amplifier provides 14 dB of gain and broadband, 50 Ω, single-

ended input and output impedances.

LOOPBACK MIXER

The loopback mixer is a Si CMOS Gilbert-cell mixer designed to

provide 10 MHz to 100 MHz of frequency translation from the

RF input to the mixer output. The mixer has 50 Ω loads at the

output for a broadband, single-ended output. The input is fed

from the SPDT GaAs pHEMT switch. The overall mixer gain is

typically −17 dB. The mixer LO input is designed to operate from

10 MHz to greater than 100 MHz.