ADL5592-EVALZ AD [Analog Devices], ADL5592-EVALZ Datasheet - Page 11

• Manufacturer Part Number: ADL5592-EVALZ
• Description: 250 MHz to 2400 MHz RF Variable Gain Amplifier
• Manufacturer: AD [Analog Devices]

THEORY OF OPERATION
Figure 16 shows a simplified schematic of the ADL5592.
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
LOOP EN
RFIN
LOIP
*COMPRISES THE DATA, CLK, AND LE PINS.
LOOP OUT
Figure 16. Simplified Schematic
ATTENUATOR
CIRCUITRY
SPI PORT*
CONTROL
ADL5592
RFOUT
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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.
ADL5592