Circuit Note

CN-0082

Circuit Designs Using Analog Devices Products

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Devices Connected/Referenced

ADL5331

RF Variable Gain Amplifier (VGA)

AD8319

45-dB RF Log Detector/Controller

AD5621

12-Bit nanoDAC®

Creating a Constant Envelope Signal Using the ADL5331 RFVGA

and AD8319 Log Detector

Rev. 0

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CIRCUIT FUNCTION AND BENEFITS

Overall performance of a transmitter, wired or wireless, is a strong

function of the output power of the amplifier. If the signal is weak,

bit error rate (BER) or modulation error rate (MER) will degrade

due to low SNR. If the signal is too strong, distortion will cause the

same issues. This circuit uses the ADL5331 VGA, the AD8319

power detector, and the AD5621 low power nanoDAC to generate

output power control accurate to 12 bits. The AD8319 has very

high temperature stability to compensate for any gain variation

over temperature of the VGA, resulting in very accurate power

control over a wide temperature range. Because the AD8319

on a volts/dB scale and the AD5621 nanoDAC has a linear transfer

function, the resulting output power control will be a linear-in-dB

vs. DAC input code.

CIRCUIT DESCRIPTION

The ADL5331 variable gain amplifier provides accurate gain

control. However, more precise regulation of output power is

achieved with an automatic gain control (AGC) loop. Figure 1

shows the ADL5331 operating in an AGC loop. The addition of

the AD8319 log amp allows the AGC to have improved

temperature stability over a wide output power control range.

To operate the ADL5331 VGA in an AGC loop, a sample of the

output RF is back to the detector (typically using a directional

coupler and additional attenuation). A setpoint voltage is

Based on the detector’s defined linear-in-dB relationship

INLO

INHI

GAIN

OPLO

OPHI

23dB

DIRECTIONAL

COUPLER

VPOS

COMM

ADL5331

+5V

+5V

+5V

+5V

COMM

VOUT

VPOS

VSET

INHI

INLO

CLPF

AD8319

LOG AMP

VOUT

SYNC

SCLK

SDIN

GND

AD5621

12-BIT DAC

VDD

RF OUTPUT

SIGNAL

RF INPUT

SIGNAL

1k

Ω

490

Ω

SETPOINT

VOLTAGE

220pF

1nF

1nF

120nH

120nH

100pF

100pF

100pF

100pF

ETC1-1-13

(M/A-COM)

ETC1-1-13

(M/A-COM)

Figure 1. ADL5331 Operating in an Automatic Gain Control Loop in Combination

with the AD8319 and AD5621 (Simplified Schematic)