CN-0082 Datasheet, PDF (1/4 Pages) Analog Devices – Creating a Constant Envelope Signal Using the ADL5331 RFVGA and AD8319 Log Detector

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Circuit Note
CN-0082
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
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
control input VSET and the output VOUT are related to the RF input
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
applied by the AD5621 DAC to the VSET input of the detector
while VOUT is connected to the GAIN pin of the ADL5331.
Based on the detector’s defined linear-in-dB relationship
between VOUT and the RF input signal, the detector adjusts the
voltage on the GAIN pin (the detector’s VOUT pin is an error
+5V
+5V
RF INPUT
SIGNAL
100pF
ETC1-1-13 ^100pF
(M/A-COM)
VPOS COMM
INHI
OPHI
ADL5331
INLO
OPLO
GAIN
120nH
120nH
RF OUTPUT
SIGNAL
100pF
100pF
23dB
ETC1-1-13 _DIRECTIONAL
(M/A-COM) _COUPLER
+5V
+5V
SETPOINT
SYNC
SCLK
SDIN
VDD
VOLTAGE
AD5621 _VOUT ^1kΩ
12-BIT DAC
GND
490Ω
VOUT VPOS
VSET
INHI
AD8319
1nF
LOG AMP
1nF
CLPF
INLO
220pF
COMM
Figure 1. ADL5331 Operating in an Automatic Gain Control Loop in Combination
with the AD8319 and AD5621 (Simplified Schematic)
Rev. 0
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