CN-0050 Datasheet, PDF (1/3 Pages) Analog Devices – Stable, Closed-Loop Automatic Power Control for RF Applications

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Circuit Note
CN-0050
Devices Connected/Referenced in this Circuit Note
ADL5330 Variable Gain Amplifier (VGA)
AD8318 70 dB Logarithmic Detector/Controller
Stable, Closed-Loop Automatic Power Control for RF Applications
CIRCUIT FUNCTION AND BENEFITS
The circuit described in this document provides closed-loop, auto-
matic power control using a VGA (ADL5330) and a log detector
(AD8318). Due to the high temperature stability of the AD8318,
this circuit provides stability over temperature because the AD8318
RF detector ensures the same level of temperature stability at the
output of the ADL5330 VGA. The addition of the log amp detector
converts the ADL5330 from an open-loop variable gain amplifier
to a closed-loop output power control circuit. Because the AD8318,
like the ADL5330, has a linear-in-dB transfer function, the POUT vs.
setpoint transfer function also follows a linear-in-dB characteristic.
CIRCUIT DESCRIPTION
Although the ADL5330 variable gain amplifier provides accurate
gain control, precise regulation of output power can be achieved
+5V
with an automatic gain control (AGC) loop. Figure 1 shows the
ADL5330 operating in an AGC loop. The addition of the AD8318
log amp allows the AGC to have improved temperature stability
over a wide output power control range.
To operate the ADL5330 VGA in an AGC loop, a sample of the
output RF must be fed back to the detector (typically using a
directional coupler and additional attenuation). A setpoint voltage
is applied by a DAC to the VSET input of the detector while VOUT
is connected to the GAIN pin of the ADL5330. 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 amplifier output) until the
level at the RF input corresponds to the applied setpoint voltage.
GAIN settles to a value that results in the correct balance between
the input signal level at the detector and the setpoint voltage.
+5V
RF INPUT
SIGNAL
100pF
100pF
ETC1-1-13
(M/A-COM)
VPSx
COMx
INHI
OPHI
ADL5330
INLO
OPLO
GAIN
120nH
120nH
100pF
RF OUTPUT
SIGNAL
100pF
DIRECTIONAL
COUPLER
ETC1-1-13
(M/A-COM)
412Ω
+5V
1kΩ
SETPOINT
VOLTAGE
VOUT
VPSx
1nF
DAC*
VSET
INHI
220pF
AD8318
LOG AMP
1nF
CLPF
INLO
CMxP
ATTENUATOR
*SEE COMMON VARIATIONS SECTION
Figure 1. ADL5330 Operating in an Automatic Gain Control Loop in Combination with the AD8318 (Simplified Schematic: Decoupling and
All Connections Not Shown)
Rev. B
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