CN-0082 Datasheet, PDF (3/4 Pages) Analog Devices – Creating a Constant Envelope Signal Using the ADL5331 RFVGA and AD8319 Log Detector

Circuit Note
CN-0082
T
AM MODULATED INPUT
T
1
AD8319 OUTPUT
2
3
ADL5331 OUTPUT
CH1 250mV Ω CH2 200mV M2.00ms
A CH4 1.80V
CH3 250mV Ω
T 0.00000s
Figure 5. Oscilloscope Screenshot Showing an
AM Modulated Input Signal
Figure 6 shows the response of the AGC RF output to a pulse
on VSET. As VSET decreases to 1 V, the AGC loop responds with
an RF burst. Response time and the amount of signal
integration are controlled by the capacitance at the AD8319
CLFP pin—a function analogous to the feedback capacitor
around an integrating amplifier. An increase in the capacitance
results in a slower response time.
The circuit must be constructed on a multilayer PC board with
a large area ground plane. Proper layout, grounding, and
decoupling techniques must be used to achieve optimum
performance (see Tutorial MT-031 and Tutorial MT-101 and
the ADL5331and AD8319 evaluation board layouts).
On the underside of the ADL5331 and AD8319 chip scale
packages, there is an exposed compressed paddle. This paddle is
internally connected to the chip’s ground. Solder the paddle to
the low impedance ground plane on the printed circuit board to
ensure the specified electrical performance and to provide
thermal relief. It is also recommended that the ground planes
on all layers under the paddle be stitched together with vias to
reduce thermal impedance.
COMMON VARIATIONS
This circuit can be used to implement a constant power out
function (fixed setpoint with variable input power) or a variable
power out function (variable setpoint with fixed or variable
input power). If a higher output power control range is desired,
the AD8318 log amp (60 dB power detection range) can be used
in place of the AD8319. For a constant output power function,
the lower dynamic range of the AD8319 will be adequate since
the loop will always servo the detector’s input power to a
constant level.
The ADL5331 VGA, which is optimized for transmit
applications, can also be replaced by the AD8368 VGA. The
AD8368 is optimized for low frequency receive applications up
to 800 MHz and provides 34 dB of linear-in-dB voltage-
controlled variable gain.
CURS1 POS
4.48µs
CURS2 POS
2.4µs
t1: 4.48µs
t2: 2.4µs
Δt: –2.08µs
1/Δt: –480.8kHz
2
MEAN(C1) 440.3mV
AMPL(C1) 3.36V
AMPL(C2) 900mV
CH2
500mV
B
W
M 4.0µs 12.5MS/s 80.0ns/pt
CH3 200mV Ω
A CH1 150mV
Figure 6. Oscilloscope Screenshot Showing the ADL5331 Output
LEARN MORE
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of"AGND" and "DGND." Analog Devices.
MT-073 Tutorial, High Speed Variable Gain Amplifiers. Analog
Devices.
MT-077 Tutorial, Log Amp Basics. Analog Devices.
MT-078 Tutorial, High Speed Log Amps. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Whitlow, Dana. Design and Operation of Automatic Gain
Control Loops for Receivers in Modern Communications
Systems. ChapterVIII, Analog Devices Wireless
Seminar (2006).
Data Sheets and Evaluation Boards
AD5621 Data Sheet.
AD8318 Data Sheet.
AD8319 Data Sheet.
AD8319 Evaluation Board.
AD8368 Data Sheet.
ADL5331 Data Sheet.
ADL5331 Evaluation Board.
REVISION HISTORY
7/09—Revision 0: Initial Version
Rev. 0 | Page 3 of 4