circuit note CN-0050 circuit designs using analog devices products apply these product pairings quickly and with confidence. for more information and/or support call 1 -800- analogd (1 -800-262-5643) or visit www.analog.com/circuit s . 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 rev. b circuits from the lab from analog devices have been designed and buil t by analog devices engineers. standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. however, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. accordingly, in no event shall analog devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any circuit from the lab . ( continued on last page) one technology way, p.o. box 9106, norwood, ma 02062 - 9106, u.s.a. tel: 781.329.4 700 www.analog.com fax: 781.461.3113 ? 2008 C 2010 analog devices, inc. all rights reserved. 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 t emperature 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 cl osed - loop output power control circuit. because the ad8318 , like the adl5330 , has a linear - in - db transfer function, the p out vs. setpoint transfer function also fo llows 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 with an automatic gain c ontrol (agc) loop. figure 1 shows the adl5330 operating in an agc loop. the addition of the ad8318 log amp allows the agc to have improve d 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 set point 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 detectors defined linear - in - db relationsh ip between vout and the rf input signal, the detector adjusts the voltage on the gain pin (the detectors 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 resul ts in the correct balance between the input signal le vel at the detector and the set point voltage. inlo etc1-1-13 (m/a-com) etc1-1-13 (m/a-com) inhi gain oplo ophi directional coupler a ttenu at or vpsx comx adl5330 +5v +5v +5v cmxp vout vpsx vset inhi inlo clpf ad8318 log amp dac* rf input signal rf output signal 412? 1k? setpoint volt age 220pf 1nf 1nf 120nh 120nh 100pf 100pf 100pf 100pf 08515-001 *see common v ari a tions section figure 1 . adl5330 operating in an automatic gain control loop in combination with the ad8318 (simplified schematic: decoupling an d all connections n ot shown)
CN-0050 circuit note rev. b | page 2 of 3 the basic connections for operating the adl5330 in an agc loop with the ad8318 are shown in figure 1 . the ad8318 is a 1 mhz to 8 ghz precision demodulating logarithmic amplifier. it offers a large detection range of 60 db with 0.5 db temperature stability. the gain control pin of the adl5330 is controlled by the output pin of the ad8318 . this voltage, vout, has a range of 0 v to near vp sx . to avoid overdrive recovery issues, the ad8318 output voltage can be scaled down using a resistive divider to interface with the 0 v to 1.4 v gain control range of the adl5330 . a coupler/attenuation of 23 db is used to match the desired maximum o utput power from the vga to the top end of the linear operating range of the ad8318 (at approximately ?5 dbm at 900 mhz). the detectors error amplifier uses c lpf , a ground - referenced capacitor pin, to integrate the error signal (in the form of a current). a capacitor must be connected to clpf to set the loop bandwidth and to ensure loop stability. 08515-002 error (db) ?4 4 3 2 1 0 ?1 ?2 ?3 setpoint vo lt age (v) 2.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 output power (dbm) 30 20 10 0 ?10 ?20 ?30 ?40 ?50 figure 2 . adl 5330 output power vs. ad8318 setpoint voltage, pin = ? 1.5 dbm figure 2 shows the transfer function of the output power vs. the vset voltage over temperature for a 900 mhz sine wave with an input power of ?1.5 dbm. note that the power control of the ad8318 ha s a negative sense. decreasing vset, which corres - ponds to demanding a higher signal from the adl5330 , tends to increase gain. the agc loop is capable of controlling signals just under the full 60 db gain cont rol range of the adl5330 . the performance over temperature is most accurate over the highest power range, where it is generally most critical. across the top 40 db range of output power, the linear conformance error is well within 0.5 db over temperature. the broadband noise added by the logarithmic amplifier is negligible. 08515-003 ad8318 output ch1 250mv ? ch2 200mv ch3 250mv ? m2.00ms 1 2 3 t 0.00000s t t adl5330 output am modul a ted input figure 3 . oscilloscope showing an am modulated input signal f or the agc loop to remain in equilibrium, the ad8318 must track the envelope of the adl5330 output signal and provide the necessary voltage levels to the adl5330 gai n control input. figure 3 shows an oscilloscope screen shot of the agc loop depicted in figure 1 . a 100 mhz sine wave with 50% am modulation is applied to the adl5330 . the output signal from the adl5330 is a constant envelope sine wave with amplitude corresponding to a setpoint voltage at the ad8318 of 1.5 v. also sho wn is the gain control response of the ad8318 to the changing input envelope. 08515-004 ch1 2.00v ch2 50.0mv ? m10.0s a ch1 2.60v 2 1 t 20.2000s t ad8318 with pulsed v set adl5330 output t figure 4 . oscilloscope showing the adl5330 output figure 4 shows the response o f 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 ad8318 c lpf pin a function analogous to the feedback capacitor around an integrating amplifier. an increase in the capacitance results in slower response time. the circuit must be constructed on a multilayer printed circuit board with a large area ground plane. proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see the mt - 031 tutorial and the mt - 101 tutorial and the adl5330 and adl8318 evaluation board layouts).
circuit note CN-0050 rev. b | page 3 of 3 on the underside of the adl5330 and ad8318 chip scale packages, there is an exposed compressed paddle. this paddle is internally connected to the chips ground. solder the paddle to the low impedance ground plane on the printed circuit board to ensure 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 lower output power control range is desired, the ad8318 log amp (60 db power detection range) can be replaced with either the ad8317 (50 db power detection range) or the ad8319 (45 db power detection range). for a constant output power function, the lowest dynamic range detector (ad8319) is adequate because the loop always servos the detector input power to a constant level. the adl5330 vga, which is optimized for transmit applications, can be replaced by the ad8368 vga. the ad8368 is optimized for receive application low frequencies of up to 800 mhz and provides 34 db of linear-in-db voltage-controlled variable gain. there are a number of dacs suitable for this application. all of the following dacs have internal references: single: ad5660 / ad5640 / ad5620 (16-bit/14-bit/12-bit), dual: ad5663r / ad5643r / ad5623r (16-bit/14-bit/12-bit) quad: ad5664r / ad5644r / ad5624r (16-bit/14-bit/12-bit) learn more dana whitlow, design and operation of automatic gain control loops for receivers in modern communications systems , analog devices wireless seminar, chapter 7, 2006. 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. data sheets adl5330 ad8318 ad8317 ad8319 adl5330 evaluation board ad8318 evaluation board revision history 9/10rev. a to rev. b changes to figure 1 .......................................................................... 1 changes to circuit description section......................................... 1 changes to common variations section ....................................... 3 11/09rev. 0 to rev. a updated format ................................................................. universal changes to circuit note title .......................................................... 1 10/08revision 0: initial release (continued from first page) "circuits from the lab" are intended only for use with analog devices products and are the intellec tual property of analog devices or its licensors. while you may use the "circuits from the lab" in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the "circuits from the lab". information furnished by analog devices is believed to be accurate and reliable. however, " circuits from the lab" are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by analog devices for their use, nor for any infringements of patents or other rights of third partie s that may result from their use. analog devices reserves the right to change any "circuits from the lab" at any time without notice, but is under no obligation to do so. trademarks and reg istered trademarks are the property of their respective owners. ?2008C2010 analog devices, inc. all rights reserved. trademarks and registered trademarks are the prop erty of their respective owners. cn08515-0-9/10(b)
|
