circuit note cn- 0082 circuit designs using analog devices products apply these product pairings quickly and with conf idence. for more information and/or support call 1 -800- analogd (1 -800-262-5643) or visit www.analog.com/circuit . devices connected /referenced adl5331 rf variable gain amplifier ( vga ) ad8319 45- db rf log detector /controller ad5621 12-b it nano dac ? creating a constant envelope signal u sing the ad l5331 rfvga and ad8319 log detector rev. 0 circuits from the lab from analog devices have been designed and buil t by analog devices engineers. standard engineer ing 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 dete rmining 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: 781.329.4700 www.analog.com fax: 781.461.3113 ? 2009 analog devices, inc. all rights reserved. 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 sign al is too strong, distortion will cause the same issues. this c ircuit uses the adl5331 vga, the ad8319 power detector, and the ad5621 low power nano dac to generate output power control accurate to 12 bits. the ad8319 has very high temperature stabilit y to compensate for any gain variation over temperature of the vga, resulting in very accurate power control over a wide temperature range . becaus e the ad8319 control input v set and the output v out are related to the rf input on a volts/db scale and the ad 5621 nano dac has a linear transfer function, the resulting output power control will be a linear - in - db vs. dac input code . circuit description t he adl533 1 variable gain amplifier provides accurate gain control. however, more precise regulation of output p ower is achieved with an automatic gain control (agc) loop. figure 1 shows the adl533 1 operating in an agc loop. the addition of the ad831 9 log amp allows the agc to have improved temperature stability over a wide output power con trol range . to operate the adl533 1 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 s etpoint voltage is applied by the ad5621 dac to the v set input of the detector w hile v out is connected to the gain pin of the adl533 1 . based on the detectors defined linear - in - db relationship between v out and the rf input signal, the detector adjusts the voltage on the gain pin (the detectors v out pin is an error inlo inhi gain oplo ophi 23db directiona l coupler vpos comm adl5331 +5v +5v +5v +5v comm vout vpos vset inhi inlo clpf ad8319 log am p vout sync sclk sdin gnd ad5621 12-bit dac vdd rf output signa l rf input signa l 1k? 490? setpoint vo lt age 220pf 1nf 1nf 120nh 120nh 100pf 100pf 100pf 100pf etc1-1-13 (m/a-com) etc1-1-13 (m/a-com) 08323-001 figure 1 . adl5331 operating in an automatic gain control loop in combination with the ad8319 and ad5621 (simplified schematic)
cn- 0082 circuit note rev. 0 | page 2 of 4 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 le vel at the detector and the set point voltage. the basic connections for operating the adl533 1 in an agc loop with the ad831 9 are shown in figure 1 . the ad831 9 is a 1 mhz to 10 ghz precision demodulating logarithmic ampli fier. it offers a detection range of 45 db with 0.5 db temperature stability. the v out pin of the ad8319 controls the gain (gain control) pin of the adl5331 . when the ad8319 is in controller mode, as it is in this application, v out on the ad8319 can drive the adl5331 gain pin over its full linear range of 0 v to 1.4 v. under very low power rf in conditions, outside the linear control range of the loop, v out on the ad8319 may be driven to its maximu m value very close to v pos . to avoid overdrive recovery issues with the adl5331 gain input, a voltage divider can be placed between v out on the ad8319 and gain on the adl5331. this may have a slight effect on the overall speed of the loop , for instance, wh en the input power to the adl5331 is stepped. a coupler/attenuation of 23 db is use d to match the desired output power range fro m the vga to the linear operating range o f the ad8319 . in this case, the desired output power range of the vga is ? 15 dbm to +1 5 dbm. with the given attenuator/coupler, the range of power to the ad8319 rf input is ? 8 dbm to ? 38 dbm, within the specified range of ? 3 dbm to ? 43dbm for a 1 db error. the dete ctors error amplifier uses clfp , a ground - referenced capacitor pin, to inte grate the error signal (in the form of a current). a capacitor must be connected to c lfp to set the loop bandwidth and to ensure loop stability. figure 2 , figure 3 , and figure 4 show the transfer function of the adl5331 output power vs. the ad5621 dac code for a 1 00 mhz sine wave with an i nput power of 0 dbm , ? 10 dbm , and ? 20 dbm. note that the power control of the ad831 9 has a negative sense. decreasing the dac code , which corresponds to demanding a higher signal from the adl533 1 , tends to increase gain. in order for the agc loop to remain in equilibrium, the ad831 9 must track the envelope of the adl533 1 output signal and provide the necessary voltage levels to the adl533 1 s gain control input. figure 5 shows an oscilloscope screenshot of the agc loop in figure 1 . a 100 mhz sine wave with 50% am modulation is applied to the adl533 1 . the output signal from the adl533 1 is a constant envelope sine wave with amplitude corresponding to a setpoint voltage at the ad831 9 of 1.5 v. also shown is the gain control respons e of the ad831 9 to the changing input envelope. ?40 ?30 ?20 ?10 0 10 20 1024 1524 2024 2524 3024 3524 ad5621 dac code adl5331 output power (dbm) ?5.0 ?2.5 0 2.5 5.0 error (dbm) power out straight line error 08323-002 figure 2 . adl5331 power out vs. ad5621 dac code with rf input signal = 0 dbm ?40 ?30 ?20 ?10 0 10 20 1280 1780 2280 2780 3280 3780 ad5621 dac code adl5331 output power (dbm) ?5.0 ?2.5 0 2.5 5.0 error (dbm) 08323-003 power out straight line error figure 3. adl5331 power out vs. ad5621 dac code with rf input signal = ? 10 dbm ?40 ?30 ?20 ?10 0 10 20 1280 1780 2280 2780 3280 3780 ad5621 dac code adl5331 output power (dbm) ?5.0 ?2.5 0 2.5 5.0 error (dbm) 08323-004 power out straight line error figure 4. adl5331 power out vs. ad5621 dac code with rf input signal = ? 2 0 dbm
circuit note cn- 0082 rev. 0 | page 3 of 4 ad8319 output ch1 250mv ? ch2 200mv ch3 250m v ? m2.00ms a ch4 1.80v 1 2 3 t 0.00000s t t adl5331 output am modul a ted input 08323-005 figure 5 . oscilloscope screenshot showing an am modulated input signal figure 6 shows the response of the agc rf output to a pulse on v set . as v set 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 ad831 9 c lfp pin a function analogous to the feed back capacitor around an integrating amplifier. an increase in the capacitance results in a slower response time. the circuit must be constructed on a multi layer pc board with a large area ground plane. proper layout, grounding, and decoupling techniques m ust be used to achieve optimum performance (see tutor ia l mt - 031 and tutori a l mt - 101 and the adl533 1 and ad831 9 evaluation board layouts). on the underside of the adl5331 and ad8319 chip scale package s , there is an exposed compressed padd le. 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 the specified electrical performance and to provide thermal relief. it is also recommended that the gro und 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 f unction (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 cons tant o utput power function, the lower dynamic range of the ad8319 will be adequate since the loop will always servo the detectors 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. ch3 200mv ? ch2 500mv b w m 4.0s 12.5ms/s 80.0ns/pt a ch1 150mv 2 t1: 4.48s t2: 2.4s t: C2.08s 1/t: C480.8khz mean(c1) 440.3mv curs2 pos 2.4s curs1 pos 4.48s ampl(c1) 3.36v ampl(c2) 900mv 08323-006 figure 6 . oscilloscope sc reenshot 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, a nalog 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
cn- 0082 circuit note rev. 0 | page 4 of 4 (continued from first page) "circuits from the lab" are intended only for use wi th analog devices products and are the intellectual 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, "circu its 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 oth er rights of third parties 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 registered trademarks are the p roperty of the ir respective owners. ? 2009 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. cn08323 - 0 - 7/09(0)
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