Circuit Note CN-0082 Devices Connected/Referenced 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. 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 CIRCUIT DESCRIPTION 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. 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 120nH 100pF VPOS COMM INHI 100pF OPHI ADL5331 INLO +5V 12-BIT DAC GND VOUT SETPOINT VOLTAGE 1kΩ VOUT VSET 1nF AD8319 490Ω 220pF VPOS INHI LOG AMP CLPF INLO COMM 1nF 08323-001 SDIN VDD AD5621 23dB ETC1-1-13 DIRECTIONAL (M/A-COM) COUPLER GAIN +5V SCLK 100pF OPLO 100pF ETC1-1-13 (M/A-COM) SYNC RF OUTPUT SIGNAL 120nH Figure 1. ADL5331 Operating in an Automatic Gain Control Loop in Combination with the AD8319 and AD5621 (Simplified Schematic) Rev. 0 “Circuits from the Lab” from Analog Devices have been designed and built 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: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2009 Analog Devices, Inc. All rights reserved. CN-0082 Circuit Note 2.5 0 –10 ERROR (dBm) 0 –20 –2.5 –30 2024 2524 AD5621 DAC CODE –5.0 3524 3024 08323-002 1524 Figure 2. ADL5331 Power Out vs. AD5621 DAC Code with RF Input Signal = 0 dBm 20 5.0 POWER OUT STRAIGHT LINE ERROR 10 2.5 0 –10 ERROR (dBm) 0 –20 –2.5 –30 –5.0 1780 2280 2780 3280 AD5621 DAC CODE 08323-003 –40 1280 3780 Figure 3. ADL5331 Power Out vs. AD5621 DAC Code with RF Input Signal = −10 dBm 20 Rev. 0 | Page 2 of 4 5.0 POWER OUT STRAIGHT LINE ERROR 10 2.5 0 0 –10 ERROR (dBm) ADL5331 OUTPUT POWER (dBm) In order for the AGC loop to remain in equilibrium, the AD8319 must track the envelope of the ADL5331 output signal and provide the necessary voltage levels to the ADL5331’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 ADL5331. The output signal from the ADL5331 is a constant envelope sine wave with amplitude corresponding to a setpoint voltage at the AD8319 of 1.5 V. Also shown is the gain control response of the AD8319 to the changing input envelope. POWER OUT STRAIGHT LINE ERROR 10 –40 1024 The detector’s error amplifier uses CLFP, a ground-referenced capacitor pin, to integrate the error signal (in the form of a current). A capacitor must be connected to CLFP 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 100 MHz sine wave with an input power of 0 dBm, −10 dBm, and −20 dBm. Note that the power control of the AD8319 has a negative sense. Decreasing the DAC code, which corresponds to demanding a higher signal from the ADL5331, tends to increase GAIN. 5.0 –20 –2.5 –30 –40 1280 –5.0 1780 2280 2780 3280 AD5621 DAC CODE 3780 Figure 4. ADL5331 Power Out vs. AD5621 DAC Code with RF Input Signal = −20 dBm 08323-004 A coupler/attenuation of 23 dB is used to match the desired output power range from the VGA to the linear operating range of the AD8319. In this case, the desired output power range of the VGA is −15 dBm to +15 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. ADL5331 OUTPUT POWER (dBm) The basic connections for operating the ADL5331 in an AGC loop with the AD8319 are shown in Figure 1. The AD8319 is a 1 MHz to 10 GHz precision demodulating logarithmic amplifier. It offers a detection range of 45 dB with ±0.5 dB temperature stability. The VOUT 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, VOUT 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, VOUT on the AD8319 may be driven to its maximum value very close to VPOS. To avoid overdrive recovery issues with the ADL5331 GAIN input, a voltage divider can be placed between VOUT on the AD8319 and GAIN on the ADL5331. This may have a slight effect on the overall speed of the loop, for instance, when the input power to the ADL5331 is stepped. 20 ADL5331 OUTPUT POWER (dBm) 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. Circuit Note CN-0082 T AM MODULATED INPUT T CURS1 POS 4.48µs 1 CURS2 POS 2.4µs t1: 4.48µs t2: 2.4µs Δt: –2.08µs 1/Δt: –480.8kHz AD8319 OUTPUT 2 2 MEAN(C1) 440.3mV AMPL(C1) 3.36V AMPL(C2) 900mV ADL5331 OUTPUT M2.00ms A CH4 T 0.00000s 1.80V CH2 500mV 08323-005 CH1 250mV Ω CH2 200mV CH3 250mV Ω CH3 200mV Ω B W M 4.0µs 12.5MS/s 80.0ns/pt A CH1 150mV Figure 6. Oscilloscope Screenshot Showing the ADL5331 Output 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. LEARN MORE 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). MT-078 Tutorial, High Speed Log Amps. Analog Devices. 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-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). 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. AD8319 Evaluation Board. COMMON VARIATIONS AD8368 Data Sheet. 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. ADL5331 Data Sheet. Data Sheets and Evaluation Boards AD5621 Data Sheet. AD8318 Data Sheet. AD8319 Data Sheet. ADL5331 Evaluation Board. REVISION HISTORY 7/09—Revision 0: Initial Version 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 voltagecontrolled variable gain. Rev. 0 | Page 3 of 4 08323-006 3 CN-0082 Circuit Note (Continued from first page) "Circuits from the Lab" are intended only for use with 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, "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 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 property of their respective owners. ©2009Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN08323-0-7/09(0) Rev. 0 | Page 4 of 4