Circuit Note CN-0366 Circuits from the Lab® reference designs are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0366. Devices Connected/Referenced ADL6010 0.5 GHz to 43.5 GHz, 45 dB Microwave Detector AD7091R 12-Bit, 1 MSPS Precision ADC A 40 GHz Microwave Power Meter with a Range from −30 dBm to +15 dBm EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards ADL6010 Evaluation Board (ADL6010-EVALZ) AD7091R Evaluation Board (EVAL-AD7091RSDZ) System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials The circuit shown in Figure 1 is an accurate 40 GHz, microwave power meter with a 45 dB range that requires only two components. The RF detector has an innovative detector cell using Schottky diodes followed by an analog linearization circuit. A low power, 12-bit, 1 MSPS analog-to-digital converter (ADC) provides a digital output on a serial peripheral interface (SPI) port. A simple calibration routine is run before measurement operation, at the particular RF frequency of interest. The user can then operate the system in measurement mode. When in measurement mode, the CN-0366 Evaluation Software displays the calibrated RF input power that is applied at the input of the detector in units of dBm. The total power dissipation of this circuit is less than 9 mW on a single 5 V supply. 5V VPOS ADL6010 DETECTOR MAXIMUM OUTPUT = 4V INPUT RANGE: 0V TO 2.5V RFCM RF INPUT POWER RFIN RFCM ANALOG SIGNAL PROCESSOR VOUT VIN 200Ω 340Ω 5V VDD AD7091R 12-BIT, 1MSPS ADC SPI 12625-001 GND COMM Figure 1. Microwave Power Meter Simplified Schematic (All Connections and Decoupling Not Shown) Rev. 0 Circuits from the Lab reference designs 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 toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage) 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 ©2014 Analog Devices, Inc. All rights reserved. CN-0366 Circuit Note 10 1 0.5GHz 1.0GHz 5.0GHz 10.0GHz 15.0GHz 20.0GHz 25.0GHz 30.0GHz 35.0GHz 40.0GHz 43.5GHz 0.1 0.01 The system must be calibrated because the output voltage is dependent on the frequency of the input waveform. A correction factor is also needed when measuring modulated signals. PC based software with a simple graphical user interface is provided to perform the computations (CN-0366 Evaluation Software). 0.001 –40 –35 –30 –25 –20 –15 –10 5 10 15 Figure 3. Transfer Function at Frequencies from 500 MHz to 43.5 GHz 10 4 VOUT = Slope × VRFIN + Intercept 3 1 2 1 0.1 0 –1 –55°C –40°C +25°C +85°C +125°C –2 –3 –4 –30 –25 –20 –15 –10 –5 0 5 10 15 0.001 20 PIN (dBm) CALIBRATION AT –20dBm, 0dBm, AND +10dBm 3 1 2 1 0.1 0 –1 –55°C –40°C +25°C +85°C +125°C 3 VPOS –2 2 VOUT OUTPUT VOLTAGE (V) ADL6010 10 4 ERROR (dB) Figure 2 shows a functional block diagram of the ADL6010. LINEARIZER 0.01 Figure 4. Transfer Function and Error at 10 GHZ for Various Temperatures where: VOUT is the voltage on the VOUT pin. Slope is approximately 5.9 V/V rms at 10 GHz. VRFIN is the rms input voltage. Intercept is the y-axis value that the data crosses if extended. RFCM 4 CALIBRATION AT –28dBm, –10dBm, AND +10dBm ERROR (dB) The ADL6010 is 45 dB envelope detector that operates from 500 MHz to 43.5 GHz. It has a linear in volts slope of approximately 5.9 V/V rms and an absolute detector input range from −30 dBm to +15 dBm or −43 dBV to +2 dBV in a 50 Ω system. The detector cell uses a proprietary eight Schottky diode array followed by a novel linearizer circuit that creates a linear voltmeter with an overall scaling factor (or transfer gain) of nominally ×5.9 relative to the rms voltage amplitude of the input. With an output averaging capacitor, the ADL6010 can detect a signal with a varying envelope, but a correction factor must be used to compensate for the change in output voltage for the same given input power. The output voltage is related to the rms input voltage by 20 PIN (dBm) Power Detector RFIN 5 0 –5 12625-003 OUTPUT VOLTAGE (V) The AD7091R 12-bit, 1 MSPS ADC samples the detector output, and the data is processed through a data capture board and sent to a PC for further processing and analysis. The ADC has an internal 2.5 V reference voltage that can be used to set the fullscale voltage. The internal reference can be overridden if a larger full-scale voltage is needed. OUTPUT VOLTAGE (V) The circuit shown in Figure 1 uses an ADL6010 RF and microwave power detector to convert an ac waveform to a scaled output voltage that corresponds to the amplitude of the input waveform. The output voltage is linear-in-voltage, having a slope with units of V/V rms. The ADL6010 can extract RF signal envelopes with bandwidths of up to 40 MHz. However, in most power meter applications, the output voltage is a settled dc value that represents the amplitude of the input waveform. Figure 3 shows the variation in the transfer function with frequency. There is approximately 300 mV of voltage deviation in the output between 1 GHz and 40 GHz. The temperature variation is less than ±0.5 dB over the entire frequency range. Figure 4 and Figure 5 show the temperature variation at 10 GHz and 40 GHz, respectively. 12625-004 CIRCUIT DESCRIPTION 0.01 –4 –30 –25 –20 –15 –10 –5 PIN (dBm) Figure 2. ADL6010 RF/Microwave Detector Functional Diagram 0 5 10 15 0.001 20 12625-005 1 COMM 12625-002 –3 RFCM 6 Figure 5. Transfer Function and Error at 40 GHz for Various Temperatures Rev. 0 | Page 2 of 8 Circuit Note CN-0366 Analog-to-Digital Converter System Transfer Function The AD7091R is a 12-bit, 1 MSPS ADC with an input voltage range between 0 V and VREF, where the reference voltage is either provided by the internal 2.5 V reference or by an external reference that overrides the internal reference. The external reference can be as high as 5 V. For a 2.5 V full-scale voltage (VREF = 2.5 V), the LSB size is The slope and intercept of the system from the input of the detector to the output of the ADC are LSB = (2.5 V)/212 = 610 μV SlopeSYS CODEHIGH CODELOW VHIGH VLOW INTSYS = CODEHIGH − (SlopeSYS × VHIGH) The output voltage of the ADL6010 is approximately 25 mV to 4 V; therefore, a 200 Ω/340 Ω resistor divider with an attenuation of approximately 1.6 reduces the amplitude of the signal so that it is always within the range of the AD7091R when using the internal 2.5 V reference. where: SlopeSYS is the system slope. CODEHIGH, CODELOW are the high and low code outputs, respectively, from the ADC. VHIGH, VLOW are the high and low RF voltages, respectively. INTSYS is the system intercept. Data Analysis The overall system transfer function is The EVAL-SDP-CB1Z system demonstration platform (SDP) board is used in conjunction with software based on the AD7091R evaluation board control software to capture the data being sampled by the ADC. The software has a power meter readout and calibration option. The power meter display shows the power applied to the input of the ADL6010. To take an accurate power measurement with the ADL6010 and the AD7091R, apply two known input powers at different levels to the input of the ADL6010, then read the corresponding output ADC code. These four values make up two points on a plot and must be stored for later use in the calibration procedure. The two points are CODE = SlopeSYS × VIN + INTSYS where VIN is the rms voltage of the input RF signal. Solve for VIN using VIN CODE INTSYS Slope SYS Therefore, the power in dBm, PIN, can be expressed as 10 3 PIN (dBm) 10 log 10 R CODE INT Slope 2 For a 50 Ω input impedance, this equation simplifies to Point 1: (VLOW, CODELOW) Point 2: (VHIGH, CODEHIGH) CODE INT PIN (dBm) 13.01 dB 20 log10 Slope From these two points, a slope and an intercept can be found and used to calibrate the system at the particular frequency of operation. (1) User Calibration Algorithm The CN-0366 Evaluation Software performs a one-time calibration at the particular frequency of operation. Calibration is achieved via the Calibration tab, shown in the window of Figure 6. The calibration routine is as follows: Figure 6 shows the software power level display. 12625-006 1. 2. 3. 4. 5. 6. 7. 8. 9. Figure 6. CN-0366 Evaluation Software Display Rev. 0 | Page 3 of 8 Set the RF power to high level (VHIGH). Measure the code from ADC (CODEHIGH). Set the RF power to low level (VLOW). Measure the code from ADC (CODELOW). Calculate the system slope (units of codes/V). Calculate the system intercept (units of codes). Store the slope and intercept as calibration coefficients. Measure the ADC code with an arbitrary input RF power. Calculate the input power using the code, slope, and intercept. CN-0366 Circuit Note 4 Measurement Results for Complete Signal Chain Including ADC 3 Using the CN-0366 Evaluation Software, measurements were taken across several frequencies. Each frequency was calibrated before the measurement took place. The results are shown in Figure 7, Figure 8, and Figure 9. In Figure 7, note the dependency of the error on the calibration power levels. Choosing the proper calibration levels may require some trial and error. ERROR (dB) 1 0 –1 Figure 7 through Figure 9 show the input power measured vs. the actual power applied to the input of the ADL6010 and the error between the two. –2 –4 –40 –40 –50 –30 –20 –10 0 10 0 –10 –1 –20 –2 –3 –30 –3 MEASURED POWER (dBm) 10 0 –10 –20 10 –4 20 4 20 25°C 70°C 0°C 25°C, ERROR 70°C, ERROR 0°C, ERROR 10 20 –20 –10 0 APPLIED POWER (dBm) Figure 10. Measured Power and Error vs. Applied Power for Various Temperatures at 1 GHz Figure 7. Measured Power and Error vs. Applied Power for Various Three-Point Calibrations at 1 GHz –30 3 2 0 1 0 –10 –1 –20 –2 –30 –3 1GHz 10GHz 20GHz 30GHz 40GHz –40 –50 –40 –40 –60 –30 –20 –10 0 APPLIED POWER (dBm) 10 20 –30 –20 –10 0 APPLIED POWER (dBm) 10 –4 20 Figure 11. Measured Power and Error vs. Applied Power for Various Temperatures at 10 GHz 12625-008 –70 –40 1 –40 –40 APPLIED POWER (dBm) 2 0 –30 –2 –4 20 3 Figure 8. Measured Power vs. Applied Power at Various Frequencies Rev. 0 | Page 4 of 8 12625-011 –60 –40 –1 10 12625-007 –15, 0, +10 –20, –5, +10 –25, –10, +5 –25, 0, +10 ERROR, –15, 0, +10 ERROR, –20, –5, +10 ERROR, –25, –10, +5 ERROR, –25, 0, +10 –30 4 25°C 70°C 0°C 25°C, ERROR 70°C, ERROR 0°C, ERROR ERROR (dB) 0 20 12625-010 –20 10 ERROR (dB) 1 0 20 MEASURED POWER (dBm) –10 ERROR (dB) 2 MEASURED POWER (dBm) MEASURED POWER (dBm) 0 –10 Figure 9. Measured Power Error vs. Applied Power at Various Frequencies 4 3 –20 APPLIED POWER (dBm) Note that some measurements were taken using an early version of the software that did not have an averaging feature, which is why there is a large ripple at the lower input power levels. 10 –30 12625-009 –3 Data was taken over temperature at 0°C, 25°C, and 70°C, and the results are shown in Figure 10, Figure 11, Figure 12, and Figure 13 for frequencies of 1 GHz, 10 GHz, 20 GHz, and 30 GHz. 20 1GHz 10GHz 20GHz 30GHz 40GHz 2 Circuit Note CN-0366 4 20 25°C 70°C 0°C 25°C, ERROR 70°C, ERROR 0°C, ERROR 3 CODE = VOUT/LSB 2 LSB = VREF/4096 0 1 0 –10 –1 ERROR (dB) MEASURED POWER (dBm) 10 The ADC transfer function (when being driven by the ADL6010) is given by where: CODE is the ADC output code, a unitless number that can range from 0 and 4096. VREF is the full-scale reference voltage of the ADC, in volts. LSB is the smallest quantized voltage that the ADC can resolve, in V/step or V/bit. –20 –2 –30 –3 –30 –20 –10 0 APPLIED POWER (dBm) 10 –4 20 12625-012 –40 –40 VOUT = CODE × LSB Substituting this expression in the previous VIN equation yields Figure 12. Measured Power and Error vs. Applied Power for Various Temperatures at 20 GHz VIN 4 20 25°C 70°C 0°C 25°C, ERROR 70°C, ERROR 0°C, ERROR 10 1 0 –10 –1 where R is the impedance on the RFIN pin of the ADL6010. Substituting VIN into the power equation yields –2 10 3 PIN (dBm) 10 log 10 R –30 10 –4 20 12625-013 –3 –20 –10 0 APPLIED POWER (dBm) Summary of Equations The following is a summary of the equations in the CN-0366, as well as additional equations to provide insight into the function of this power meter circuit. The transfer function of the ADL6010 is given by VOUT = SlopeDET × VIN + INTDET where: VOUT is the dc output voltage of the detector. SlopeDET is the gain/slope of the detector, in V/V rms. VIN is the rms voltage of the input RF signal. INTDET is the intercept of the detector, in volts. VIN VOUT INTDET Slope DET CODE LSB INTDET Slope DET 2 Simplifying the equation yields CODE LSB INTDET PIN (dBm) 30 dB 10 log 10 R 20 log10 SlopeDET Figure 13. Measured Power and Error vs. Applied Power for Various Temperatures at 30 GHz Solve for VIN using (2) V 2 PIN (dBm) 10 log10 103 IN R –20 –30 SlopeDET Referring everything back to input power yields 2 0 –40 –40 CODE LSB INTDET 3 ERROR (dB) MEASURED POWER (dBm) Solve for VOUT using If R = 50 Ω, the equation can be simplified even further to CODE LSB INTDET PIN (dBm) 13.01 dB 20 log10 Slope DET (3) This equation is in terms of the ADL6010 detector output slope and intercept, which is useful for conceptual purposes to show the interactions of the system. However, for practical purposes, the transfer function of the system is required in terms of the overall system slope and intercept, where the slope has units of codes/V and the intercept has units of codes. The final transfer function can be derived as follows. The derivation of the system transfer function in terms of system slope (slopeSYS) and system intercept (INTSYS) begins with Equation 2, which has the input power in terms of detector slope (slopeDET) and detector intercept (INTDET). This derivation is achieved by multiplying both the numerator and denominator of the VIN expression (Equation 2) by 1/LSB, as follows: VIN Rev. 0 | Page 5 of 8 CODE LSB INTDET SlopeDET 1 LSB 1 LSB CN-0366 Circuit Note Substituting this expression for VIN into Equation 2 yields Equation 1. INTDET has units of volts, SlopeDET has units of V/V rms, and LSB has units of V/codes. Multiplying by 1/LSB converts INTDET into its equivalent ADC code and converts the detector slope into the system slope with unit of codes/V rms, yielding the following relations: INTSYS = INTDET/LSB Slopesys = SlopeDET/LSB If a known voltage is applied to the input of the coupler (generalized as VCOUPLER in Figure 14), the general relationship between the input of the coupler and the input of the detector is VIN = VCOUPLER × Voltage Loss where Voltage Loss is a constant attenuation factor in fractional form. For example, a 20 dB voltage loss from the input of the coupler to the input of the detector is a voltage loss factor of 1/10. If the voltage levels at the input of the coupler are used instead of the input of the detector, a new expression for the system slope that includes the coupler and transmission line loss is A complete set of design files, including schematics, layouts, Gerbers, and bill of materials for the CN-0366 circuit are available in the CN-0366 Design Support Package. Slope SYS = COMMON VARIATIONS In power monitoring and VGA applications, a common practice is to tap some power off of a transmission line with a coupler and then feed the signal into the RF/microwave detector, as shown in Figure 14. where the VHIGH’ and VLOW’ are the high and low calibration voltages, respectively, at the input of the coupler. The same equation in terms of the input voltages of the detector and the loss of the coupler and transmission lines is TO NEXT STAGE COUPLER Slope SYS = Voltage Loss × + VIN DETECTOR ADC CODE 12625-014 – When calibrating this setup, the previously described calibration routine does not change. The loss of the coupler and transmission lines are calibrated out and accounted for in the slope and intercept equations. Use the voltage level applied to the input of the coupler when computing the system slope and intercept. The system slope and intercept using the input voltage to the detector and the ADC output code are as follows: MEASURED POWER (dBm) Figure 14. Generic Application of Power Meter Using a Coupler Slope SYS = VHIGH − VLOW Figure 15 shows the transfer function at 1 GHz and 5 GHz for a system using a coupler with 10 dB loss. VOLTAGE LOSS VLOG OR VRMS CODE HIGH − CODE LOW 30 3 20 2 10 1 0 0 1GHz 5GHz 1GHz ERROR 5GHz ERROR –10 –1 –20 CODE HIGH − CODE LO W VHIGH − VLOW –30 –20 INTSYS = CODEHIGH − (SlopeSYS × VHIGH) –2 –10 0 10 APPLIED POWER (dBm) where: VHIGH, VLOW are the high and low voltages, respectively, applied to the input detector (generalized as VIN in Figure 14). CODEHIGH, CODELOW are the high and low outputs of the ADC. ERROR (dB) VCOUPLER VHIGH ' − VLOW ' 20 –3 30 12625-015 FROM PREVIOUS STAGE CODE HIGH − CODE LO W Figure 15. Measured Power and Error vs. Applied Power at 1 GHz and 5 GHz for System Using 10 dB Coupler Rev. 0 | Page 6 of 8 Circuit Note CN-0366 CIRCUIT EVALUATION AND TEST Functional Block Diagram Equipment Needed Figure 16 shows the functional block diagram of the test setup that was used for testing the receive chain. The following equipment is needed to perform the evaluations described in the CN-0366: To set up and test the microwave power meter system, take the following steps: The ADL6010-EVALZ evaluation board. The EVAL-AD7091RSDZ evaluation board. The EVAL-SDP-CB1Z SDP board. The Agilent E8257D signal generator. The Agilent 34410A digital multimeter. A PC running Windows® 7 connected to the SDP board via a USB cable (supplied with the EVAL-SDP-CB1Z). A 5 V power supply to supply ADL6010-EVALZ board. A 9 V ac-to-dc, wall-mounted converter to supply the EVAL-AD7091RSDZ evaluation board (supplied with the EVAL-AD7091RSDZ). Note that the EVAL-SDP-CB1Z is powered from a regulator on the EVAL-AD7091RSDZ. The CN-0366 Evaluation Software. 1. Turn on all test equipment and wait for the test equipment to warm up. 2. Connect the ADL6010-EVALZ evaluation board input to the Agilent signal generator (connecting directly from the signal generator to the evaluation board via a barrel connector is recommended). 3. Connect the ADL6010-EVALZ evaluation board output to the input of the EVAL-AD7091RSDZ evaluation board. 4. Connect the EVAL-SDP-CB1Z SDP board to the EVALAD7091RSDZ evaluation board. 5. Connect the SDP board to a PC via the USB cable that is provided with the SDP board 6. Download and install the CN-0366 Evaluation Software onto the PC that is connected to the SDP control board. 7. After the software is installed properly, run the executable. 8. Turn on the signal generator and set it to a power and frequency within the operating limits of the ADL6010. 9. To obtain an accurate power reading, run the calibration routine in the software. 10. The software GUI now calculates and displays the correct power that is applied to the input of the ADL6010. Getting Started Make the following modifications and link settings to the ADL6010-EVALZ evaluation board and the EVAL-AD7091RSDZ evaluation board to implement the circuit shown in Figure 1. For the ADL6010-EVALZ, replace R1 with a 200 Ω resistor (0402 size). For the EVAL-AD7091RSDZ, replace R1 with a 0 Ω resistor (0603 size), and replace C13 with a 340 Ω resistor (0603 size). For the EVAL-AD7091RSDZ link settings, set LK1 to Position C, set LK2 to Position C, and leave LK3 and LK4 open. 5V POWER SUPPLY 9V POWER SUPPLY PC USB VPOS RF GENERATOR RFIN GND ADL6010-EVALZ 120 J1 VOUT J5 J4 EVAL-AD7091RSDZ CON A OR CON B EVAL-SDP-CB1Z Figure 16. Functional Block Diagram for Testing RF and Microwave Power Meter Rev. 0 | Page 7 of 8 12625-016 Setup and Test CN-0366 Circuit Note LEARN MORE Data Sheets and Evaluation Boards CN-0366 Design Support Package. ADL6010 Data Sheet and Evaluation Board Ardizzoni, John. A Practical Guide to High-Speed Printed-CircuitBoard Layout. Analog Dialogue 39-09, September 2005. AD7091R Data Sheet and Evaluation Board UG-409. EVAL-AD7091RSDZ Evaluation Board User Guide. Analog Devices. REVISION HISTORY 10/14—Revision 0: Initial Version ADIsimRF Design Tool. CN-0178 Circuit Note. Software-Calibrated, 50 MHz to 9 GHz, RF Power Measurement System. 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 (VGAs). Analog Devices. MT-101 Tutorial. Decoupling Techniques. Analog Devices. (Continued from first page) Circuits from the Lab reference designs 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 reference designs 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 reference designs. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab reference designs 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 reference designs at any time without notice but is under no obligation to do so. ©2014 Analog Devices, Inc. All rights reserved. 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