Circuit Note CN-0366 Circuits from the Lab® reference designs are engineered and Devices Connected/Referenced tested for quick and easy system integration to help solve today’s ADL6010 0.5 GHz to 43.5 GHz, 45 dB Microwave Detector analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0366. 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 The circuit shown in Figure 1 is an accurate 40 GHz, microwave ADL6010 Evaluation Board (ADL6010-EVALZ) power meter with a 45 dB range that requires only two AD7091R Evaluation Board (EVAL-AD7091RSDZ) components. The RF detector has an innovative detector cell System Demonstration Platform (EVAL-SDP-CB1Z) using Schottky diodes followed by an analog linearization circuit. Design and Integration Files A low power, 12-bit, 1 MSPS analog-to-digital converter (ADC) Schematics, Layout Files, Bill of Materials 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 MAXIMUM INPUT RANGE: 5V ADL6010 DETECTOR OUTPUT = 4V 0V TO 2.5V VDD RFCM ANALOG RFIN VOUT VIN AD7091R SPI RF INPUT SIGNAL 12-BIT, POWER PROCESSOR 200Ω 1MSPS ADC RFCM 340Ω GND COMM 12625-001 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due Tel: 781.329.4700 www.analog.com to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 ©2014 Analog Devices, Inc. All rights reserved. CN-0366 Circuit Note CIRCUIT DESCRIPTION Figure 3 shows the variation in the transfer function with The circuit shown in Figure 1 uses an ADL6010 RF and frequency. There is approximately 300 mV of voltage deviation microwave power detector to convert an ac waveform to a in the output between 1 GHz and 40 GHz. The temperature scaled output voltage that corresponds to the amplitude of the variation is less than ±0.5 dB over the entire frequency range. input waveform. The output voltage is linear-in-voltage, having Figure 4 and Figure 5 show the temperature variation at 10 GHz a slope with units of V/V rms. The ADL6010 can extract RF and 40 GHz, respectively. signal envelopes with bandwidths of up to 40 MHz. However, in 10 most power meter applications, the output voltage is a settled dc value that represents the amplitude of the input waveform. 1 The AD7091R 12-bit, 1 MSPS ADC samples the detector output, and the data is processed through a data capture board and sent 0.5GHz AGE (V) 1.0GHz LT to a PC for further processing and analysis. The ADC has an 0.1 5.0GHz 10.0GHz internal 2.5 V reference voltage that can be used to set the full- 15.0GHz scale voltage. The internal reference can be overridden if a larger 20.0GHz 25.0GHz OUTPUT VO OUTPUT full-scale voltage is needed. 0.01 30.0GHz 35.0GHz The system must be calibrated because the output voltage is 40.0GHz 43.5GHz dependent on the frequency of the input waveform. A correction factor is also needed when measuring modulated signals. PC 0.001 12625-003 –40 –35 –30 –25 –20 –15 –10 –5 0 5 10 15 20 based software with a simple graphical user interface is provided PIN (dBm) to perform the computations (CN-0366 Evaluation Software). Figure 3. Transfer Function at Frequencies from 500 MHz to 43.5 GHz Power Detector 4 10 CALIBRATION AT –28dBm, –10dBm, AND +10dBm The ADL6010 is 45 dB envelope detector that operates from 3 500 MHz to 43.5 GHz. It has a linear in volts slope of approximately 5.9 V/V rms and an absolute detector input 2 1 range from −30 dBm to +15 dBm or −43 dBV to +2 dBV in a 1 50 Ω system. The detector cell uses a proprietary eight Schottky AGE (V) LT diode array followed by a novel linearizer circuit that creates a 0 0.1 linear voltmeter with an overall scaling factor (or transfer gain) ERROR (dB) –1 of nominally ×5.9 relative to the rms voltage amplitude of the –55°C –40°C VO OUTPUT –2 +25°C 0.01 input. With an output averaging capacitor, the ADL6010 can +85°C +125°C detect a signal with a varying envelope, but a correction factor –3 must be used to compensate for the change in output voltage for the same given input power. The output voltage is related to the –4 0.001 –30 –25 –20 –15 –10 –5 0 5 10 15 20 rms input voltage by P (dBm) 12625-004 IN VOUT = Slope × VRFIN + Intercept Figure 4. Transfer Function and Error at 10 GHZ for Various Temperatures where: 4 10 CALIBRATION AT –20dBm, 0dBm, AND +10dBm VOUT is the voltage on the VOUT pin. 3 Slope is approximately 5.9 V/V rms at 10 GHz. 2 1 VRFIN is the rms input voltage. Intercept is the y-axis value that the data crosses if extended. 1 AGE (V) Figure 2 shows a functional block diagram of the ADL6010. LT 0 0.1 ADL6010 ERROR (dB) –1 –55°C RFCM 4 3 VPOS –40°C VO OUTPUT –2 +25°C 0.01 +85°C RFIN 5 LINEARIZER 2 VOUT +125°C –3 RFCM 6 1 COMM –4 0.001 12625-002 –30 –25 –20 –15 –10 –5 0 5 10 15 20 Figure 2. ADL6010 RF/Microwave Detector Functional Diagram PIN (dBm) 12625-005 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 The slope and intercept of the system from the input of the range between 0 V and VREF, where the reference voltage is detector to the output of the ADC are either provided by the internal 2.5 V reference or by an external CODE CODE reference that overrides the internal reference. The external Slope HIGH LOW SYS VV reference can be as high as 5 V. For a 2.5 V full-scale voltage HIGH LOW (VREF = 2.5 V), the LSB size is INTSYS = CODEHIGH − (SlopeSYS × VHIGH) LSB = (2.5 V)/212 = 610 μV where: The output voltage of the ADL6010 is approximately 25 mV to SlopeSYS is the system slope. 4 V; therefore, a 200 Ω/340 Ω resistor divider with an CODEHIGH, CODELOW are the high and low code outputs, attenuation of approximately 1.6 reduces the amplitude of the respectively, from the ADC. signal so that it is always within the range of the AD7091R VHIGH, VLOW are the high and low RF voltages, respectively. when using the internal 2.5 V reference. INTSYS is the system intercept. Data Analysis The overall system transfer function is The EVAL-SDP-CB1Z system demonstration platform (SDP) CODE = SlopeSYS × VIN + INTSYS board is used in conjunction with software based on the where VIN is the rms voltage of the input RF signal. AD7091R evaluation board control software to capture the data Solve for VIN using being sampled by the ADC. The software has a power meter readout and calibration option. The power meter display shows CODE INTSYS VIN the power applied to the input of the ADL6010. To take an SlopeSYS accurate power measurement with the ADL6010 and the Therefore, the power in dBm, PIN, can be expressed as AD7091R, apply two known input powers at different levels to 2 the input of the ADL6010, then read the corresponding output 103 CODE INT ADC code. These four values make up two points on a plot and PIN log10(dBm) 10 R Slope must be stored for later use in the calibration procedure. The two points are For a 50 Ω input impedance, this equation simplifies to Point 1: (VLOW, CODELOW) CODE INT P log20dB01.13(dBm) (1) Point 2: (VHIGH, CODEHIGH) IN 10 Slope From these two points, a slope and an intercept can be found User Calibration Algorithm and used to calibrate the system at the particular frequency of operation. The CN-0366 Evaluation Software performs a one-time calibration at the particular frequency of operation. Calibration is achieved Figure 6 shows the software power level display. via the Calibration tab, shown in the window of Figure 6. The calibration routine is as follows: 1.