
CN0366: A 40 GHz Microwave Power Meter with a Range from −30 dBm to +15 dBm10/1/2014PDF311 K
Overview
Design Resources
Design & Integration Files
 Schematic
 Bill of Materials
 Gerber Files
 PADS Files
 Assembly Drawing
Evaluation Hardware
Part Numbers with "Z" indicate RoHS Compliance. Boards checked are needed to evaluate this circuit.
 ADL6010EVALZ ($615.25) ADL6010 Evaluation Board
 EVALAD7091RSDZ ($72.60) AD7091R Evaluation Board
 EVALSDPCB1Z ($116.52) Eval Control Board
Device Drivers
Software such as C code and/or FPGA code, used to communicate with component's digital interface.
AD7091R  NoOS Driver for Renesas Microcontroller Platforms
AD7091R  NoOS Driver for Microchip Microcontroller Platforms
Features & Benefits
 40GHz Power Meter
 45dB Dynamic Range
 Low Power
Markets and Technologies
Parts Used
Documentation & Resources
Circuit Function & Benefits
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, 12bit, 1 MSPS analogtodigital 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 CN0366 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.
Circuit Description
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 linearinvoltage, 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.
The AD7091R 12bit, 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 fullscale voltage is needed.
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 (CN0366 Evaluation Software).
Power Detector
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
where:
V_{OUT} is the voltage on the V_{OUT} pin.
Slope is approximately 5.9 V/V rms at 10 GHz.
V_{RFIN} is the rms input voltage.
Intercept is the yaxis value that the data crosses if extended.
Figure 2 shows a functional block diagram of the ADL6010.
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.
AnalogtoDigital Converter
The AD7091R is a 12bit, 1 MSPS ADC with an input voltage range between 0 V and V_{REF}, 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 fullscale voltage (V_{REF} = 2.5 V), the LSB size is
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.
Data Analysis
The EVALSDPCB1Z 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
 Point 1: (V_{LOW}, CODE_{LOW})
 Point 2: (V_{HIGH}, CODE_{HIGH})
From these two points, a slope and an intercept can be found and used to calibrate the system at the particular frequency of operation.
Figure 6 shows the software power level display.
System Transfer Function
The slope and intercept of the system from the input of the detector to the output of the ADC are
where:
Slope_{SYS} is the system slope.
CODE_{HIGH}, CODE_{LOW} are the high and low code outputs, respectively, from the ADC.
V_{HIGH}, V_{LOW} are the high and low RF voltages, respectively.
INT_{SYS} is the system intercept.
The overall system transfer function is
where V_{IN} is the rms voltage of the input RF signal.
Solve for V_{IN} using
Therefore, the power in dBm, PIN, can be expressed as
For a 50 Ω input impedance, this equation simplifies to
User Calibration Algorithm
The CN0366 Evaluation Software performs a onetime 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:
 Set the RF power to high level (V_{HIGH}).
 Measure the code from ADC (CODE_{HIGH}).
 Set the RF power to low level (V_{LOW}).
 Measure the code from ADC (CODE_{LOW}).
 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.
Measurement Results for Complete Signal Chain Including ADC
Using the CN0366 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.
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. 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. 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.
Summary of Equations
The following is a summary of the equations in the CN0366, 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
where:
V_{OUT} is the dc output voltage of the detector.
Slope_{DET} is the gain/slope of the detector, in V/V rms.
V_{IN} is the rms voltage of the input RF signal.
INT_{DET} is the intercept of the detector, in volts.
Solve for V_{IN} using
The ADC transfer function (when being driven by the ADL6010) is given by
CODE = V_{OUT}/LSB
LSB = V_{REF}/4096
where:
CODE is the ADC output code, a unitless number that can range from 0 and 4096.
V_{REF} is the fullscale reference voltage of the ADC, in volts. LSB is the smallest quantized voltage that the ADC can resolve, in V/step or V/bit.
Solve for V_{OUT} using
V_{OUT} = CODE × LSB
Substituting this expression in the previous V_{IN} equation yields
Referring everything back to input power yields
where R is the impedance on the RFIN pin of the ADL6010. Substituting V_{IN} into the power equation yields
Simplifying the equation yields
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 (slope_{SYS}) and system intercept (INT_{SYS}) begins with Equation 2, which has the input power in terms of detector slope (slope_{DET}) and detector intercept (INT_{DET}). This derivation is achieved by multiplying both the numerator and denominator of the V_{IN} expression (Equation 2) by 1/LSB, as follows:
Substituting this expression for V_{IN} into Equation 2 yields Equation 1.
INT_{DET} has units of volts, Slope_{DET} has units of V/V rms, and LSB has units of V/codes. Multiplying by 1/LSB converts INT_{DET} into its equivalent ADC code and converts the detector slope into the system slope with unit of codes/V rms, yielding the following relations:
INT_{SYS} = INT_{DET}/LSB
Slope_{sys} = Slope_{DET}/LSB
A complete set of design files, including schematics, layouts, Gerbers, and bill of materials for the CN0366 circuit are available in the CN0366 Design Support Package.
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.
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:
where:
V_{HIGH}, V_{LOW} are the high and low voltages, respectively, applied to the input detector (generalized as V_{IN} in Figure 14).
CODE_{HIGH}, CODE_{LOW} are the high and low outputs of the ADC.
If a known voltage is applied to the input of the coupler (generalized as V_{COUPLER} in Figure 14), the general relationship between the input of the coupler and the input of the detector is
V_{IN} = V_{COUPLER} × 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
where the V_{HIGH}’ and V_{LOW}’ 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
Figure 15 shows the transfer function at 1 GHz and 5 GHz for a system using a coupler with 10 dB loss.
Circuit Evaluation & Test
Equipment Needed
The following equipment is needed to perform the evaluations described in the CN0366:
 The ADL6010EVALZ evaluation board.
 The EVALAD7091RSDZ evaluation board.
 The EVALSDPCB1Z 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 EVALSDPCB1Z).
 A 5 V power supply to supply ADL6010EVALZ board.
 A 9 V actodc, wallmounted converter to supply the EVALAD7091RSDZ evaluation board (supplied with the EVALAD7091RSDZ). Note that the EVALSDPCB1Z is powered from a regulator on the EVALAD7091RSDZ.
 The CN0366 Evaluation Software.
Getting Started
Make the following modifications and link settings to the ADL6010EVALZ evaluation board and the EVALAD7091RSDZ evaluation board to implement the circuit shown in Figure 1.
For the ADL6010EVALZ, replace R1 with a 200 Ω resistor (0402 size).
For the EVALAD7091RSDZ, replace R1 with a 0 Ω resistor (0603 size), and replace C13 with a 340 Ω resistor (0603 size). For the EVALAD7091RSDZ link settings, set LK1 to Position C, set LK2 to Position C, and leave LK3 and LK4 open.
Functional Block Diagram
Figure 16 shows the functional block diagram of the test setup that was used for testing the receive chain.
Setup and Test
To set up and test the microwave power meter system, take the following steps:
 Turn on all test equipment and wait for the test equipment to warm up.
 Connect the ADL6010EVALZ evaluation board input to the Agilent signal generator (connecting directly from the signal generator to the evaluation board via a barrel connector is recommended).
 Connect the ADL6010EVALZ evaluation board output to the input of the EVALAD7091RSDZ evaluation board.
 Connect the EVALSDPCB1Z SDP board to the EVALAD7091RSDZ evaluation board.
 Connect the SDP board to a PC via the USB cable that is provided with the SDP board
 Download and install the CN0366 Evaluation Software onto the PC that is connected to the SDP control board.
 After the software is installed properly, run the executable.
 Turn on the signal generator and set it to a power and frequency within the operating limits of the ADL6010.
 To obtain an accurate power reading, run the calibration routine in the software.
 The software GUI now calculates and displays the correct power that is applied to the input of the ADL6010.
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