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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)
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Tel: 781.329.4700
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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.
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registered trademarks are the property of their respective owners.
CN12625-0-10/14(0)
Rev. 0 | Page 8 of 8