CN-0150 [ADI]

Software-Calibrated, 1 MHz to 8 GHz, 60 dB RF Power Measurement System Using a Logarithmic Detector; 软件校准的1 MHz至8 GHz的60 dB RF功率测量系统使用对数检测器


型号：	CN-0150
厂家：	ADI
描述：	Software-Calibrated, 1 MHz to 8 GHz, 60 dB RF Power Measurement System Using a Logarithmic Detector 软件校准的1 MHz至8 GHz的60 dB RF功率测量系统使用对数检测器
文件：	总5页 (文件大小：247K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

Circuit Note

CN-0150

Devices Connected/Referenced

1 MHz to 8 GHz, 70 dB, Logarithmic

Detector/Controller

Circuits from the Lab™ reference circuits 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/CN0150.

AD8318

2.7 V to 5.25 V, Micropower, 2-Channel,

125 kSPS, 12-Bit ADC in 8-Lead MSOP

AD7887

ADR421

Precision, Low Noise, 2.5 V Reference

Software-Calibrated, 1 MHz to 8 GHz, 60 dB RF Power

Measurement System Using a Logarithmic Detector

A simple two-point system calibration is performed in the

digital domain.

EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

The AD8318 maintains accurate log conformance for signals of

1 MHz to 6 GHz and provides useful operation to 8 GHz.

The device provides a typical output voltage temperature

stability of 0.ꢀ dB.

CN-0150 Circuit Evaluation Board (EVAL-CN0150A-SDPZ)

System Demonstration Platform (EVAL-SDP-CB1Z)

Design and Integration Files

Schematics, Layout Files, Bill of Materials

The AD7887 ADC can be configured for either dual or single

channel operation via the on-chip control register. There is a

default single-channel mode that allows the AD7887 to be operated

as a read-only ADC, thereby simplifying the control logic.

CIRCUIT FUNCTION AND BENEFITS

This circuit measures RF power at any frequency from

1 MHz to 8 GHz over a range of approximately 60 dB. The

measurement result is provided as a digital code at the output of

a 12-bit ADC with serial interface and integrated reference. The

output of the RF detector has a glueless interface to the ADC and

uses most of the ADC’s input range without further adjustment.

Typical data is shown for the two devices operating over a

−40°C to +8ꢀ°C temperature range.

+5V

POS

499Ω

0.1µF

10µF

0.1µF

100pF

SERIAL

INTERFACE

CMIP CMIP TADJ VPSO

13 TEMP

CMOP

VSET

VOUT

CLPF

AD7887

PULSED RF

INPUT

OUT

C1 1nF

C2 1nF

AIN0

14 INHI

SCLK

52.3Ω

AD8318

µC/µP

FIN

SEE

TEXT

DOUT

DIN

15 INLO

16 ENBL

AIN1/

REF

0.1µF

CMIP CMIP VPSI VPSI

GND

0.1µF

100pF

0.1µF

POS

Figure 1. Software-Calibrated RF Measurement System (Simplified Schematic: All Connections Not Shown)

Rev. C

Circuits from the Lab™ circuits 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 totheuseof anyCircuits from the Lab circuits. (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

CN-0150

Circuit Note

Using the two known input power levels, PIN_1 and PIN_2,

and the corresponding observed ADC codes, CODE_1 and

CODE_2, SLOPE_ADC, and INTERCEPT can be calculated

using the following equations:

CIRCUIT DESCRIPTION

The RF signal being measured is applied to the AD8318. The

device is configured in its so-called measurement mode, with

the VSET and VOUT pins connected together. In this mode,

the output voltage vs. the input signal level is linear-in-dB

(nominally −24 mV/dB) and has a typical output voltage range

of 0.ꢀ V to 2.1 V.

SLOPE_ADC = (CODE_2 − CODE_1)/(PIN_2 − PIN_1)

INTERCEPT = PIN_2 − (CODE_2/SLOPE_ADC)

Once SLOPE_ADC and INTERCEPT are calculated and stored (in

nonvolatile RAM) during factory calibration, they can be used

to calculate an unknown input power level, PIN, when the

equipment is in operation in the field using the equation

The AD8318 output is connected directly to the AD7887, 12-bit

ADC. The ADC uses its internal reference and is configured for

a 0 V to 2.ꢀ V input, resulting in an LSB size of 610 μV. With the RF

detector providing a nominal −24 mV/dB, the digital resolution

is 39.3 LSBs/dB. With this much resolution, there is little value

in trying to scale the 0.ꢀ V to 2.1 V signal from the RF detector

to exactly fit the 0 V to 2.ꢀ V range of the ADC.

PIN = (CODE_OUT/SLOPE_ADC) + INTERCEPT

Figure 3 through Figure 8 show how the system transfer function

deviates from this straight line equation, particularly at the

endpoints of the transfer function. This deviation is expressed

in dB using the equation

The transfer function of the detector can be approximated by

the equation

Error (dB) = Measured Input Power − True Input Power =

(CODE_OUT/SLOPE_ADC) + INTERCEPT – PIN_TRUE

_OUT= SLOPE × (P_IN− INTERCEPT)

where SLOPE is in mV/dB (−24 mV/dB nominal); INTERCEPT is

the x-axis intercept with a unit of dBm (20 dBm nominal);

and P_INis the input power expressed in dBm. A typical plot

of detector output voltage vs. input power is shown in Figure 2.

where:

CODE_OUT is the ADC output code.

SLOPE_ADC is the stored ADC slope in codes/dB.

INTERCEPT is the stored intercept.

PIN_TRUE is the exact (and unknown) input level.

2.4

2.1

1.8

1.5

1.2

0.9

0.6

0.3

2.0

1.5

1.0

0.5

25°C

OUT

ERROR 25°C

The plots shown in Figure 3 through Figure 8 show the typical

system performance that can be obtained using the AD8318 and

AD7887BR in an RF power measurement system. The graphs

depict the RF input power in dBm vs. the ADC output code and

output error in dB (scaled on the axes on the right side of the

plots). They were generated from data taken with various input

power levels, frequencies, and temperatures and with both internal

and external ADC voltage references. The charts show improved

system performance and lower temperature drift with the use of

a low drift external ADC voltage reference. (See the Common

Variations section for more details about the use of an external

reference.

–0.5

–1.0

–1.5

RANGE OF

CALCULATION

OF SLOPE AND

INTERCEPT

–65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5

10 15

INTERCEPT

(dBm)

Figure 2. Typical Output Voltage vs. Input Signal Level for the AD8318

At the output of the ADC, the equation can be written as

CODE_OUT = SLOPE_ADC × (PIN − INTERCEPT)

A complete design support package for this circuit note can be

found at www.analog.com/CN01ꢀ0-DesignSupport.

4.0k

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

3.5k

where SLOPE_ADC is in codes/dB and PIN and INTERCEPT

are in dBm. Figure 3 shows a typical detector power sweep in

terms of input power and observed ADC codes.

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

CODE_2

CODE_1

Because the slope and intercept of the system vary from device

to device, a system level calibration is required. A calibration is

performed by applying two known signal levels close to the

endpoints of the AD8318 linear input range and measuring the

corresponding output codes from the ADC. The calibration

points chosen should be well within the linear operating

range of the device (−10 dBm and −ꢀ0 dBm in this case).

–1

–2

–3

–4

–70

–60 –50

–40

–30

–20 –10

INPUT POWER (dBm)

PIN_2

PIN_1

Figure 3. Input = 900 MHz, ADC Using an Internal 2.5 V Reference

Rev. C | Page 2 of 5

Circuit Note

CN-0150

4.0k

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

4.0k

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

–1

–2

–3

–1

–2

–3

–4

–70

–60

–50

–40

–30

–20

–10

–70

–60

–50

–40

–30

–20

–10

INPUT POWER (dBm)

Figure 4. Input = 900 MHz, ADC Using an External 2.5 V Reference

Figure 7. Input = 2.2 GHz, ADC Using an Internal 2.5 V Reference

4.0k

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

4.0k

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

–1

–2

–3

–1

–2

–3

–4

–70

–60

–50

–40

–30

–20

–10

–70

–60

–50

–40

–30

–20

–10

INPUT POWER (dBm)

Figure 5. Input = 1.9 GHz, ADC Using an Internal 2.5 V Reference

Figure 8. Input = 2.2 GHz, ADC Using an External 2.5 V Reference

4.0k

COMMON VARIATIONS

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

The AD7887 is a 2-channel, 12-bit ADC with an SPI interface.

The second input channel of this device can be connected to the

AD8318 TEMP pin. This provides a convenient measure of the

ambient temperature around the AD8318. Like the AD8318

power measurement output, the TEMP voltage output should

also be calibrated.

–1

–2

–3

If the end application requires only a single channel, the 12-bit

AD749ꢀ can be used. In multichannel applications that require

multiple ADCs and DAC channels, the AD7294 can be used.

In addition to providing four 12-bit DAC outputs, this subsystem

chip includes four uncommitted ADC channels, two high-side

current sense inputs, and three temperature sensors. Current

and temperature measurements are digitally converted and

available to read over the I²C-compatible interface.

–4

–70

–60

–50

–40

–30

–20

–10

INPUT POWER (dBm)

Figure 6. Input = 1.9 GHz, ADC Using an External 2.5 V Reference

The temperature stability of the circuit can be improved using an

external ADC reference. The AD7887 internal 2.ꢀ V reference has

a ꢀ0 ppm/°C drift, which is approximately 1ꢀ mV over a 12ꢀ°C

range. Because the detector has a slope of −24 mV/dB, the ADC

reference drift contributes approximately 0.3 dB to the temperature

drift error budget. The AD8318 temperature drift is approximately

0.ꢀ dB over a similar temperature range. (This varies with

frequency. See the AD8318 data sheet for more details.)

Rev. C | Page 3 of 5

CN-0150

Circuit Note

If an external voltage reference is to be used, the ADR421 2.ꢀ V

reference is recommended. Its 1 ppm/°C temperature drift results

in a reference voltage variation of only 312 μV from −40°C to

+8ꢀ°C. This has a negligible effect on the overall temperature

stability of the system.

6 V wall wart can be connected to the barrel connector on the

board and used in place of the 6 V power supply. Connect the USB

cable supplied with the SDP board to the USB port on the PC.

Note: Do not connect the USB cable to the mini USB connector

on the SDP board at this time.

If a less dynamic range is required, the AD8317 (ꢀꢀ dB) or AD8319

(4ꢀ dB) log detector can be used. If a true rms responding power

measurement is required, the AD8363 (ꢀ0 dB) or ADLꢀ902

(6ꢀ dB) can be used.

Test

Apply power to the 6 V supply (or wall wart) connected to

EVAL-CN01ꢀ0A-SDPZ circuit board. Launch the evaluation

software and connect the USB cable from the PC to the USB

mini connector on the SDP board.

CIRCUIT EVALUATION AND TEST

Once USB communications are established, the SDP board can

now be used to send, receive, and capture serial data from the

EVAL-CN01ꢀ0A-SDPZ board.

This circuit uses the EVAL-CN01ꢀ0A-SDPZ circuit board and

the EVAL-SDP-CB1Z System Demonstration Platform (SDP)

evaluation board. The two boards have 120-pin mating connectors,

allowing for the quick setup and evaluation of the circuit’s

performance. The EVAL-CN01ꢀ0A-SDPZ board contains the

circuit to be evaluated, as described in this note, and the SDP

evaluation board is used with the CN01ꢀ0A evaluation software to

capture the data from the EVAL-CN01ꢀ0A-SDPZ circuit board.

The data in this circuit note were generated using a Rohde &

Schwarz SMT-03 RF signal source and an Agilent E3631A power

supply. The signal source was set to the frequencies indicated in

the graphs, and the input power was stepped and data recorded

in 1 dB increments.

Equipment Needed

Temperature testing was performed using a Test Equity Model 107

environmental chamber. The EVAL-CN01ꢀ0A-SDPZ evaluation

board was placed in the chamber via a slot in the test chamber

door, with the SDP evaluation board extending outside.

•

PC with a USB port and Windows® XP or Windows Vista®

(32-bit), or Windows 7 (32-bit)

•

EVAL-CN01ꢀ0A-SDPZ Circuit Evaluation Board

EVAL-SDP-CB1Z SDP Evaluation Board

CN01ꢀ0A Evaluation Software

Power supply: 6 V or 6 V wall wart

Environmental chamber

Information and details regarding how to use the evaluation

software for data capture can be found in the CN01ꢀ0A

evaluation software readme file.

Information regarding the SDP board can be found in the SDP

User Guide.

LEARN MORE

RF signal source

CN01ꢀ0 Design Support Package:

Coaxial RF cable with SMA connectors

http://www.analog.com/CN01ꢀ0-DesignSupport

Getting Started

SDP User Guide

Load the evaluation software by placing the CN01ꢀ0A evaluation

software CD in the CD drive of the PC. Using My Computer,

locate the drive that contains the evaluation software CD and

open the readme file. Follow the instructions contained in the

readme file for installing and using the evaluation software.

MT-031 Tutorial, Grounding Data Converters and Solving the

Mystery of “AGND” and “DGND,” Analog Devices.

MT-077 Tutorial, Log Amp Basics, Analog Devices.

MT-078 Tutorial, High Speed Log Amps, Analog Devices.

MT-101 Tutorial, Decoupling Techniques, Analog Devices.

Functional Block Diagram

See Figure 1 of this circuit note for the circuit block diagram

and the EVAL-CN150A-SDPZ-SCH-Rev0.pdf file for the

circuit schematics. This file is contained in the CN01ꢀ0

Design Support Package.

Whitlow, Dana. Design and Operation of Automatic Gain

Control Loops for Receivers in Modern Communications

Systems. Chapter 8. Analog Devices Wireless Seminar. 2006.

Data Sheets and Evaluation Boards

CN-01ꢀ0 Circuit Evaluation Board (EVAL-CN01ꢀ0A-SDPZ)

System Demonstration Platform (EVAL-SDP-CB1Z)

AD7887 Data Sheet

Setup

Connect the 120-pin connector on the EVAL-CN01ꢀ0A-SDPZ

circuit board to the CON A connector on the EVAL-SDP-CB1Z

evaluation (SDP) board. Use nylon hardware to firmly secure

the two boards, using the holes provided at the ends of the 120-pin

connectors. Using an appropriate RF cable, connect the RF signal

source to the EVAL-CN01ꢀ0A-SDPZ board via the SMA RF

input connector. With power to the supply off, connect a 6 V power

supply to the +6V and GND pins on the board. If available, a

AD7887 Evaluation Board

AD8318 Data Sheet

AD8318 Evaluation Board

ADR421 Data Sheet

Rev. C | Page 4 of 5

Circuit Note

CN-0150

REVISION HISTORY

2/12—Rev. B to Rev. C

Changed 70 dB to 60 dB in Circuit Note Title ..............................1

3/11—Rev. A to Rev. B

Added Evaluation and Design Support Section............................1

Added Circuit Evaluation and Test Section...................................4

8/10— Rev. 0 to Rev. A

Changes to the Circuit Function and Benefits Section ................1

Changes to the Circuit Description Section ..................................2

Changes to the Common Variations Section ................................4

4/10—Revision 0: Initial Version

I²C refers to a communications protocol originally developed by Philips Semiconductors (now NXP Semiconductors).

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registered trademarks are the property of their respective owners.

CN08967-0-2/12(C)

Rev. C | Page 5 of 5

CN-0150 [ADI]

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