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One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Image Rejection Calibration on the
INTRODUCTION
Heterodyne radios, such as the
use a mixer to downconvert received RF signals to an inter-
mediate frequency (IF). The output of the mixer contains the
wanted frequency component along with an unwanted
component at the image frequency. Unwanted signals present
at the image frequency may cause desensitization of the
receiver, resulting in loss of signal on the wanted channel.
In theory, transceivers employing an IQ receive architecture
can be configured to infinitely reject the effect of the image
frequency. This theory assumes that the gain balance and the
phase orthogonality of the mixer quadrature paths are perfectly
aligned. In practice, some imbalance exists due to imperfections
in the mixer. The image calibration process adjusts the gain and
phase of the mixer via a digital control register until the
RFINB
ADF7021
family of transceivers,
AGC_MODE
LNA_GAIN
LNA_CURRENT (BIAS)
RFIN
LNA
MUX
INTERNAL
SIGNAL
SOURCE
IR_CAL_SOURCE ÷ 2
IR_CAL_SOURCE_DRIVE_LEVE L
IR_PHASE_ADJUST_DIRECTION
IR_PHASE_ADJUST_MAG
Figure 1. Image Calibration Functional Block Diagram
ADF7021, ADF7021-N, and
by Michael Dalton
quadrature signals are optimally balanced, providing maximum
image rejection.
This application note provides information on the mechanism
which generates the image frequency and describes how image
calibration can be implemented on the ADF7021, ADF7021-N,
and ADF7021-V.
ABBREVIATIONS
The following abbreviations are used in this application note:
•
Intermediate frequency (IF)
•
In-phase and quadrature components of a signal (IQ)
•
Image rejection (IR)
•
Radio frequency (RF)
•
Received signal strength indication (RSSI)
ADF7021/ADF7021-N/ADF7021-V
FILTER_GAIN
IF_BW
POLYPHASE
IF FILTER
IR_GAIN_ADJUST_UP/DN
IR_GAIN_ADJUST_IQ
IR_GAIN_ADJUST_MAG
PHASE ADJUST
Q
I
FROM LO
SERIAL
INTERFACE
4
PHASE ADJUST
REGISTER 5
4
GAIN ADJUST
REGISTER 5
MICROCONTROLLER
I/Q GAIN/PHASE ADJUST AND
RSSI MEASUREMENT
ALGORITHM
Rev. 0 | Page 1 of 12
APPLICATION NOTE
ADF7021-V
RSSI/
LOG AMP
7-BIT ADC
RSSI READBACK
AN-1258
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Related Manuals for Analog Devices AFDF7021
Summary of Contents for Analog Devices AFDF7021
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Page 1: Introduction
AN-1258 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com ADF7021, ADF7021-N, and ADF7021-V Image Rejection Calibration on the by Michael Dalton INTRODUCTION Heterodyne radios, such as the ADF7021 family of transceivers, quadrature signals are optimally balanced, providing maximum... -
Page 2: Table Of Contents
AN-1258 Application Note TABLE OF CONTENTS Introduction ..................1 Internal Tone Setup ...............5 Abbreviations ..................1 Brute Force Sweep ................7 Revision History ................2 Gradient Descent Algorithm Overview .........9 Implementing Image Calibration ........... 3 Gradient Descent Algorithm Terminating Conditions ..... 10 Image Frequency Mechanism ............. -
Page 3: Implementing Image Calibration
Application Note AN-1258 IMPLEMENTING IMAGE CALIBRATION ω – ω IMAGE FREQUENCY MECHANISM Figure 2 shows two signals applied to the input of a quadrature receiver. ω – ω 100kHz 100kHz cos (ω cos (ω ) + cos (ω ω ω ω... -
Page 4: Register Settings
AN-1258 Application Note REGISTER SETTINGS Table 1 gives an overview of the primary registers adjusted during the calibration process. Recommended values are provided in the Notes column of this table. The parameters are also displayed in Figure 3. Table 1. Register Settings Setting Register Notes... -
Page 5: Internal Tone Setup
Application Note AN-1258 INTERNAL TONE SETUP The internal tone is generated from integer multiples of the Although the internal tone is not produced at precisely DEMOD_CLK. The power of the tone varies with the reference 868 MHz, results taken at 870.84 MHz are a good estimate of crystal and the RF band used. - Page 6 AN-1258 Application Note START START WRITE TO REGISTER 1 READBACK RSSI INCREASE (DELAY 700µs) (AVERAGE OVER 10 READBACKS) IR_CAL_SOURCE_DRIVE_LEVEL TO HIGH (DECIMAL 3) SET GAIN AND PHASE TO 15 ON I CHANNEL WRITE TO REGISTER 3 SETS DEMOD CLOCK TONE PRESENT? ALREADY WRITE TO REGISTER 0...
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Page 7: Brute Force Sweep
Figure 7 shows an example array of results obtained using the Note that if gain mode is used, instead of attenuate mode, the IR_Sweep_Cal software, available from Analog Devices, Inc., resulting array will be 63 × 31 = 1953 RSSI values. However, in installed with the evaluation software. - Page 8 AN-1258 Application Note Figure 7. Brute Force Example Results Rev. 0 | Page 8 of 12...
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Page 9: Gradient Descent Algorithm Overview
Application Note AN-1258 GRADIENT DESCENT ALGORITHM OVERVIEW If the image rejection at Point B is better than Point A, then This algorithm is a more efficient alternative to performing a ΔGain > 0 and the gain should be increased by 1. If ΔGain < 0, brute force sweep. -
Page 10: Gradient Descent Algorithm Terminating Conditions
AN-1258 Application Note START ΔPHASE > SET IR CALIBRATION VALUES (ΔGAIN + 2)? FOR POINT A MEASURE RSSI ΔGAIN > 0? SET IR CALIBRATION VALUES FOR POINT B (GAIN + 1) INCREMENT DECREMENT GAIN GAIN MEASURE RSSI SET IR CALIBRATION VALUES ΔGAIN >... -
Page 11: Other Considerations
Application Note AN-1258 OTHER CONSIDERATIONS START IR calibration evaluation software is provided by Analog Devices. This is included with the ADF7xxx evaluation software. RSSI VALUE AT POINT A < BEST? The optimum calibration values vary with temperature. Figure 11 shows the image rejection varying with changes INCREMENT in temperature. - Page 12 AN-1258 Application Note NOTES ©2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN11774-0-11/13(0) Rev. 0 | Page 12 of 12...
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