Espressif ESP8266 Series User manual

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Version 1.0
Copyright © 2016
ESP8266
Hardware Matching Guide

About This Guide
This document introduces frequency offset tuning and antenna impedance matching for
ESP8266 in order to achieve optimum RF performance.
Release Notes
Chapter
Title
Subject
Chapter 1
Overview
Relevance of frequency offset adjustment and impedance
matching.
Chapter 2
Frequency Offset Tuning
Introduction to frequency offset testing and tuning methods.
Chapter 3
Antenna Matching
Introduction to antenna impedance matching.
Date
Version
Release note
2016.06
V1.0
First release.

Table of Contents
1. Overview!1".................................................................................................................................
2. Frequency Offset Tuning!2".........................................................................................................
2.1. Testing Frequency Offset!2"...........................................................................................................
2.2. Adjusting Frequency Offset!2"........................................................................................................
3. Impedance Matching!4..............................................................................................................

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1. Overview
1. Overview"
ESP8266 is the most integrated System-on-a-Chip (SoC) with only 7 external components.
In order to achieve the optimum performance of the chip, customers only need to tune the
frequency offset and match the antenna impedance according to the crystal oscillator and
antenna.
The lack of frequency offset tuning and antenna impedance matching may reduce the RF
performance, resulting in AP scanning issue, unstable connection and unstable data
transmission.
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2. Frequency Offset Tuning
2. Frequency Offset Tuning"
2.1. Testing Frequency Offset
The frequency offset can be tested in one of the following ways:
1. GPIO0 outputs crystal oscillator clock signals by default. Customers can compare the
calculated frequency against the standard frequency and get the offset value.
2. Customers can use AT command firmware and send AT + CWLAP command. The
last parameter is the offset value. However, it is a relative value. Customers can use a
standard device for comparison.
3. Customers can use FCC/CE test firmware to send standard wave and get the offset
value with IQView equipment. IQView equipment is as shown in Figure 2-1. FCC/CE
certification and test document can be downloaded from:"
http://www.espressif.com/en/support/download/other-tools.
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Figure 2-1. IQView Equipment
2.2. Adjusting Frequency Offset
The frequency offset can be adjusted in two ways:
1. Adjust the regulating capacitors on both sides of crystal oscillator.
-Increase capacitance if the frequency offset is positive, for example +50 ppm.
-Decrease capacitance if the frequency offset is negative, for example -50 ppm.
-Generally, the capacitances should be matched and adjusted at the same time.
2. Modify the frequency offset in ESP8266 DOWNLOAD TOOL. Click LoadInitBin to
load esp_init_data_default.bin, modify PraticalFreqOffset and click GenInitBin to
generate a new esp_init_data_default.bin to download to Flash. Please refer to
Figure 2-2.
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2. Frequency Offset Tuning
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Figure 2-2. ESP8266 DOWNLOAD TOOL - RFConfig Tab
📖Note:
ESP8266 DOWNLOAD TOOL can be downloaded from:"
http://www.espressif.com/en/support/download/other-tools.
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3. Impedance Matching
3. Impedance Matching"
The impedance of ESP8266 PA output end is 39+j6 Ω, so the matched impedance is 39-j6
Ω(from antenna to the chip).
For EMC test, the π-type impedance matching circuit of external antenna should be as
shown in Figure 3-1.
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Figure 3-1. Antenna Impedance Matching
5
5
4
4
3
3
2
2
1
1
D D
C C
B B
A A
Antenna
GND GND
Title
Size
Document Number Rev
Date: Sheet of
<Doc> <RevCode>
<Title>
A
11Tuesday, May 26, 2015
Title
Size
Document Number Rev
Date: Sheet of
<Doc> <RevCode>
<Title>
A
11Tuesday, May 26, 2015
Title
Size
Document Number Rev
Date: Sheet of
<Doc> <RevCode>
<Title>
A
11Tuesday, May 26, 2015
C1
2.4pF
L1 NC
C2
NC
📖Notes:
•C1 must be a 2.4 pF capacitor for filtering harmonics twice.
•L1, C2, along with C1 perform 39-j6 Ωimpedance matching for antenna.
•L1 and C2 can be inductors or capacitors as required.
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Disclaimer and Copyright Notice
Information in this document, including URL references, is subject to change without
notice.
THIS DOCUMENT IS PROVIDED AS IS WITH NO WARRANTIES WHATSOEVER,
INCLUDING ANY WARRANTY OF MERCHANTABILITY, NON-INFRINGEMENT, FITNESS
FOR ANY PARTICULAR PURPOSE, OR ANY WARRANTY OTHERWISE ARISING OUT
OF ANY PROPOSAL, SPECIFICATION OR SAMPLE.
All liability, including liability for infringement of any proprietary rights, relating to use of
information in this document is disclaimed. No licenses express or implied, by estoppel or
otherwise, to any intellectual property rights are granted herein.
The Wi-Fi Alliance Member logo is a trademark of the Wi-Fi Alliance. The Bluetooth logo is
a registered trademark of Bluetooth SIG.
All trade names, trademarks and registered trademarks mentioned in this document are
property of their respective owners, and are hereby acknowledged.
Copyright © 2016 Espressif Inc. All rights reserved.
Espressif IOT Team"
www.espressif.com
