AN421: Antenna Selection Guide for the Si4020 and

AN421
Antenna Selection Guide for the
Si4020 and Si4320 ISM Band
FSK Transmitter/Receiver Chipset
Application Note
Version 1.5
1
AN421 REV 1.5 0504
© 2009, Silicon Laboratories, Inc.
Silicon Labs, Inc.
400 West Cesar Chavez
Austin, Texas 78701
Tel: 512.416.8500
Fax: 512.416.9669
Please visit the Silicon Labs Technical Support web page:
https://www.silabs.com/support/pages/
contacttechnicalsupport.aspx
and register to submit a technical support request.
The information is provided “as is” without any express or implied warranty of any kind,
including warranties of merchantability, non-infringement of intellectual property, or fitness
for any particular purpose. In no event shall Silicon Laboratories, Inc., or its suppliers be
liable for any damages whatsoever arising out of the use of or an inability to use the
materials. Silicon Laboratories, Inc., and its suppliers further do not warrant the accuracy or
completeness of the information, text, graphics, or other items contained within these materials.
Silicon Laboratories, Inc., may make changes to these materials, or to the products described
within, at any time, without notice.
© 2009 Silicon Laboratories, Inc. All rights reserved. Silicon Laboratories is a trademark
of Silicon Laboratories, Inc. All trademarks belong to their respective owners.
i
ABOUT THIS GUIDE
The antenna selection guide for the Si4020 and Si4320 ISM Band FSK Transmitter/Receiver Chipset is designed to give product
designers a quick time-to-market approach for on-board antenna selection. The guide is designed to address geographic regulations
covering the standard ISM FSK band frequencies; 315MHz, 434MHz, 868MHz, and 915MHz and to address the approximate rangeversus-bandwidth to given antenna pairs.
Designers wishing to develop custom antennas can refer to the Antenna Development Guide: AN422.
For further information on the devices used in this publication, see the following datasheets:
Si4020 Universal ISM Band Transmitter datasheet:
Si4020-DS
Si4320 Universal ISM Band Receiver datasheet:
Si4320-DS
ii
INTRODUCTION
DESCRIPTION
This document is an Antenna Selection Guide for the universal, four-band (315 MHz, 434 MHz, 868 MHz, 915 MHz) Si4020
transmitter and Si4320 receiver.
Within this document two main antenna groups are referenced:
Loop antennas
Modified Inverted (IFA) antennas, the
so-called “back IFA” antennas
For further information regarding antenna types and RF link properties, please visit our Web site: http://www.silabs.com
http://www.silabs.com, and
download the Antenna Development Guide: AN422.
1
1. ANTENNA PAIRS AND RANGES
The range is estimated from the measured EIRP (Equivalent Isotropic Radiated Power) and the sensitivity of the transmitter and
the receiver with the different antennas, respectively. The definition of EIRP is given in Appendix A. During the range calculations,
ideal free space propagation conditions were assumed. The given range corresponds to a transmitter (TX) with a two-sided FSK
deviation of 120 kHz (with data rate of 9600 bps) and 180 kHz (with data rate of 57470 bps). The receiver (RX) baseband filter
bandwidth was adjusted to 135 kHz. The EIRP data of the TX with different antennas as well as the measured RX sensitivities at
10-2 BER (Bit Error Rate) are given in detail in Appendix A. The free space range calculation method is described in Appendix B. The
measurement setup of the TX and RX measurement is detailed in Appendix C.
In some cases, the allowed radiation power (given either in ERP (Equivalent Radiated Power - see Appendix A for definition) or field
strength), set by the U.S. FCC or European ETSI guidelines, is lower than the maximum power of the transmitter. In those cases, the
range corresponding to the allowed radiation power is given together with the necessary power reduction.
As the impedance of the loop antennas are much higher compared to that of the IFA antennas, the output current of the TX with loop
antennas must be reduced so as not to exceed the maximum allowed differential voltage swing (4 Vpp) on the outputs. The given
ranges of loop TX antennas correspond to the properly reduced currents.
The receiver sensitivity was measured in the presence of strong interference (GSM, TV etc.) signals with frequencies close to the
used bands. The electric field of the interference signals around 900 MHz during the sensitivity measurements were between 60 and
80 mV/m; it is approx. 40-60 dB higher than the useful signal’s electric field. As the receiver sensitivity is approx. 6-8 dB better in an
interference-free environment (i.e., if a narrow band saw filter is used at the receiver input), the distance is about 2 times higher in
that circumstance.
The typical range to achieve a BER (Bit Error Rate) of 10-2 in the case of various transmitter-receiver antenna pairs is presented for
9600 and 57450 bps data rate at each frequency. The antenna layouts together with the antenna dimensions are also given.
In the case of Back IFA antennas, the frame shows the nearest allowed cutting edge of the PCB. If the cutting is closer to the arms of
the antenna, significant de-tuning — and thus reduction of the radiated power — will occur.
2
U.S. REGULATIONS: 915 MHz, 434 MHz, 315 MHz BAND
Tables 1.1, 1.2, and 1.3 give the typical ranges in meters for different TX and RX antenna pairs for the U.S. 915 MHz, 434 MHz, and
315 MHz band, respectively. The transmitted power is regulated by part 15 of the FCC standards (Note 2). It gives restrictions to the
allowed field strength at 3 m distances. The allowed field strengths are 50, 11, and 6 mV/m at 915, 434 and 315 MHz, respectively.
In the case of spread spectrum transmission, the maximum allowed TX power is 1 W at 915 MHz, which can be achieved only with
an external booster stage.
RX tapped loop (see Fig 2.5)
RX back IFA dev (see Fig 2.6)
915 MHz U.S. band
TX loop (see Fig 2.1)
(3dB reduced power state)
TX tapped loop (see Fig 2.2)
TX back IFA (see Fig 2.3)
EIRP
dBm)
(max. power: ERP≈-1
TX back IFA dev (see Fig 2.4)
(max. power: EIRP
ERP≈ 4 dBm, allowed
only in case of spread spectrum
transmission)
9600 bps
127
9600 bps
502
57470 bps
67
57470 bps
317
9600 bps
246
9600 bps
980
57470 bps
131
57470 bps
618
9600 bps
632
9600 bps
2118
57470 bps
336
57470 bps
1589
9600 bps
1164
9600 bps
4835
57470 bps
618
57470 bps
2924
Table 1.1 Range [m] in the 915 MHz U.S. unlicensed band, in the presence of strong interference signals
assuming ideal free space propagation conditions (see Appendix D).
Note 1: In an interference-free environment, the estimated ranges are approximately two times higher. In the case of non-ideal
propagation, the ranges can dramatically decrease (see Appendix E for details).
Note 2: For further details on FCC part 15, see “Understanding the FCC Regulations for Low-Power, Non-Licensed Transmitters,” by
.fcc.go
the Federal Communications Commission, available through the FCC Web site, http://www
http://www.fcc.go
.fcc.govv.
3
U.S. REGULATIONS: 915 MHz, 434 MHz, 315 MHz BAND (CONTINUED)
RX tapped loop (see Fig. 2.8)
434 MHz U.S. band
TX loop (see Fig. 2.7)
(6dB reduced power state)
9600 bps
44
57470 bps
23
Table 1.2 Range [m] in the 434 MHz U.S. unlicensed band, in the presence of strong interference signals
assuming ideal free space propagation conditions (see Appendix D).
RX tapped loop (see Fig. 2.10)
315 MHz U.S. band
TX loop (see Fig. 2.9)
(6dB reduced power state)
9600 bps
56
57470 bps
33
Table 1.3 Range [m] in the 315 MHz U.S. unlicensed band, in the presence of strong interference signals
assuming ideal free space propagation conditions (see Appendix D).
Note 1: In an interference-free environment, the estimated ranges are approximately two times higher. In the case of non-ideal
propagation, the ranges can dramatically decrease (see Appendix E for details).
4
EUROPEAN REGULATIONS: 868 MHz BAND AND 434 MHz BAND
The typical ranges for the 868 MHz and 434 MHz European unlicensed bands are given in Tables 1.4 & 1.5, respectively.
The normal and the cross-tapped loop antenna for 868 MHz are identical to that of the 915 MHz band, as the automatic tuning
circuitry allows multi-band operation.
The allowed transmitter ERP is between 7-27dBm (corresponding to 9.14-29.14 dBm EIRP) at 868 MHz depending on the subchannel frequency. The allowed ERP is 10 dBm (corresponding to 12.14 dBm EIRP) at 434 MHz. (Note 2).
RX tapped loop (see Fig 2.13)
RX back IFA dev (see Fig 2.14)
868 MHz E.U. band
TX loop (see Fig 2.1)
(3dB reduced power state)
TX tapped loop (see Fig 2.2)
TX back IFA (see Fig 2.11)
TX back IFA dev (see Fig 2.12)
9600 bps
107
9600 bps
372
57470 bps
62
57470 bps
232
9600 bps
302
9600 bps
1049
57470 bps
174
57470 bps
645
9600 bps
893
9600 bps
3095
57470 bps
514
57470 bps
1932
9600 bps
1680
9600 bps
5830
57470 bps
970
57470 bps
3640
Table 1.4 Range [m] in the 868 MHz E.U. unlicensed band, in the presence of strong interference signals
assuming ideal free space propagation conditions (see Appendix D).
Note 1: In an interference-free environment the estimated ranges are approximately two times higher. In the case of non-ideal
propagation, the ranges can dramatically decrease (see Appendix E for details).
Note 2: For further details on ERC/REC devices, see “Relating to the Use of Short Range Devices,” available through the European
Radio Communications Office website, http://www.ero.dk
http://www.ero.dk.
5
EUROPEAN REGULATIONS: 868 MHz BAND AND 434 MHz BAND (CONTINUED)
RX tapped loop (see Fig. 2.8)
434 MHz E.U. band
TX loop (see Fig. 2.7)
(6dB reduced power state)
9600 bps
44
57470 bps
23
Table 1.5 Range [m] in the 434 MHz E.U. unlicensed band, in the
presence of strong interference signals assuming ideal free space
propagation conditions (see Appendix D).
Note 1: In an interference-free environment, the estimated ranges are approximately two times higher. In the case of non-ideal
propagation, the ranges can dramatically decrease (see Appendix E for details).
6
2. ANTENNA LAYOUTS
All antennas are connected to the Si4020 Transmitter IC output or Si4320 Receiver IC input pins through 0.25 mm wide and 1 mm
long leads. They are shown only in the sized figures of the normal loop TX antennas (see Fig. 2.1, Fig. 2.5 and Fig. 2.9) and in the
figure of the cross-tapped loop TX antenna (see Fig. 2.2). The large grounding metal plate comprises all the necessary circuitry that
would be used in production PCBs, and can be observed in the figures of the back IFA antennas. In case of loop and cross-tapped loop
antennas, the boundary of the large metal plate (PCB with components) should be situated at the end of the 0.5 mm wide connection
leads (the 0.25 mm-wide connection leads shown for example in Fig. 1 are situated inside the PCB).
All antenna layouts were created on an 0.5 mm-thick FR4 substrate with an epsilon of 4.7.
The impedance of the BIFA antenna is very sensitive to the electrical length of the arms, and therefore sensitive to the variation of
the dielectric constant. This effect can be compensated only slightly by the automatic tuning function of the TX. In the case of RX
antennas, the sensitivty is lower due to the low RX quality factor (Q). The physical edge of the PCB should be at least 2 mm away from
the cut arms of the BIFA antennas. If it is closer, the electrical length of the arms will change and cause significant detuning,
especially in the TX, and thus power or RX sensitivity degradation will occur (i.e. in the case of 0.5 mm distance, the power
degradation of the TX is approximately 2 dB).
In the case of loop antennas, the epsilon change has negligible effect, thus the PCB cutting close to the antenna metalization is
allowed. However, due to the printed capacitor, the tapped antennas have some epsilon dependency, which is usually significant only
in the TX due to the higher Q. The automatic tuning function can compensate for the detuning.
The open collector outputs of the transmitter require a DC path to the VCC. Thus, all TX antennas have a narrow bias line
situated at the symmetrical axis of the antenna at the bottom layer of the PCB and connected by a ‘via’ hole to the antenna
metallization.
The receiver inputs do not require DC path. However, in the case of the cross-tapped loop antennas (for the cross-tapping and for the
symmetrical printed capacitance at the main loop) the use of the bottom layer is necessary.
At the end of the arms of several BIFA antennas printed edge tuning capacitors are used to reduce the length of the arms.
7
915 MHz BAND
Fig 2.1 915-868 MHz dual band loop TX antenna (dimensions in mm)
Top and bottom view
Fig 2.2a 915-868 MHz dual band tapped loop TX antenna (dimensions in mm), this antenna is
designed to be a two-layer PCB design, as seen in fig 2.2b below
8
915 MHz BAND (CONTINUED)
Fig 2.2b
As shown in Fig 2.2a and Fig 2.2b above, the antenna makes use of the capacitance characteristics of the PCB. This capacitor is
generated by printing the PCB, as shown below in Fig 2.2c.
Top and bottom view
Fig 2.2c Printed capacitor of the 915MHz TX cross-tapped loop antenna
9
915 MHz BAND (CONTINUED)
Fig 2.3 915 MHz TX back IFA antenna (dimensions in mm)
Fig 2.4 915 MHz TX back IFA antenna 2 (dimensions in mm)
10
915 MHz BAND (CONTINUED)
Top and bottom view
Fig 2.5a 915 MHz RX cross-tapped loop antenna (dimensions in mm)
This antenna is designed to be a two layer PCB design, as such the overall look of the antenna can be seen in the diagram
below in Fig 2.5b
Fig 2.5b
11
915 MHz BAND (CONTINUED)
As shown in Fig 2.5a and Fig 2.5b above, the antenna makes use of the capacitance characteristics of the PCB. This
capacitor is generated by printing the PCB, as shown below in Fig 2.5c.
Top and bottom view
Fig 2.5c Printed capacitor of the 915MHz RX cross-tapped loop antenna
12
915 MHz BAND (CONTINUED)
Fig 2.6 915 MHz RX BIFA antenna (dimensions in mm)
13
434 MHz BAND
Fig 2.7 434 MHz TX loop antenna (dimensions in mm)
14
434 MHz BAND (CONTINUED)
Top and bottom view
Fig 2.8a 434 MHz RX cross-tapped loop antenna (dimensions in mm)
This antenna is designed to be a two layer PCB design. The overall look of the antenna can be seen in the diagram below,
Fig 2.8b
Fig 2.8b
15
434 MHz BAND (CONTINUED)
As shown in Fig 2.8a and Fig 2.8b, the antenna makes use of the capacitance characteristics of the PCB. This capacitor is
generated by printing the PCB, as shown below in Fig 2.8c.
Top and bottom view
Fig 2.8c Printed capacitor of the 434 MHz RX cross-tapped loop antenna
16
315 MHz BAND
Fig 2.9 315 MHz TX loop antenna (dimensions in mm)
17
315 MHz BAND (CONTINUED)
Top and bottom view
Fig 2.10a 315 MHz RX cross-tapped loop antenna (dimensions in mm)
This antenna is designed to be a two-layer PCB design. The overall look of the antenna can be seen in the diagram
below, Fig 2.10b.
Fig 2.10b
18
315 MHz BAND (CONTINUED)
As shown in Fig 2.10a and Fig 2.10b above, the antenna makes use of the capacitance characteristics of the PCB. This
capacitor is generated by printing the PCB, as shown below in Fig 2.10c.
Top and bottom view
Fig 2.10c Printed capacitor of the 315 MHz RX cross-tapped loop antenna
19
868 MHz BAND
The 915-868 MHz multiband normal loop and cross-tapped loop antenna is given above by Fig. 2.1 and Fig 2.2, respectively.
Fig 2.11 868 MHz TX back IFA antenna (dimensions in mm)
Fig 2.12 868 MHz TX back IFA antenna (dimensions in mm)
20
868 MHz BAND (CONTINUED)
Top and bottom view
Fig 2.13a 868 MHz RX cross-tapped loop antenna (dimensions in mm)
This antenna is designed to be a two layer PCB design, as such the overall look of the antenna can be seen in the diagram
below, Fig 2.13b
Fig 2.13b
21
868 MHz BAND (CONTINUED)
As shown in Fig 2.13a and Fig 2.13b above, the antenna makes use of the capacitance characteristics of the PCB. This
capacitor is generated by printing the PCB, as shown below in Fig 2.13c.
Top and bottom view
Fig 2.13c Printed capacitor of the 868 MHz RX cross-tapped loop antenna
22
868 MHz BAND (CONTINUED)
Fig 2.14
868 MHz RX back IFA antenna (dimensions in mm)
23
APPENDIX
APPENDIX A
The radiated power can be described either by ERP (Equivalent Radiated Power) or EIRP (Equivalent Isotropic Radiated Power).
The EIRP is the power that would be radiated by a hypothetical isotropic antenna with a radiation intensity equal to the maximum
radiation intensity of the characterized antenna-TX configuration.
In the case of ERP, instead of the isotropic antenna, a perfectly matched half wavelength dipol is used as a reference. The gain
(relative to isotropic antenna) of the dipol is 2.14 dB, i.e. ERP[dBm]=EIRP[dBm]-2.14
Antenna type
ERP
EIRPmax [dBm]
Loop
Tapped
loop
Back IFA
Back IFA dev.
board
915 MHz
-15.3
(-3dB state)
-9,5
-1.2
4.4
868 MHz
-20
(-3dB state)
-11
-1.6
3.9
434 MHz
-18.3
(-6dB state)
N/A
N/A
N/A
315 MHz
-20.4
(-6dB state)
N/A
N/A
N/A
Table A.1 Maximum EIRP in dBm of the Si4020 transmitter IC with the previously given
TX antennas
For high impedance loop antennas, the current must be reduced due to the allowed maximum voltage swing on the outputs. At low bands
(315, 434MHz) a 6 dB reduction is necessary, whereas at high bands, a 3 dB reduction is enough due to the lower Q of the TX chip.
Antenna type
Emin r.m.s. [mV/m]
Tapped loop
Back IFA dev.
9600 bit/s
57470 bit/s
9600 bit/s
57470 bit/s
915 MHz
0.24
0.44
0.06
0.09
868 MHz
0.16
0.28
0.05
0.07
434 MHz
0.48
0.9
N/A
N/A
315 MHz
0.29
0.5
N/A
N/A
Table A.2 Required r.m.s. electric field strength at the antenna of the Si4320
receiver IC in mV/m to achieve a BER of 10-2 in case of strong interference
In an interference-free environment, half of the values are enough (6 dB better sensitivity). The applied two sided deviation is 120 kHz
and 180 kHz at 9600 and 57470 bps rates, respectively.
24
APPENDIX
APPENDIX B
Range calculations:
From the EIRP (in watts) the power density (denoted by S) at a given d distance (in meters) can be calculated by Equation 1:
S
ERP ª W º
EIRP
4Sd 2 «¬ m 2 »¼
The r.m.s. electric field strength can be calculated from the S by Equation 2:
E
120SS
ªV º
«¬ m »¼
From Equation 1 and Equation 2, the range can be derived if the EIRP of the TX and the required r.m.s. electric field strength at the
RX antenna is known:
d
30 ERP
EIRP
>[email protected]
E
25
APPENDIX
APPENDIX C
TX EIRP Measurement Setup
The TX measurement setup is shown in Fig C.1. The wideband dipole is used as a reference RX antenna and is connected to a
spectrum analyzer. The transmitting testboard is controlled by the Silicon Labs Wireless Development Software (WDS). The distance
and the height of the testboard is slightly changed to find the maximum and minimum of the received power. Knowing the gain of the
reference antenna, the power spectral density and thus the EIRP or the field strength at 3m can be calculated from the product mean
(average in dB) of the received maximum and minimum power.
d=2m
Si4020
Testboard
IA4220
TXTX
Testboard
with
with
loadboard
loadboard
Wideband dipole as
reference RX antenna
dh for
max. and
min search
HP4202B
SPA
WDS on PC
h=1.7m
dd for
max. and
min search
Fig C.1. TX EIRP measurement setup
26
APPENDIX
APPENDIX C (CONTINUED)
RX Sensitivity Measurement Setup
The RX measurement setup is shown in Fig C.2. The wideband dipole is used as a reference TX antenna and is connected to a signal
generator. The receiving testboard is controlled by the Silicon Labs Wireless Development Software (WDS). The HP 4432B signal
generator generates the FSK modulated signal. The clock and data recovered by the Si4320 RX chip is fed back to the generator for
the BER measurements and monitored by an oscilloscope. The distance and the height of the testboard is slightly changed to find the
maximum and minimum of the measured BER. Knowing the gain of the reference TX antenna, the power spectral density and thus
the required field strength by the measured RX antenna to achieve the desired BER value can be calculated. As a final value the
product mean (average in dB) of the reqired maximum and minimum electric field is given.
d=2m
Wideband dipole
as reference TX antenna
RX Testboard with
loadboard
dh for
max. and
min search
E4432B
signal gen
54645
D
scope
WDS on PC
h=1.7m
dd for
max. and
min search
Recovered data and clock for BER measurement
Fig C.2. RX sensitivity measurement setup
27
APPENDIX
APPENDIX D
Jamming Signal Levels During Receiver Sensitivity Measurements
The receiver sensitivity is measured in the presence of strong interferences.
Fig. D. shows the measured spectra when applying a wideband dipole (G=-1.5..-0.5 dB with cable) as a receiver antenna at
the place of the sensitivity measurement.
18:04:57 Jan 19, 2004
Ref -20 dBm
#Peak
1
Log
10
dB/
Mkr3 903 MH
-41.39 dBm
Atten 5 dB
3
2
Marker
903.000000 MHz
-41.39 dBm
Start 50 MHz
#Res BW 300 kHz
Marker
1
2
3
Trace
(1)
(1)
(1)
VBW 300 kHz
Type
Freq
Freq
Freq
X Axis
108 MHz
935 MHz
903 MHz
Stop 1 GHz
Sweep 13.6 ms (401 pts
Amplitude
-40.02 dBm
-40.79 dBm
-41.39 dBm
Fig. D.
As one can observe, the highest jamming signals are around 900 MHz and 100 MHz. At 900 MHz, the level of the jamming signals are
between -40 and -50 dBm (which corresponds to approximately 70 to 20 [mV/m] r.m.s. electric field strength). It is approximately 3050 dB higher than the useful signal’s electric field during the sensitivity measurements. The interference is also very high, around 100
MHz (radio) and 1.8 GHz (GSM, not shown). The signal level of the other jamming signals is approximately –60 to -70 dBm, corresponding
to several mV/m electric field strength.
28
APPENDIX
APPENDIX E
Range Calculations in the Case of Non-Ideal Propagation Conditions
In the case of real propagation conditions (urban area, indoors, etc.), the practical range can be significantly different from the
calculated free space range. According to the literature (Note 1), the average path loss for an arbitrary d TX-RX distance can be described
by power function either for an indoor or an outdoor environment:
d 
a d    
 d0 
n
e.1.a
or
d 
a dB  d   a dB  d 0   10n log  
 d0 
e.1.b
where d0 is a reference close in distance (but already in the far field of the TX antenna ) with a known path loss, and n is the
propagation exponent that strongly depends on the channel properties. Table 2.1 gives the n for several typical environments. Typical
value of d0 is 2-3 m. A calculation example is given below.
It should be emphasized that the actual path loss can strongly deviate from the given average above due to variation of the clutter and
obstacle positions in the environment. Hence, the path loss behaves as a random statistical variable with log-normal (normal in dB)
distribution around the mean value given above (for more details please refer to Note 1).
Environment
Free space
Urban area cellular radio
In building line-of-sight (in case of duct effect)
Obstructed in building
Obstructed in factories
Propagation Exponent, n
2
2.7 .. 3.5
1.6 .. 1.8
4 .. 6
2 .. 3
Note 1: T.S. Rappaport, Wireless Communications, Principles and Practice.
29
APPENDIX
APPENDIX E (CONTINUED)
Method of Range Calculation Using the Free Space Range Data as a Starting Point
Taking the applied 2 m distance of the measurements as a reference distance (d0), the power margin at d0 can be calculated by
Equation e.2. from the previously given free space range data.
m
dB
 did 

 d0 
 d id   20 log 
e.2
During the increase of the distance this margin is used in the link, i.e.:
d
n10 log  nreal
 d0

 did 
  20 log 


 d0 
e.3
By rearranging equation e.3 the resulting distance (dnreal) in nonideal environment can be derived:
n2
d nreal  n did2 d 0
e.4
Calculation Example
Using the BIFA antennas of Fig 2.12 and 2.14 as a TX and RX antenna in a 868 MHz communication link at 9600 bps bit rate,
the resulted ideal free space range in the presence of strong interferences is around 5830 m (Table 1.4). In an obstructed indoor
environment (n=4), the resulted average range is:
d nreal  4 58302 22  108m
e.5
30
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