ABSTRACT In order to increase the higher competition in lowpower wireless network communication market, a highperformance and low-cost product is necessary to distinguish the difference with others. Through integrating the system performance with suitable L-shape impedance-match circuit assisting with some network analyzer, this target with a 2.4 GHz radio-frequency (RF) product in long-distance data transportation seems to be promisingly implemented. In short-distance data transportation, the ideal outputlink transportation rate (~ max. 54 Mb/sec) is slightly influenced by impedance mismatch between power amplifier (PA) and antenna port. However, it is tremendously reduced at long-distance condition and the transportation rate is decreased to ~ 24 Mb/sec. Using the attenuator to attenuate the real input signal to -70dB to simulate the real signal transportation, the packet error rate (PER) is less than 10% at a physical sublayer service data unit (PSDU) length of 1000 bytes under the communication 802.11g spec. as the real transmission rate is 20 Mb/sec. If the impedance of the transmission line is shifted, the long-distance transportation rate will be reduced to, almost, 20 x 24 / 54 = 8.8 Mb/sec. The transportation performance is greatly deducted. With the delicate design and the feasible component arrangement, the impedance mismatch influencing the long-distance (~ 100 m) data transportation is overcome and reduced to the acceptable range. In this investigation using 3.3 V power supply, we observe that the selection of electronic components with miniaturization is also an art to reduce the radiation side-effect

Chia-Hao Tu

Chung-Hsiao E Rd

Mu-Chun Wang

R O C Taiwan

Shu-Han Chao

Shuang-Yuan Chen

Taipei

Zhen-Ying Hsieh

CiteSeerX

  
 
1 Co
Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 06/30/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-us(b       
 
 
 
 
 
 
Correcting Front-end RF Impedance Mismatch for 2.4GHz Wireless Long-
Distance Data Transmission 
 
 
Mu-Chun Wang1,*, Zhen-Ying Hsieh1, Shu-Han Chao*, Chia-Hao Tu1, Shuang-Yuan Chen1
 
 
1Graduate Institute of Mechatronic Engineering, National Taipei University of Technology 
No. 1, Sec. 3, Chung-Hsiao E. Rd., Taipei, Taiwan, R.O.C. 
*Dept. of Electronic Engineering, Ming Hsin University of Science & Technology 
No. 1 Hsin-Hsing Road, Hsin-Fong, Hsin-Chu, Taiwan, R.O.C. 
*Tel.: 886-3-559-3142, FAX: 886-3-559-1402, E-mail: mucwang@must.edu.tw  
Proceedings of MicroNano08 
MicroNano2008 
June 3-5, 2008, Clear Water Bay, Kowloon, Hong Kong 
MicroNano2008-70092ABSTRACT 
In order to increase the higher competition in low-
power wireless network communication market, a high-
performance and low-cost product is necessary to 
distinguish the difference with others. Through 
integrating the system performance with suitable L-shape 
impedance-match circuit assisting with some network 
analyzer, this target with a 2.4 GHz radio-frequency (RF) 
product in long-distance data transportation seems to be 
promisingly implemented. 
In short-distance data transportation, the ideal output-
link transportation rate (~ max. 54 Mb/sec) is slightly 
influenced by impedance mismatch between power 
amplifier (PA) and antenna port. However, it is 
tremendously reduced at long-distance condition and the 
transportation rate is decreased to ~ 24 Mb/sec. 
Using the attenuator to attenuate the real input signal 
to -70dB to simulate the real signal transportation, the 
packet error rate (PER) is less than 10% at a physical 
sublayer service data unit (PSDU) length of 1000 bytes 
under the communication 802.11g spec. as the real 
transmission rate is 20 Mb/sec. If the impedance of the 
transmission line is shifted, the long-distance 
transportation rate will be reduced to, almost, 20 x 24 / 
54 = 8.8 Mb/sec. The transportation performance is 
greatly deducted.  
With the delicate design and the feasible component 
arrangement, the impedance mismatch influencing the 
long-distance (~ 100 m) data transportation is overcome 
and reduced to the acceptable range. In this investigation 
using 3.3 V power supply, we observe that the selection 
of electronic components with miniaturization is also an 
art to reduce the radiation side-effect.  
1. Introduction 
An excellent design in RF route can attribute the 
success to the good performance of transmission line and 
matching impedance circuit [1], as shown in Fig. 1. Most 
of them are composed by resistor (R), capacitor (C) and 
inductor (L). In route circuit, if the component length is 
close to one-tenth of radiated wavelength, then the 
radiation phenomenon is usually generated. To avoid this 
radiation effect, the RF components on printed circuit 
board (PCB) are miniaturized.  
 
 
 
Fig. 1 Schematic of (a) match impedance circuit, and (b) 
mrcrostrip transmission line. 
 
In the meantime, the matching impedance of 
transmission line must be, as possible as, approximated 
to the loaded transmission line 50 ohm (ZL = 50 Ω) for 
applying to the 2.4 GHz ~ 2.5 GHz transmitted band [2]. 
Therefore, the transmission-line path and the line pitch 
also must be precisely calculated. In addition, there are 
some reserved component pads, closed to the transmitted 
Matching 
Impedance 
Circuit
Zin
ZL
Zin
ZL
hεr 
(a)
)
pyright © 2008 by ASME
e
         
Downloaded Fend on PCB, to adjust the shift of final matching 
impedance during the PCB fabrication. In this study, the 
matching impedance circuit can be generally designed by 
L-type matching network [3-5], collocating with the 
short and open circuit transmission line and quarter-wave 
transmission line. The L-type matching network with the 
passive discrete components of resistor, inductor and 
capacitor is typically employed. One of advantages of L-
type matching network is that the L-type passive 
components are discrete such as the pair of R and L, or L 
and C, etc.  
 
2. L-type Matching Network 
The useful microstrip technology allows the 
transmission line to fabricate directly on the printed 
circuit board. On PCB layout technology with computer, 
the impedance value is obtained, in advance, by 
simulation, as shown in Fig. 2. The widths, W1 and W2, 
the interval D1, the trace (tiny electrical line) thickness 
T1 and the layer stack of transmission line were 
presented in this Fig. 2.  
 
 
 
Fig. 2 Schematic cross-section profile of transmission 
line in simulation. 
 
Additionally, before shipping RF products, the 
impedance of transmission line must be tested to make 
sure the fabrication error from PCB fabrication less than 
5% under the requirement of 50 ohm impedance. In order 
to compensate the small uncertainty impedance shift, the 
L-type matching network on layout reserves some pin 
pads to insert some passive components, as shown in Fig. 
3. 
 
 
 
(a) 2
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(b) 
 
 
 
                                            (c) 
 
Fig. 3 L-type component location on PCB (a) top-view 
layout (b) top view of real fabrication pattern (c) 
magnification of part (b). 
 
Next, the L-type matching network is presented in Fig. 
4, where E1 and E2 are referred as resistor (R), capacitor 
(C) or inductor (L). However, the series or parallel 
connection is not fixed, but determined by the Smith 
chart [1]. In the combined ZY-Smith chart [6], as 
presented in Fig. 5, a dotted line and real line belongs to 
impedance-plane (Z-plane) and admittance (Y-plane), 
respectively. The Z-plane referring to a component is 
parallel with ZS, and the Y-plane is series with ZS.  
 
 
 
Fig. 4 Schematic circuit of L-type matching network. 
 
When the measured impedance locates at point A and, 
the impedance change with a matching network, 
reaching the impedance at point B, can be observed and 
calculated. Of course, there are several analysis paths 
from point A to point B such as A-C-B, A-H-B, A-E-H-B 
or A-D-F-B, etc., which are suitable to the matching 
network. For instance, the impedance change is the A-C-
G path. Considering a suitable capacitor or an adequate 
inductor, a typical way is to justify the trend of fall 
curves or rise curves when the specified point moves 
along on real line and dotted line. For a fall curve, it 
always adds a capacitor. 
E1ZS 
E2 
ZL Copyright © 2008 by ASME
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Downloaded F 
 
Fig. 5 Usage of the Smith chart to determine the pas ve 
components of L-type match  network. 
rts an inductor. 
ence, the A-C path is along with dotted line and, 
th
imental Measurement 
hm as a short-
 RF trace with 
A
si
ing
 
Again, for a rise curve, it frequently inse
H
erefore, Zs is parallel with an inductor. Then, the C-B 
path moves along on real line on complex-variable 
coordinate, and E1 is parallel with an inductor. Similarly, 
the C-B path moves along on dotted line, and E2 is series 
with an inductor, too. In this case, the bandwidth (BW) 
of this L-matching network is 2.4GHz if the loaded 
quality factor QLD (= central carrier frequency / BW3dB) 
is set at 1. 
 
3. Exper
First, the load impedance is set at 0 o
type transmission line to test the designed
gilent 8719ES network analyzer. Regardless of 
anticipated target, the whole RF circuit response or 
characteristic trace was depicted in Fig. 6. Because the 
input reflection coefficient S11 shows the negative phase, 
this measurement mode with zero ohm at 2.4 ~ 2.5 GHz 
operation is a capacitor-like behavior. Furthermore, the 
signal magnitude with LOG format is presented in Fig. 7. 
 
 
 
Fig. 6 S11 distribution of short-type transmission l e 
un er 2.4-2.5GHz operation on Smith chart.  
in
d
 
A 
B 
C G 
D 
F 
E
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Fig. 7 LOG MAG value is about -10dB with short-type 
transmission line under 2.4~2.5GHz operation. 
 
Then, using a vector signal analyzer, the error vector 
magnitudes (EVM) of the transmitted signals [7-8] on 
orthogonal frequency-division multiplexing (OFDM) 
constellation [9], as shown in Fig. 8, can be sensed. The 
energy in each read point, circled by a yellow-dash circle, 
is dispersed. This means that the transmitted signal 
overlap is serious. It is a warning sign. 
Again, using an attenuator to simulate the attenuated 
degree of short-type transmitted signal under a certain 
distance is a feasible method. In this case, a 70dBm 
attenuation is treated as a long-distance transmission. 
The transmitting data rate through some elapsed time 
was decreased, as depicted in Fig. 9.  The red-dash 
rectangle in Fig. 9 indicated the throughput of 
transmitted data after 20 sec was gradually degraded. 
Using the same method, ten-time repeatable 
measurement was shown at the Table I. The test results 
show a large standard deviation about 3.680 Mbps. 
 
 
 
Fig. 8 Relative constellation error on OFDM 
constellation is about -24.4 dB. 
 
Based on these test results, the degradation or the 
vibration of data throughput seriously affected the 
transmitted quality of transmitting data and caused some 
data loss. In ideal calculation, the transmission speed 
should be greater than 20 Mbps. After analyzing these 
results at Table I, this weak performance was chiefly 
generated by impedance mismatch. Using the Smith 3 Copyright © 2008 by ASME
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Downloaded Fchart as an auxiliary, the capacitor-like phenomenon in 
this short-type transmission line can be adjusted as an 
inductor-like behavior, as shown in Fig. 10 and Fig. 11. 
 
 
Fig. 9 Degradation of transmitted signal throughput 
(Mbps) after some elapsed time (h:mm:ss). [10]. 
 
Table I. The short-type transmitted throughput for 10-
time repeated test before adjusting. 
 
Times Throughput  (Mbps) 
1 16.51 
2 7.22 
3 7.58 
4 12.85 
5 8.11 
6 13.43 
7 15.9 
8 15.24 
9 14.9 
10 15.52 
AVG. 12.726 
Std. Dev. 3.680 
 
To compensate the front-end RF impedance mismatch, 
a pico-level capacitance was shunted in this circuit to 
correct the amplitude in Fig. 10. The shifting path from 
point A to point B is path 1. To obtain a predicted 
shifting path from point B to point C, called path 2, a 
nano-level inductance was series to adjust the phase of 
transmission line toward the inductor-like behaviour on 
Smith chart, as shown in Fig. 11. The LOG MAG after 
adjustment also demonstrates in Fig. 12. The S11 value 
indeed reduced to -11~ -20 dB. The acceptable spec. for 
S11 in front-end of wireless transmission products is 
basically less than -10 dB. 
While the same test method, comparing it with the 
previous case, with a vector signal analyzer to measure 
the error vector magnitude on OFDM constellation, the 
relative constellation error also decrease to -26.0 dB, as 
shown in Fig. 13. Each data in Fig. 13 depicts the 
inseparable shape. It indicates the transmitted signal 
energy is not dispersed. The signal overlap issue is not 
critical. 4
rom: https://proceedings.asmedigitalcollection.asme.org on 06/30/2019 Terms of  
C
A 
B 
 
Fig. 10 Predicting impedance change from capacitor-like 
behaviour to inductor-like characteristic with S11. 
 
 
 
Fig. 11 S11 parameter of short-type transmission line at 
2.4 ~ 2.5 GHz operation after adjusting. 
 
 
Fig. 12 S11 value showing with LOG MAG format at 2.4 
~ 2.5 GHz operation.  
 
Copyright © 2008 by ASME
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Downloaded Fro 
 
Fig. 13 Relative constellation error on OFDM 
constellation is about -26.0 dB. 
 
Continuously, repeating the transmission test for short-
type transmission line at 70 dBm attenuation, the low 
transmission-speed phenomenon after 3 minutes was not 
observed, as shown in Fig. 14. The transmission 
throughput was generally maintained over 20 Mbps. 
 
 
 
Fig. 14 Data transmission test under 70 dBm attenuation 
for transmission line after L-value and C-value 
compensation. 
 
Table II. The result of transmitted speed for 10-time 
repeated test after adjusting.  
 
Times Throughput  (Mbps) 
1 21.12 
2 22.01 
3 21.86 
4 21.55 
5 21.38 
6 22.1 
7 21.67 
8 20.92 
9 22.07 
10 22.08 
AVG. 21.676 
Std. Dev. 0.425 
 
5
m: https://proceedings.asmedigitalcollection.asme.org on 06/30/2019 Terms of UAfter impedance correction, repeating ten-time 
throughput test was also executed. The test results were 
demonstrated at Table II.  
 
4. Test results and discussion 
According to these two tests, the wireless transmission 
rate after component adjustment is really better than that 
without adjustment in short-type transmission line, as 
shown in Fig. 15. All of test values after adjustment are 
over 20 Mbps. The average value is about 21.68 Mbps at 
Table II. Again, the measurement variation after 
adjustment is also smaller. The standard deviation is only 
0.425 Mbps. It means that this passive component 
compensation in impedance match is valuable and 
stabilizes the output transmission rate. In the meantime, 
the improvement of transmission rate is about 8.95 Mbps. 
In general, the S21 value (forward voltage gain) and the 
S12 value (reverse voltage gain) are also important in 
impedance design. However, the impedance mismatch is 
concerned in this study. The S11 value is sufficient to 
describe the matching quality. 
 
 
 
Fig. 15 Comparison of transmission rate before and after 
passive component adjustment during 10-time test. 
 
5. Conclusion 
Through this experiment, good impedance match 
between power amplifier and antenna port in front-end 
RF part indeed and strongly influences the data 
transmission quality. Not only the transmission quality, 
but also the transmission distance at the same RF output 
power. The adequate simulation and calculation adding 
some passive components improve the data transmission 
rate, increasing about 8.95 Mbps. Again, the 
transmission characteristic is more stable, too.  
Deliberately designing high-efficiency wireless 
communication products is very important in marketing 
competition. This design is suitable to this requirement 
increasing the 2.4GHz long-distance data transmission 
under a low-power 3.3V operation. It also satisfies IEEE 
802.11b/g spec. and can be applied to network cards with 
2.4GHz central carrier frequency. 
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10
Test number 
Mbps
After 
Before Copyright © 2008 by ASME
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Downloaded FroAcknowledgments  
The authors sincerely thank AboCom Systems, Inc. in 
Taiwan for offering the PCB fabrication and the 
instrument resources.  
 
References 
[1] Reinhold Ludwing and Pavel Bretchko, 2000 : RF 
Circuit Design--- Theory and Applications, Prentice-
Hall, New Jersey,  pp. 42-132. 
[2] P. Sjoblom, et al, 2005, “An adaptive impedance 
tuning CMOS circuit for ISM 2.4-GHz band,” IEEE 
Transactions on Circuits and Systems I, vol. 52, issue 
6, pp. 1115 – 1124. 
[3] Y. S. Lin, et al, 2004, “Lumped-element impedance-
transforming uniplanar transitions and their antenna 
applications,” IEEE Transactions on Microwave 
Theory and Techniques, vol. 52, iss. 4, pp. 1157-
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[4] G. Smith, 1977, “Efficiency of electrically small 
antennas combined with matching networks,” IEEE 
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[5] H.M. Elkamchouchi, et al, 1996, “Smith chart based 
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[6] C. Thimaporn, et al, 2004, “Applications of the ZY 
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[7] Pak Kay Tang, et al, 2004, “PER and EVM 
measurements of a radio-over-fiber network for 
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[8] J. K. Stuart, et al, 2003.06, “Wireless LAN Medium 
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[9] Behzad Razavi, 1998 : RF Microelectronics, 
Prentice-Hall, New Jersey,  pp. 313-314. 
[10]AboCom Systems, Inc., the web-site     
“http://www.abocom.com.tw” 6 Copyright © 2008 by ASME
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Correcting Front-end RF Impedance Mismatch for 2.4GHz Wireless Long- Distance Data Transmission

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Correcting Front-end RF Impedance Mismatch for 2.4GHz Wireless Long- Distance Data Transmission

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