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Microcoaxial Cable Assembly for High-speed
Transmission in Cellular Phones
Takashi Matsukawa1, Masako Ito1, Korenari Higashi1,
Tomoyuki Shinohara1 and Yasushi Nakagawa2
In recent years, cellular phones and other mobile electronic devices have been miniaturized
rapidly.
In line with the technical tendency, internal wiring material must also be miniaturized and
should have a high-speed transmission characteristics. We have developed and produced a
microcoaxial cable with the high-speed transmission characteristics. In this report, the highspeed transmission characteristics of the microcoaxial cable for cellular phones and, further,
the wiring material for the high-speed transmission as a step toward the next generation are
discussed.
1. Introduction
In recent years, microcoaxial cable assembly is
attracting worldwide attention as the internal wiring
material of cellular phones. It has a stable transmission characteristic with a small diameter and flexible
structure. The microcoaxial assembly meets the
design and functional needs of cellular phones. With
the high resolution of Liquid Crystal Display (LCD),
the transmission speed becomes high. But it is
expected to increase the noise. The microcoaxial
cable is advantageous compared with other materials.
Fujikura developed the assembly as a mass production product. We report the microcoaxial cable characteristics and the high-speed cable.
2. The technical trend of cellular-phone wiring
material
olution LCDs. Because in order to use a high resolution LCD, the transmission speed reaches hundreds
of megabytes per second, it is necessary to prevent
noise.
As it is difficult to apply complete shielding at the
hinge area, shielding of FPC is adopted. (Fig.2)
The microcoaxial cable structure has a complete
Data rate
QVGA (Quarter VGA)
240 × 320 dot
200 Mbps
WQVGA (Wide Quarter VGA)
400 x 240 dot
400 Mbps
HVGA (Half VGA)
320 x 480 dot
2.1. Wiring material for cellular phones
In Japan, the clamshell-type cellular phone is commonly used and its popularity is spreading quickly
further in overseas markets. Flexible Printed
Circuit(FPC) is widely adopted as the wiring material
of the clamshell-type cellular phones, in order to connect the display part with the main part by a hinge.
We also supply FPC to the cellular phone market.
VGA (Video Graphics Array)
640 x 480 dot
SVGA (Supper VGA)
800 x 600 dot
900 Mbps
Fig.1. The tendency of LCD panel for cellular phones.
2.2. Demand of high-speed transmission
A demand for a high-resolution and multicolor display is spreading quickly in the cellular phone market
(Fig.1). Currently, Quarter Video Graphic
Array(QVGA) LCD is in use. However, the development of HVGA and SVGA is progressing as high-res1 Electronic Component R&D Department, Electronic Components R&D Center
2 Fujikura Thailand Ltd.
Fujikura Technical Review, 2007
Fig.2. FPC for cellular phones.
47
Silver-coated
copper alloy
wires
PFA
Hair
φ 0.29 mm
Tin-coated
copper alloy
wires
Conductor
Fig.4. The structure of microcoaxial cable.
Table 1. The structure of microcoaxial cable (AWG42).
Item
Fig.3. Microcoaxial cable assembly for cellular phones.
shielding structure. It has shield characteristics of
more than that of the shielding of FPC.
Silver-coated copper alloy
Construction
7 / 0.025 mm
Diameter
0.075 mm
Material
PFA
Dielectric constant
2.15
Conductor
Insulation
Diameter
0.165 mm
Material
Tin-coated copper alloy
Construction
φ0.03 mm spiral
Diameter
0.225 mm
Material
PFA
Dielectric constant
2.15
Diameter
0.29 mm
2.3. Adaptability to various cellular-phone designs
The design of a cellular phone is an important factor of individuality. To satisfy customer needs , various types of hinges are adopted by the designs such
as clamshell-type and rotation-type. And the hinge
parts are miniaturized in line with the tendency
toward having miniature and light weight cellular
phones. It is hard to miniaturize the shield of FPC. In
order to secure the bending characteristic, it is necessary to provide a safe space. Moreover, FPC does not
have flexibility in the twist direction. It is hard to
adopt a compound mechanism such as folding and
rotation.
The microcoaxial cable has the advantage of bending and twisting characteristics. It is adopted for cellular phones of complicated designs. (Fig.3)
Table 2. Electrical property of microcoaxial cable (AWG42).
Item
Method
Specification
Conductor resistance
JIS C3102
Max. 7.5 Ω/m
Insulation resistance D.C 250 V 1 min.
Min. 100M Ω/km
Dielectric strength A.C 250 V 1 min.
No break down
Characteristic
impedance
TDR
Nom. 45 Ω
Attenuation
-
Nom. 8.0 dB/m (at 1 GHz)
The amount of transmission data and the number
of internal wiring are on the increase. Hence, the system produces a noisy environment. To alleviate the
noise, the case design is restricted. The reliability of
the system is affected by the noise.
The microcoaxial cable is adopted for a high-speed
transmission with high flexibility. Currently, it is used
as a wiring harness with around 40 ~ 50 wires.
However, the demand for cable miniaturization and
reduction of the number of cables by the serial transmission will increase. In order to realize further highspeed transmission by the serial transmission, the differential transmission system will be adopted to
improve the Signal-to-Noise (SN) ratio.
center conductor (diameter 25µm × 7 wires) and a
spiral shield structure. (Fig.4) The outer diameter of
the cable is 0.29mm. To produce the cable, it is necessary to control the wire tension with high precision. It
uses a high technology such as the Fujikura optical
fiber.
Because a fluorine compound has low dielectric
constant (Table 1), it is adopted for insulation and
jacket. It is suitable for transmission at a high frequency. Although the microcoaxial cable for the notebook PC market has a thin PET tape as the jacket, we
adopted a fluorine compound for the jacket to
improve the flexibility. To make a thin insulation
layer, a special extruding machine has to be used. We
maintain a secure control of the high quality of each
process such as the insulation, shield, and jacket.
3. Microcoaxial cable
3.2. The electrical property of a microcoaxial cable
3.1. Structure of a microcoaxial cable
The general electrical property of a microcoaxial
cable is shown in Table 2.
2.4. Demand for future transmission
We developed AWG 42 microcoaxial cable with a
48
Outer conductor
Jacket
Structure , Material
Material
Table 3. Available cable alignment pitch and cable diameter.
bending performance: attenuation
0.00
−2.00
0.0
0.5
1.0
1.5
2.0
2.5
3.0
GHz
3.5
Alignment pitch
AWG size
Cable diameter
0.25 mm
AWG # 46
0.24 mm
before test
−4.00
after bending (200K times)
−6.00
0.3 mm
−8.00
0.4 mm
−10.00
−12.00
−14.00
dB
0.5 mm
Microcoaxial cable
Diameter of a mandrel: 4 mm
Fig.5. Bending performance of microcoaxial cable (AWG42).
40 wires
AWG # 46
0.24 mm
AWG # 42
0.29 mm
AWG # 46
0.24 mm
AWG # 42
0.29 mm
AWG # 42
0.325 mm
AWG # 46
0.24 mm
AWG # 42
0.29 mm
AWG # 42
0.325 mm
assembly:
(1) Technology of cable arrangement by the same
pitch (Table 3), for example, 0.3 mm and 0.4 mm
pitch.
(2) Insulation and jacket strip technology
(3) Cable soldering technology to connect with connector or PCB
(4) Cable bundling technology to make suitable
shape
In addition, we realized the capability by making
not only the cable but also the assembly in an overseas factory.
4. Transmission characteristic
2.5 mm
4.1. High-speed transmission characteristic of a
microcoaxial cable
Coaxial cable 3C-2V
Fig.6. The size of microcoaxial cable assembly
(AWG42 40 wires).
3.3. Bending property
The general demand for a cellular phone model in
its bending characteristics is around 100,000 times to
200,000 times. When opening and closing of a cellular
phone are repeated 50 times every day, it is used for
approximately 200,000 times in 10 years. Examination
after 200,000 times shows an attenuation characteristic. (Fig.5) It turns out that there is no change before
and after the examination. It is thought that there is
sufficient durability.
3.4. Assembly of a microcoaxial cable
We supply a microcoaxial cable assembly as wiring
material for cellular phones. The bundle diameter is
around 2.5 mm by using AWG42 × 40 wires. It is suitable for space-saving wiring. (Fig.6)
We developed the following technologies for this
Fujikura Technical Review, 2007
The transmission quality is influenced by speed
and distance. The high-speed transmission characteristic by changing the cable length is shown in Fig.7.
1. In a measurement of AWG42 cable with a length of
0.5m, the transmission characteristic is good at
3Gbps because the jitter is around 0.1UI.
2. In a measurement of AWG42 cable with a length of
1.0m, the transmission characteristic is not good at
3Gbps because the jitter reaches 0.25UI and the
eye pattern also collapses.
As a result, the AWG42 cable with 0.5m can support 3Gbps transmission speed.
4.2. The high-speed transmission characteristic of a
microcoaxial cable
Figure 8 shows high-speed transmission characteristic of a microcoaxial cable assembly. The measurement shows the following results:
1. The transmission characteristic of the assembly is
less than that of the cable.
2. With the connector, there is some difference in the
transmission characteristic. To perform a highspeed transmission, the connector design is an
important parameter. Figure 9 shows the connection impedance of the connector.
3. Although transmission characteristic is good at 3
Gbps with a cable length of 300 mm, the jitter
49
UI
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Jitter
AWG42 L=0.5m and L=1m
AWG42
L=1m
AWG42
L= 0.5m
0 500 1,000 1,500 2,000 2,500 3,000 3,500
Mbps
EYE Height
AWG42 L=0.5m and L=1m
AWG42
L= 0.5m
AWG42
L=1m
mV
100
80
60
AWG42
L=500 mm
L=1,000 mm
500 Mbps
2.5 Gbps
3.125 Gbps
40
20
0
0 500 1,000 1,500 2,000 2,500 3,000 3,500
Mbps
Fig.7. Eye pattern for microcoaxial cable (AWG42).
Microcoaxial connector
Maker A
L=300 mm
Microcoaxial connector
Maker B
L=500 mm
L=300 mm
L=500 mm
500 Mbps
2.5 Gbps
3.125 Gbps
condition: Input 100 mV, PRBS: 223–1
UI
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Jitter: The comparison of
assembled product
Connector A
AWG42
L=500 mm
L=300 mm
Connector B
L=500 mm
L=300 mm
0 500 1,000 1,500 2,000 2,500 3,000 3,500 Mbps
Eye height: The comparison of
assembled product
mV
Connector A
AWG42
100
L=500 mm
90
L=300 mm
80
Connector B
70
L=500 mm
L=300 mm
60
50
40
30
20
10
0
0
500 1,000 1,500 2,000 2,500 3,000 3,500 Mbps
Fig.8. Eye pattern for microcoaxial cable assembly with connector.
50
Table 4. The structure of Twinax cable.
reaches 0.25 UI with 500 mm of cable length.
Because there is a distortion of the eye pattern, limited length is used.
The general transmission speed in a cellular phone
is 250 MHz ~ 500 MHz. Moreover, the general cable
length is about 100 mm.
The microcoaxial cable assembly has sufficient performance for current cellular phones.
Item
Conductor
Insulation
Outer conductor
Characteristic impedance: 50 ps
(Ω)
100
90
80
70
60
50
40
30
Measuring MC
20
(TDR)
10
0
34.8
35.3
Connector
Jacket
Test
fixture
Structure, Material
Material
Silver-coated copper alloy
Construction
7/0.025 mm
Diameter
0.075 mm
Material
PFA
Dielectric constant
2.15
Diameter
0.165 mm
Material
Tin-coated copper alloy
Construction
Braiding 0.03 mm wire
Diameter
0.305 mm × 0.47 mm
Material
PFA
Dielectric constant
2.15
Diameter
0.37 mm × 0.54 mm
Cable
35.8
36.3
(ns)
Connector A (50ps)
Connector B (50ps)
Fig.9. Impedance for microcoaxial cable assembly
with connector.
Microcoaxial cable
Twinax cable
Non-noise
environment
Non-noise
environment
Size
AWG42
L=500 mm
mV
200
180
160
140
120
100
80
60
40
20
0
Noise
environment
AWG42
L=500 mm
L=500 mm
500 Mbps
500 Mbps
2 Gbps
2 Gbps
3.125 Gbps
3.125 Gbps
Eye height
Microcoaxial cable vs Twinax cable
AWG42 L=500 mm
(Comparison in the noise environment)
Twinax :Normal
Twinax :Noisy environment
Microcoaxial :Normal
Microcoaxial :Noisy environment
0
Size
500 1,000 1,500 2,000 2,500 3,000 3,500 Mbps
UI
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Noise
environment
L=500 mm
Jitter
Microcoaxial cable vs Twinax cable
AWG42 L=500 mm
(Comparison in the noise environment)
Twinax :Normal
Twinax :Noisy environment
Microcoaxial :Normal
Microcoaxial :Noisy environment
0
500 1,000 1,500 2,000 2,500 3,000 3,500 Mbps
Fig.10. Comparison of a transmission characteristic under noise environment. (Microcoaxial cable and Twinax cable)
Fujikura Technical Review, 2007
51
Table 5. Electrical property of Twinax cable (AWG42).
Item
Method
Conductor resistance
JIS C3102
Insulation resistance D.C 250 V 1min.
Specification
Max. 7.5 Ω/m
Min. 100MΩ/km
Dielectric strength
A.C 250 V 1min.
No break down
Characteristic
impedance
TDR
Nom. 100 Ω
(Differential)
Attenuation
-
Nom. 8.0 dB/m (at 1 GHz)
4.3. For further high-speed transmission
Reduction demand for the number of cables is
expected to be met by miniaturization of the hinge.
The high-speed serial transmission is assumed as the
solution. In order to perform the high-speed serial
transmission, it is necessary to consider not only the
cable transmission but also the SN ratio. Generally,
for the high-speed serial transmission, the differential
transmission is adopted to improve the SN ratio. We
are developing a microshield pair coaxial cable
(Twinax) for further high-speed transmission. Table 4
shows the cable structure, and Table 5 shows the
electrical property. Figure 10 shows the high-speed
transmission characteristic under noisy environment.
The transmission characteristic of the Twinax cable
52
is clearly superior to the microcoaxial cable in terms
of the noisy environment.
4.4. Conclusion for transmission characteristic
In the current transmission speed (about 500
Mbps) and wiring length (about 100 mm), the microcoaxial cable assembly has enough capability.
However, the demand for cable miniaturization and
complicated wiring will also increase, the serial
transmission will also be required. High-speed transmission will produce a noisy environment, the micro
Twinax cable demand is expected to alleviate the
noisy environment. Currently, the micro Twinax connector is under development. The present evaluation
of transmission characteristic and alleviation performance is not enough. We plan to improve the Twinax
cable assembly and reevaluate the transmission characteristic.
5. Conclusion
The microcoaxial cable and assembly can realize
high-speed transmission and space-saving wiring with
high flexibility. In view of alleviating the increasing
noisy environment in cellular phones, the micro
Twinax cable and assembly will become useful.