信号意味着温度-台光电子

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When “Signal” meets “Temperature”
Albert Chen, Wei Ning Kuo, Figaro Ho, Peter Tsai Elite Material Co., Ltd. 18 Ta-Tung First Rd., Kuan Yin Industrial District, Taoyuan County, Taiwan 32849, ROC albert_chen@https://www.wendangku.net/doc/e65742066.html,
ABSTRACT
The high speed transmission applications in the electronic product become inevitable developing trend. We could find that OEM design faster chip in the server, communication and high speed transmission interface and device application. A high reliable PCB board is not only considering conventional thermal reliability but also signal integrity under high frequency today. In recent years, the OEM defines accurately low loss material criteria and specifies the insertion loss measurement standard for PCB board. The trend prompt that CCL manufacturers develop the low loss material and PCB fabricator improve the process and more delicately handle the PCB board, besides that both CCL and PCB fabricator begin to set up electrical lab and study the insertion loss phenomenon in a PCB board. All of these efforts are to make sure that insertion loss performance could achieve the OEM design requirement. Due to the actual running temperature for a finished assembly board with component is quite different at room temperature. The OEMs are also interested in the signal loss behavior under higher temperature. We will discuss the signal integrity through impedance, S21 parameter and eye diagram analysis to understand the signal loss performance under different temperature effect. Through the paper discussion, hoping it can help us to learn more about the effect of signal bandwidth at different temperature operations.
(a) Relative dielectric constant
(b) Relative dielectric loss Figure 1 Temperature dependant complex permittivity for glass epoxy
INTRODUCTION
For a general communication model, it needs message and media between sender and receiver, the current message transmission development trend is to need high frequency (GHz) signal between two components, it means that signal integrity will play a very important role during the transmission process today. Most of signal loss studies are done at room temperature. We are very interested in the signal loss phenomena for a transmission line when the PCB board is working at higher temperature condition. According to Koksushika University study [1], the results shows that dielectric constant and dissipation factor would be increasing linearly as temperature increasing in some certain materials.
On the other hand, the copper resistance would be increasing linearly with temperature effect as shown below equation. R2=R(1+α(T 2- 20)) While R2 means resistance at temperature T2, R means resistance at temperature 20 degree Celsius, α means temperature coefficient of copper at 20 degree Celsius and the coefficient is given 0.00393. We roughly calculate the copper resistance will increase about 23.6% when temperature is raised from 20 degree Celsius to 80degree Celsius.
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transmits down the trace from the sender to the receiver. The lower S21 represents that less energy loss during the signal transmission process.
Port1
Port2
Figure 2 Equivalent circuit model of a transmission line (RLGC model) R represents series resistance per unit, G represents shunt conductance per unit; L represents series inductance per unit; and C represents shunt capacitance per unit.. The transimiision line impedance could be expressed as below formula,
Incident wave Reflected wave
DUT
Transmitted wave
Figure 4 S parameter descriptions Eye Diagram Eye diagram is a useful way to visualize the effects caused by loss line. A typical data string consists of 1(high) and 0 (low) bit value. It will form a longer data string when data bit increase. A larger number of data bits pass a point and superimposed them in the screen of oscilloscope, which we call it as eye diagram. The transition between logic states should be very clear for an ideal eye diagram. It means that eye is open enough which has appropriate eye height and eye width. A 3 bits data has 2^3 combination of data string could form eye diagram as below figure shown (2).
R and G are energy loss elements for conducror loss and dielectric loss indivally, L and C are energy stored elements. A typical stripline stack up is as below figure shown,
Figure 3 Offset stripline stack up Any change in the dielectric constant, dielectric thickness and line width will also change the impedance, the impedance is the function of material dielectric constant and pattern geometry.
Figure 5 all of possible combination, superimposed of 3 bit passing by a point of circuit. We could judge the signal integrity degree by jitter and eye height. The jitter means that time difference between actual and ideal rise time. The eye height could observe the difference in the voltage. Both of jitter and eye height is caused by energy attenuation in the transmission line. The fewer jitter and large eye height means better signal integrity in the transmission process. A higher Gbps will have a closer eye compare to lower Gbps at room temperature.
Signal Loss Measurement
S parameter The signal loss can be expressed by S parameter (S means scatter). S11 represents the part of signal that is going into port 1 and reflected back to port 1. S21 represents part of wave that is going on from port 1 toward port 2.
S11 is also called as return loss, it stands for reflection coefficient in the VNA frequency domain , it has the same meaning like the ρin the TDR time domain analysis tool. The S21 is also called insertion loss. It is a very important indicator to measure how many percent of electric energy loss when the signal
Figure 6 Example of eye diagrams at 3Gbps and 5Gbps under room temperature.
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DESIGN OF EXPERIMENT
We prepare a FR-4 material A with VLP and RTF copper foil for the same stack up. Transmission line is using offset stripline with line length 16”, line width 8mil and impedance controlled 50 ohms. Analysis instrument: Agilent PNA Calibration method: TRL (Through Reflection Line) method Place the DUT coupon into a temperature control chamber for measurement. Heating Chamber
PNA
ohm 60 58 56 54 52 50 48 46 44 0
Impedance of material A_RTF at 23 and 80 degree C
Material A_RTF @ 23 ℃ Material A_RTF @ 80 ℃
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2
3
4
5
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7
time (ns)
Computer
Figure 9 Impedance of material A_RTF at 23 and 80 degree Celsius The material A has lower impedance about 0.55-0.60 ohm at 80 degree Celsius compare to the impedance at 23 degree Celsius .It maybe comes from the result of the material dielectric constant increasing at higher temperature. Eye diagram and discussion
Eye Jitter change of percentage from 23 to 80 degree C
50.00% 40.00%
Figure 6 PNA measurements setting at high temperature environment.
TEST RESULTS AND DISCUSSION
S21 measurement result and discussion
Material A S21 change of percentage from 23 to 80 degree C
35.00% 30.00% 25.00% 20.00% 15.00% 10.00% 5.00% 0.00% 3GHz 5GHz 10GHz 15GHz Material A_VLP Material A_RTF
30.00% 20.00% 10.00% 0.00% 3Gbps 8Gbps
Material A_VLP Material A_RTF
Figure10 eye diagram jitter increase percentage from 23 to 80 degree Celsius
Eye height change of percentage from 23 to 80 degree C
3Gbps -40.00% Material A_VLP Material A_RTF 8Gbps
Figure 7 Material A S21 loss increase percentage from 23 to 80 degree Celsius We could see that insertion loss S21 increases obviously at higher temperature 80 degree Celsius. The change of percentage tends to be also increasing when frequency increases. No great benefit observes at the copper foil type effect for reducing the insertion loss attenuation degree under high temperature. Impedance measurement and discussion
Impedance of material A_VLP at 23 and 80 degree C
ohm 60 58 56 54 52 50 48 46 44 0 1 2 3 4 5 6 7 time (ns) Material A VLP @ 23℃ Material A_VLP @ 80 ℃
-30.00%
-20.00%
-10.00%
0.00%
Figure11 eye height decrease percentage from 23 to 80 degree C We could also see that jitter increase and eye height decrease at eye diagram from 23 to 80 degree Celsius. It means that eye become closer and more signal energy attenuate at higher temperature. The eye diagram at higher Gbps and higher temperature will become worse than that of lower temperature and lower Gbps. There are almost the same attenuation result in the eye diagram of both RTF and VLP copper foils under the same temperature change.
Figure 8 Impedance of material A_VLP at 23 and 80 degree Celsius
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Conclusion 1.
2. 3. The higher temperature will cause higher signal loss from the previous measurement of S21 and eye diagram. No significant variation found between copper foil VLP and RTF on signal loss under higher temperature effect. The impedance of transmission line at 80 degree Celsius is lower than at 23 degree Celsius. It maybe comes from the result of the material dielectric constant increasing at higher temperature.
REFERENCES
[1] [2] Yoshio Nikawa and Kaiduo Zhao Graduate school of Engineering, Kokushikan University “ Dynamic measurement of complex permittivity in microwave” Douglas Brooks “Signal integrity issues and printed circuit board design”
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