Tools > SI123 User Manual
도구/SI Calculator 2012. 7. 11. 20:53SI123 let you know the follow things.
1. Impedance, propagation speed of PCB trace
2. Reflection coefficient at boundary of two different medium
3. Crosstalk between two neighboring traces
4. Resistance of a conductor(PCB trace)
5. Capacitance of conductor plates of PCB(parasitic capacitance of power planes)
6. Reactance(Real impedance of real capacitor)
7. Differential Impedance of traces
8. Ampacity of a trace
9. Number of capacitors for decoupling
10. Unit Conversion
SI123 is base on empirical equations, so it can produce somewhat different results compared to real measured values. It will be recommended to use SI123 as a rough estimation tool.
After SI123 is installed, you can see the icon like following picture. For execution, click it.
Main UI is configured of buttons. Each buttons invokes each calculator. The name of buttons means the function of calculator. you can scroll the display to view all buttons.
when scrolled, hidden buttons are appeared.
Trace Characteristics
This calculator let you know the characteristics of PCB trace. there are two kinds of traces, MS(Microstrip) and SL(Stripline). In first run, the calculator runs at MS mode. if you want to change to SL mode, click the picture.
The characteristics of a trace are defined by its physical structure. Width and thickness of a trace, height between trace and return path, and permtitivity of insolator are the key components of the structure. Like a following picture, you must input these values(black). Then the characteristic values are automatically calculated(non-black).
Auto-calculated values are impedance(Zo), capacitance per unit(Co), inductance per unit(Lo) and propagation time of signal. In MS mode, effective permittivity is calculated, too.
when scrolled, hidden items are appeared.
When you change the length of a trace, the propagation time for a signal to travel the trace is changed automatically. If you change the frequency, the wave length at the frequency is re-calculated. Knowing wave length is helpful to decide the trace is a transmission line or not. If you click the picture, run mode is changed to SL mode like below.
Reflection Coefficient
It shows you the reflection coefficient at boundary between two mediums(conductors). If a reflection coefficient is 0, it means impedance is matched and there will be no reflection. If a reflection coefficient is negative, the amplitude of transmitted signal through the boundary would be small. On the hand, if a reflection coefficient is positive, the amplitude of transmitted signal through the boundary would be large. Maximum reflection coefficient is 1, and in that case, amplitude of transmitted signal would be double of injected one.
Crosstalk
The basic factors of crosstalk are space(H) between trace and reference, and space(S) between traces. There are two calculation mode, MS and SL mode. If you want to change the mode, just click the picture.
Rise time of a signal traveling the trace and traveling time during propagation are factors of crosstalk, too. If a victim trace is terminated, the effect of crosstalk will be reduced.
When the picture is clicked, the mode is changed to SL mode.
Resistance
In some case, it is helpful to know the conductor loss. A resistance is a function of the resistivity, cross-sectional area(W x T), and the length of a conductor. When AC signal flows through the conductor, the resistance of the conductor will be increase because of the skin effect.
Capacitance
This calculator is helpful to know the parasitic capacitance formed by power and ground planes. A capacitance is a function of the area of the planes(A), the distance between planes(D), and the permittivity of the insulator.
Reactance
This calculate the combination impedance of a capacitor and a inductor. A real capacitor for decoupling has ESL, At low frequency it looks like a capacitor, but at high frequency it looks like a inductor. A real capacitor can be modeled as a series combination circuit of a capacitor and a inductor. For the power distribution network, at low freqeuncy it looks like a inductor, but at high frequency it looks like a capacitor. It can be modeled as a parallel combination circuit of a capacitor and a inductor.
The capacitance and the inductance are function of a frequency, so frequency must be input.
You can see the impedance graph of a combination circuit by clicking the 'Graph' button.
Differential Pair
A differential impedance is a function of the space between the traces(S) and the distance from reference to trace(H). If the space between traces are far enough, the differential impedance would be twice of single line impedance(Zo).
You can change to SL mode by clicking the picture.
Trace Ampaicity
The ampacity of a conductor is a function of cross-sectional area(W x T) of the conductor. When current flows a conductor, heat is generated because of resistance. The more current flows, the more heat is generated. If a surrounding material of a conductor absorb the heat quickly, more current could flow at the same atmosphere temperature. For that reason MS can flow more current than SL.
Following picture shows that 0.456A could flow through the PCB trace with 1oz-thickness and 4mil-width in the condition of 10 degree temperature rise than normal temperature.
In SL mode, smaller current would flow in a trace with same dimension.
Local Decoupling
The voltage level of power sourcing chips like a FPGA, a MCU, or a memory must be kept stable from DC to the frequency of maximum signaling or operating. The change of dynamic current flowing into chip disturbs the the voltage level and results mal-function of the chip. To prevent this, appropriate decoupling capacitors are needed.
The size of capacitor to keep the voltage level stable is a function of allowed ripple level(Supply Voltage x Allowed Ripple), amount of change of dynamic current flowing into a chip(Dynamic Current), and the time in which the amount of current is changed(Transition Time). In following example, the voltage value of a power is 1.5V and allowed ripple is 4%. therefore the voltage level must be kept within 1.44~1.56V If the amount of current is changed up to 1A during 0.6ns, which means the maximum frequency of current is 556MHz, the capacitance of 5nF(Min. Capacitance) is needed to keep the voltage within 1.44~1.56V level. The capacitor must be placed within 671mils(Effective Radius) from noise source(chips) for effective decoupling.
In the following, 100nF, 3nH, and 30mOhm is used for the value of real capacitor. The mounting inductance must be included to ESL. In the condition of using this capacitor, resonance frequency of this capacitor is 9.2MHz, and impedance of this capacitor at 556MHz is 10.469ohm. One this capacitor can drive 0.143A stably during 8.164ns at that frequency. The effective radius of the capacitor is 9129mils. For keeping voltage level within target(1.44~1.56V), 7 of this kind of capacitor must be used. If you can reduce the ESL of the capacitor, you can reduce the number of the capacitors.
Global Decoupling
For stable operation, A PDN impedance must be kept low through the board. the basic principle of global decoupling is same as the local decoupling. But In here, mid- or low-range frequencies are considered. Therefore effective radius has no meaning any more. Rather, a new factor like a temperature must be considered. A capacitor is effected by a temperature and a dissipation factor(DF), so that kinds of factors must be input.
In the following example, the purpose is to keep the ripple of voltage with 4%. If the amount of current change is 3A during 1us(this may be slowest operating period of a chip in the board or multiple of that period), the target frequency is 333KHz and a 100uF capacitance is needed to keep voltage level stable.
If the values of real capacitor are 100uF, 3nH, 89mOhm, and DF=5%, 3 capacitors are needed at normal temperature.
Power Distribution Network
This tool shows the graph of a PDN impedance which must be kept below the target impedance to the target frequency. This feature is supported at future version.
Unit Conversion
There are 3 unit conversion services. length conversion(English <-> Metric), ratio conversion, AWG calculation. 4 mil is same as 0.1016 mm, -3dB is same as 0.5 power ratio and 0.71 amplitude ratio, AWG26 is same as 16 mil of diameter of a conductor.
Units
This sets the units of input of the each calculator.
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