# Design of PCB and MCM for high speed

Design of PCB and MCM for high speed digital systems The art of compromise Torstein Gleditsch SINTEF Electronics and Cybernetics Page 1 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

What this course is and what it is not • This course is an introductory course to designing Printed Circuit Boards and Multi Chip Modules for high frequency digital applications. • It will introduce you to the basic concepts of transmission lines and how utilize this theory into practical designs. • It will introduce you to modeling of lines, drivers and receivers to help you get a better understanding of the effects of the different measures. • It will not make you a proficient SPICE user. • At last it will make you understand why high speed digital design is difficult and how to improve a design. Page 2 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

What is so special with high-speed digital • Broad band – Must cover every frequency from DC to GHz – No “Dirty tricks” possible • • Higher order harmonics is necessary for edge integrity. High number of critical signals Digital signals is more tolerant to distortion Switching create high currents in short periods Page 3 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

A digital signal • Frequency 50 MHz • Risetime 1 ns ( 10% - 90% ) • Falltime 1 ns ( 10% - 90% ) • Bandwidth 350 MHz Page 4 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Spectrum of a single pulse 10 ns risetime Page 5 1 ns risetime Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Spectrum of a pulse train Page 6 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

The effect of missing harmonics 1 st harmonic only 1 st and 3 rd harmonic only Page 7 1 st to 5 th harmonic 1 st to 21 st harmonic Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Why transmission line calculations • Transmission line theory describe the behavior of the signals on a PCB or MCM • Simple models like RC-calculation do not apply at higher frequencies • It is possible to predict the quality of the signal. • It is possible to experiment with different types of termination. Page 8 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

When do I have to take into account transmission lines effects Trace length (mm) • Thumb rule: When the rise time is less 2. 5 times the time delay of the signal on the trace. (Transit time) • Another: If the trace length is larger than 1/7 of the largest wavelength of the signal. (At upper frequency edge) • Example with two different effective dielectric constants Risetime (ns) Page 9 Risetime (ns) Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Reflections from a 1 ns risetime signal 40 mm non terminated line Page 10 300 mm non terminated line Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Crosstalk 40 mm non terminated line Page 11 300 mm non terminated line Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Transmission Lines A brief introduction Page 12 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Basic Transmission Line Types Microstrip W t H Stripline er W t tand Page 13 H Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Dielectric constant (Relative permittivity) (er ) • Describe a materials ability to hold charge compared to vacuum when used in a capacitor. • The permittivity of a vacuum is: e 0 = 8. 85419 · 10 -10 • The permittivity of a material is: e = e 0 er • If we put a dielectric material between two capacitor plates of area A and a distance D the capacitance in Farad is: A C= D Page 14 [F] Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Loss tangent ( tand ) • Describe the materials “resistance” to change of polarization • s is the conductivity of the dielectric at frequency . (S/m) • e is the permittivity of the material. (F/m) tan = 2 f Page 15 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Magnetic permeability (m) The magnetic permeability describe a magnetic property of a material. It is the ratio between the magnetic flux density (B) and the external field strength (H). = B [H/m] H The relative permeability of a material is the permeability relative to vacuum. µ = µr µ 0 4 µ 0 = 107 Page 16 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Properties of glass reinforced materials Page 17 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Basic transmission line diagram Driver Page 18 Transmission line Termination resistor Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Some Transmission Line Properties • Signal velocity (m/s) – Influenced by dielectric constant of materials • Impedance – Mostly influenced by dielectric constant of material, width of line and distance to ground plane(s) • Loss – Influenced by conductivity of conducting material, frequency of signal and loss tangent of the material. • Dispersion – Influenced by frequency dependant dielectric constant Page 19 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

The four describing parameters • • R C L G Resistance per unit length [ohm / m] Capacitance [ F / m ] Inductance [ H / m] Conductance [ S / m ] Page 20 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Signal velocity ( ) • The signal velocity is measured in m/s • For a practical calculations the signal velocity is only dependant on the relative dielectric constant er • With c 0 the velocity of light in vacuum, we get: v= Page 21 c 0 r [m/s] Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Impedance ( Z 0 ) • Impedance is measured in Ohms • Impedance describes the AC resistance a driver will see when driving a signal into a indefinitely long transmission line. Z 0 = R + L j G + C j [ohm] For a loss less line this simplifies to: Z 0 = Page 22 L [ohm] C Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Loss ( a ) • Loss a is measured in d. B / m • Loss is the reduction of signal voltage along a line due to resistance and leakage • The resistive losses is due to resistance in conductor and ground plane. • The dielectric losses is due to the energy needed to change polarization of the dielectric material. • The radiation losses is the energy sent from the conductor acting as an antenna. Page 23 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Resistive losses ( a. C ) The resistance is based on the cross-section of the conductor and the bulk resistivity of the conductor material: R= 1 w t [ohm] s = bulk conductivity [S/m] w = line width [m] t = line thickness [m] The loss factor is then calculated by: = Page 24 8, 68589 R 2 Z 0 [d. B/m] Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Skin depth ( ds ) At higher frequencies only a thin layer of the conductor transport the current. The thickness where the current density is reduced to 1/e is called the skin depth ds. s= Page 25 1 µf [m] Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Resistance at higher frequencies When the conductor is much thicker than the skin depth one have to substitute the skin depth for the thickness in the resistance formula: R= R= 1 [ohm] w s 1 w 1 µf The resistance has now become frequency dependent ! Page 26 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Dielectric loss ( a. D ) The dielectric loss a. D is due to the energy needed to change the polarization of the dielectric material. The conductance is then: G = 2 f C tan [S/m] The dielectric loss is then: D = Page 27 1 2 8, 68589 G Z 0 [d. B/m] Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Radiation loss • All lines are radiating more or less • Radiation loss is often negligible from a signal integrity standpoint but important from a EMC standpoint. • Radiation loss is difficult to calculate Page 28 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Dispersion • Dispersion is an effect caused by different velocities for the different frequency components. The result is a different phase change for each of the frequency components. • The reason for this is that the dielectric constant of the material vary with frequency. • Dispersion is one of many sources of signal distortion. • It is not easy to calculate dispersion because the lack of frequency dependant material data. • Dispersion may cause trouble when exact edge position is important. Page 29 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Crosstalk is coupling of a signal form one line to another There are two key parameters used to describe crosstalk Kf the forward crosstalk coefficient Kb the backward crosstalk coefficient (normally the largest) Td is the transit time delay Kf = - Kb = - Page 30 ( 2 Z 1 Lm 0 Lm Z 0 ) - Cm Z 0 4 Td Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Reflections In this case Z 1 = R 2 = 25 ohm, and Z 0 = 50 ohm. Then r = -0. 333 => -1. 66 overshoot Page 31 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Via modeling • Inductance and capacitance of via’s is difficult to calculate without 3 D field analysis tools. • A 1 n. H inductance and a 1 p. F capacitance can be used as a start • Careful use of coaxial line equations can also give an indication of the values. Page 32 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Building a good board Some hints for the high performance designer Page 33 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

What is important for a good board • • Symmetrical around center Tolerant for etch variations (+/- 1 mil is not unusual) Lines spaced for acceptable crosstalk Standard dielectric thickness’ Tolerant for variation in dielectric thickness Acceptable high loss. (The loss may be your friend) Acceptable total thickness Power and ground layers close together (typically 100 um) Page 34 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Laminate tolerances Dictionary: Toleranse = Tolerance Tykkelse = Thickness Klasse = Class Glassvevtype = Glass fabric type Page 35 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Prepreg Tolerances Page 36 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Impedance tolerance to line width As line width increases, dependence on absolute tolerance decreases Border lines on +-1 mils absolute line width tolerance Page 37 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Impedance tolerance to laminate tolerance Page 38 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Minimum line widths vs. er and dielectric thickness Curves on equivalent dielectric thickness Page 39 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

8 layer high-speed lay-up example 1 Dictionary: Kobber = Copper rent = pure Page 40 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

8 layer high-speed lay-up example 2 Page 41 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Pack 32 A desktop calculator for transmission lines Page 42 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Idea behind the program • • Easy to use PC based ( Windows 95 or NT ) Good enough results (better than textbook formulas) Easy to organize material, component and lay-ups Easy for the developers to add new functions. Data is stored one place, no tedious typing Fast, no calculation take more than one second Page 43 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Workflow for transmission line analysis 1. Enter material data if not in database 2. Define a Lay-up 3. Calculate the line properties for a single line 4. Adjust Lay-up and re-calculate until satisfied 5. Calculate for double lines to check for crosstalk and dual line impedance. 6. Generate a SPICE model for a critical net in the design, single or double lines as appropriate 7. Simulate and adjust line parameters (width, spacing, lay-up) 8. Use the obtained parameters as design rules for your critical nets. Page 44 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Material database Sorry! You have not got the English version! For transmission line calculation, one need: • Electrical conductivity • Dielectric constant • Dielectric loss factor • Magnetic permeability Page 45 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Lay-up definition Sorry! Norwegian text! Page 46 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Single line analysis Stripline Microstrip Buried microstrip Page 47 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Dual Line Analysis Page 48 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Exercise Design a lay-up with these properties: • 50 Ohms impedance • FR-4 Dielectric • No Solder resist • 4 Signal layers • Two Power layers • Two Ground layers • No more than 3 d. B loss for a 10 cm line at 1 GHz • Tolerant for +/- 1 mil etch error. Page 49 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Analog simulation of digital signals Time for sacrifice Page 50 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

The two main simulation approaches • Time domain simulators – – – Easy to model nonlinear circuits such as digital drivers Has no possibility to handle frequency dependent parameters Convergence problems Relatively slow simulation Typical product: SPICE • Frequency domain simulators – – Easy to model circuits with frequency dependent parameters Only possible to model linear circuits Fast and easy convergence Typical product: HP-EEsof Series IV Linear simulator What we really need does not exist Page 51 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Aim-SPICE • • Based on Berkeley SPICE version 3. E 1 Good user interface Good post processor Cheap Student version is limited but usable Designed for Windows 95 / NT Text based, no schematic editor Page 52 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

A SPICE example First line. MODEL Line. Model LTRA L=460 N C=94 P R=10 LEN=. 04 V 1 in 0 PULSE(0, 10 n, 1 n, 10 n, 20 n) R 1 in lin 50 O 1 lin 0 out 0 Line. Model R 2 out 0 50 Page 53 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

General • All values are in basic SI units [Ohm, Farad, Henry, Volt, Ampere] • Postfixes are allowed: m = 10 -3 , u= 10 -6 , n= 10 -9 , p= 10 -12 k= 103 , meg= 106 , g= 109 • Element names must be unique and is interpreted by ASCII value • Node names can be numbers or names and is interpreted by ASCII • Node 0 is the system ground and MUST be connected. • The first line MUST be the model name • Blank lines are allowed On the following pages some important elements is shown, refer to the online SPICE manual for details and more options. Page 54 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Passive elements Resistor Rxxx <N 1> <N 2> <Value> Capacitor Cxxx <N 1> Inductor Lxxx <N 1> Page 55 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Transmission lines Loss-less transmission line Txxx <N 1> <Gnd> <N 2> <Gnd> <Z 0=Value> [Td=Value] Lossy transmission line. MODEL Line. Model LTRA L=460 N C=94 P R=10 LEN=. 04 Oxxx <N 1> <Gnd> <N 2> <Gnd> <Name> Page 56 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Voltage sources Voltage source Vxxx <N+> <N-> <Type> Some useful Types for transient analysis For a pure DC source: DC <Value> For a pulse train generator PULSE(<High voltage>, <Low voltage>, <Delay>, <Risetime>, <Fall-time>, <High time>, <Cycle time>) Page 57 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Exercise Simulate a 15 cm long microstrip with these specs: • 125 um wide • 17. 5 um thick • Dielectric is polyimide • Dielectric thickness is 100 um • Source impedance is 70 Ohm • Termination resistance is 50 Ohms • Rise and fall times is 0. 8 ns, frequency is 200 MHz 1. What is the maximum over / undershoot 2. Insert a capacitor of 50 p. F in parallel with the terminator, see what happens. Page 58 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Subcircuits SPICE have a nice feature called sub-circuits which work like subroutines. The syntax is: . SUBCKT <Name> <Node 1> <Node 2>. . [Node n] <Spice code with the node names as I/O>. ENDS You call this subcircuit with Xxxx <N 1> <N 2>. . . [Nn] <Name> Page 59 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Distributed models An other way of simulating transmission lines is by using distributed models. Selecting a high number of segments will give excellent results but may cause problems in initialization. The Student version can only handle 10 of these segments due to the maximum of 50 elements. Page 60 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

PACK model generation PACK can generate the models for you Page 61 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Pack Generated Distributed Model Page 62 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Exercise Make a distributed model of the last exercise and compare results Page 63 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Exercise A board needs 60 Ohms lines in the inner layers. Design a board for 60 Ohm lines and generate a single LTRA model of 5 cm. Make a three segment line with a 60 Ohm driver. After the first segment the line splits into the two other segments. Each of the splits is terminated in 60 ohms. Page 64 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Modeling of drivers and receivers When is enough Page 65 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Not all models have long hair and blue eyes Driver models are difficult to obtain, usually you have three options: 1. Use vendor supplied SPICE models of the output stage 2. Use vendor supplied IBIS models 3. Roll your own simple models based on rise time and output impedance. Page 66 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Vendor supplied SPICE models • Difficult to obtain, vendors generally do not want to disclose their design. • Most vendors require non disclosure agreement • Difficult to run. Vendors usually use H-SPICE, board designers do not. • Models are to detailed, unnecessary accuracy give slow simulation. • Difficult to set the right environment for drivers etc. So, vendor supplied SPICE models is generally not a good idea. Page 67 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Vendor supplied IBIS models IBIS solves most of the problems related to vendor models except: • Not all SPICE simulators run IBIS models • Not all vendors supply IBIS models, although this is rapidly increasing IBIS models is sufficiently accurate for package and module simulation. So, use IBIS models (If you can) Page 68 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

IBIS model Page 69 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

IBIS I/V curves (ABT 244) Pull-up curve ( Referenced to Vcc ) Y-axis: Current [A] X-axis: Voltage [V] Pull-down curve Page 70 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

IBIS viewing tool Page 71 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Make your own models • Simple models are sufficient to study transmission line effects and to develop design rules. • They are not sufficient for optimizing a specific net. • A very simple model: Page 72 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Exercise Study the supplier SPICE model of an “IBIS” driver. Adjust the data to fit an SN 74 ALVCH 16244 Compare with data sheets and the timing article Page 73 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Routing of high speed signals There is no easy way ( but some auto-routers are really good ) Page 74 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Main Issues for Routing • • Routing topologies and loading Impedance Rise and fall time degradation Signal Skew (Signal delay difference) First incident clocking Crosstalk Signal return path Termination Page 75 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Point to Point Topology • This is the ideal topology but not very efficient in that it requires a lot of extra buffers. • Always use this topology for critical clock trees. • Use low skew clock buffers • Skew can be compensated with delay lines Page 76 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Fan / Star topology • This topology is routing efficient but put heavy load on the driver, especially where several lines fan out. • Use drivers with low output impedance. • Terminate properly at each receiver or reflections will propagate back and forth in the net. • Be careful with the high power consumption of many terminated lines Page 77 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

T - Topology • This split will cause a 33% negative reflection if all lines are of same impedance. • All ends (also driver) should be terminated properly for larger nets • The driver will have to drive twice the amount of DC into the termination resistors. Page 78 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Daisy Chain • • • The preferred topology for high speed digital. Terminate in both ends. Allow no stubs Keep tap load low. Keep distance between loads so high that the signal can recover. Page 79 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Rise and fall time degradation Any lossy transmission line acts as a filter which filters the high frequency components first. Since the high frequency components is damped more the signal will show slower rise times at the end of the line. Page 80 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Signal skew is the delay difference at inputs. Keeping low signal skew in a clock system is a challenge and there are several effects to consider. • Keep every signal trace equally long • Use low skew drivers • Rise time degradation may pose a problem in calculated delay daisy chains. • Signals travel faster at outer layers, so trace lengths must be compensated. Or - use only inner layers. • Poor decoupling influence rise time. • Use only first incident clocking or better, reduce reflections to zero. Page 81 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Signal return path The high frequency signals follow a mirror image of the trace on the ground plane. Do not degrade this return by: • Changing to layers which have another ground-plane without placing a ground-ground via close to the signal via. If the new reference plane is a power plane a low inductance decoupling is placed close to the signal via • Using split reference planes. • Using one small via for several returns, this may cause common mode problems. Page 82 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

How To Design Delay Lines • Remember loss • Use meander structure, be careful to avoid inductor effects. • Keep meander lines at least 4 times the distance to ground plane apart to avoid distortion. • Preferably use inner layers, this reduce distortion and keep inductance low. Page 83 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Line Termination Terminating lines driven with drivers that do not allow termination Page 84 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Why terminate the signals • Termination is a method to match the driver, line and receiver in such way that no reflections are generated. • Generally the ideal would be to do a parallel termination in the line impedance • Terminate critical lines that are longer than 1/3 of the rise time. On FR-4 this mean that a 1 ns rise signal, the longest unterminated line is 5 cm. • Reflections can cause double trigging, if using nonterminated nets do not use data before the reflections have settled. Page 85 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Series Termination • Used to match driver impedance • Slow rise and fall times (Some times good, for instance to achieve low crosstalk) • Absorb reflections if matched to line Page 86 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Parallel Termination • The termination of choice for high speed digital, especially for ECL, PECL and other technologies intended for termination. • High power consumption. • 100% clean signals possible Page 87 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

RC Termination • AC termination, better for technologies not intended for termination. • Low power consumption (No DC consumption) • Be careful with inductive capacitors, select capacitors with care. Page 88 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Thevenin Termination • • Ideal for bus termination or lines with 3 -State drivers. 330 Ohm & 220 Ohm is often used. Faster switching from 3 -state Does NOT correctly terminate the line, reflections will occur Page 89 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Diode Termination • Kills large over and undershoot • Not generally useful in high-speed systems. (A design needing this type of termination have probably greater problems elsewhere) Page 90 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Power distribution and de-coupling How to supply the right voltage at the right place at the right time Page 91 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Power distribution • The power and ground system serves two purposes. – First to serve as a return path for the signal. – Second to supply the devices with power. • The high frequency components of today is using a lot of power at a low voltage. The result is very high currents. • A modern FPGA can draw several amperes the first nanosecond after switching. • To meet these challenges one need a low inductance, low resistance and high capacitance power system. Page 92 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Why Decoupling • The high switching currents in today's components create a need for a local current supply with sufficient charge to avoid a severe voltage drop at the power pins. • When these resources are exhausted on need larger supplies that are closer than the power supply. • Closely spaced power planes give a good high frequency, low inductance decoupling but it can not hold much charge. Page 93 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

A Capacitor is Not a Capacitor fres = 1 2 LC • • A capacitor is a passive device dominated by capacitance. A 100 n. F capacitor can have C=84 n. F, L=1. 3 n. H, R=0. 13 ohm A capacitor is often described with its resonance frequency Different dielectrics NPO, X 7 R or Z 5 U have different properties. Usually thin and wide X 7 R chips is the best choice for decoupling. To be sure on have to measure the inductance which is difficult To compare two capacitors of same nominal capacitance chose the one with the highest resonance frequency. • So called high frequency capacitors is not necessarily less inductive Page 94 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Selecting Decoupling Capacitors Selecting a decoupling capacitor is not easy but some general rules apply: • Select the smallest value that is sufficient to decouple the device. If you do not know what is sufficient choose one 100 n. F capacitor for each power pin • Use only chip capacitors, leaded capacitors is to inductive to be of any help. • Decouple in levels with the smallest value (i. e. 100 n. F) close to the power pin, the medium values(i. e. 10 m. F) evenly distributed, the higher values (i. e. 47 m. F tantalum) close to the connector power pins. Page 95 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Where to place decoupling capacitors • Get as close as possible to power and ground pins. • Keep distance to pin very SHORT and the trace WIDE. • Set the via as close as possible to the capacitor, preferably in the pad. • One capacitor for every power (and ground) pin is a good rule of thumb, you do not have to mount all of them in production if the system works with less. • Calculate inductance. If too high - forget the capacitor. • Do not forget to place decoupling capacitors close to termination resistors, it is important to keep the reference voltages steady. Page 96 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Splitting power planes • Be very careful with splitting ground planes. • If you have to split, keep the split narrow and secure an AC return path. • Splits act as slot antennas and will radiate heavily. • Remember that digital signals are broad band that filters are narrow band. Filters over the gap may cause problems. Page 97 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

Microstrip Over a Slotted Ground Plane 1 2 3 4 IEEE Circuits & Devices Nov. 1997 Page 98 Leonardo Insight II - Design of PCB and MCM for high speed digital systems

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