Wave transmission lines and networks – Coupling networks – With impedance matching
Reexamination Certificate
2003-01-15
2004-01-06
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
With impedance matching
C333S012000, C333S02200F, C361S738000
Reexamination Certificate
active
06674338
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electronic systems, and more particularly to electrical interconnecting apparatus forming electrical power distribution structures.
2. Description of the Related Art
A power distribution network of a typical printed circuit board (PCB) includes several capacitors coupled between conductors used to convey direct current (d.c.) electrical power voltages and ground conductors. For example, the power distribution network of a digital PCB typically includes a bulk decoupling or “power entry” capacitor located at a point where electrical power enters the PCB from an external power supply. The power distribution network also typically includes a decoupling capacitor positioned near each of several digital switching circuits (e.g., digital integrated circuits coupled to the PCB). The digital switching circuits dissipate electrical power during switching times (e.g., clock pulse transitions). Each decoupling capacitor typically has a capacitance sufficient to supply electrical current to the corresponding switching circuit during switching times such that the d.c. electrical voltage supplied to the switching circuit remains substantially constant. The power entry capacitor may, for example, have a capacitance greater than or equal to the sum of the capacitances of the decoupling capacitors.
In addition to supplying electrical current to the corresponding switching circuits during switching times, decoupling capacitors also provide low impedance paths to the ground electrical potential for alternating current (a.c.) voltages. Decoupling capacitors thus shunt or “bypass” unwanted a.c. voltages present on d.c. power trace conductors to the ground electrical potential. For this reason, the terms “decoupling capacitor” and “bypass capacitor” are often used synonymously.
As used herein, the term “bypass capacitor” is used to describe any capacitor coupled between a d.c. voltage conductor and a ground conductor, thus providing a low impedance path to the ground electrical potential for a.c. voltages.
A typical bypass capacitor is a two-terminal electrical component.
FIG. 1
is a diagram of an electrical model
10
of a capacitor (e.g., a bypass capacitor) valid over a range of frequencies including a resonant frequency f
res
of the capacitor. Electrical model
10
includes an ideal capacitor, an ideal resistor, and an ideal inductor in series between the two terminals of the capacitor. The ideal capacitor has a value C equal to a capacitance of the capacitor. The ideal resistor has a value equal to an equivalent series resistance (ESR) of the capacitor, and the ideal inductor has a value equal to an equivalent series inductance (ESL) of the capacitor. The series combination of the capacitance (C) and the inductance (ESL) of the capacitor results in series resonance and a resonant frequency f
res
given by:
f
res
=
1
2
π
(
ESL
)
(
C
)
.
FIG. 2
is a graph of the logarithm of the magnitude of the electrical impedance (Z) between the terminals of electrical model
10
versus the logarithm of frequency f. At frequencies f lower than resonant frequency f
res
, the impedance of electrical model
10
is dominated by the capacitance, and the magnitude of Z decreases with increasing frequency f. At the resonant frequency f
res
of the capacitor, the magnitude of Z is a minimum and equal to the ESR of the capacitor. Within a range of frequencies centered about resonant frequency f
res
, the impedance of electrical model
10
is dominated by the resistance, and the magnitude of Z is substantially equal to the ESR of the capacitor. At frequencies f greater than resonant frequency f
res
, the impedance of electrical model
10
is dominated by the inductance, and the magnitude of Z increases with increasing frequency f.
When a desired electrical impedance between a d.c. voltage conductor and a ground conductor is less than the ESR of a single capacitor, it is common to couple more than one of the capacitors in parallel between the d.c. voltage conductor and the ground conductor. In this situation, all of the capacitors have substantially the same resonant frequency f
res
, and the desired electrical impedance is achieved over a range of frequencies including the resonant frequency f
res
.
When the desired electrical impedance is to be achieved over a range of frequencies broader than a single capacitor can provide, it is common to couple multiple capacitors having different resonant frequencies between the d.c. voltage conductor and the ground conductor. The ESRs and resonant frequencies of the capacitors are selected such that each of the capacitors achieves the desired electrical impedance over a different portion of the range of frequencies. In parallel combination, the multiple capacitors achieve the desired electrical impedance over the entire range of frequencies.
A digital signal alternating between high and low voltage levels includes contributions from a fundamental sinusoidal frequency (i.e., a first harmonic) and integer multiples of the first harmonic. As the rise and fall times of a digital signal decrease, the magnitudes of a greater number of the integer multiples of the first harmonic become significant. As a general rule, the frequency content of a digital signal extends to a frequency equal to the reciprocal of &pgr; times the transition time (i.e., rise or fall time) of the signal. For example, a digital signal with a 1 nanosecond transition time has a frequency content extending up to about 318 MHz.
All conductors have a certain amount of electrical inductance. The voltage across the inductance of a conductor is directly proportional to the rate of change of current through the conductor. At the high frequencies present in conductors carrying digital signals having short transition times, a significant voltage drop occurs across a conductor having even a small inductance. Transient switching currents flowing through electrical impedances of d.c. power conductors cause power supply voltage perturbations (e.g., power supply “droop” and ground “bounce”). As signal frequencies increase, continuous power supply planes (e.g., power planes and ground planes) having relatively low electrical inductances are being used more and more. The parallel power and ground planes are commonly placed in close proximity to one another in order to further reduce the inductances of the planes.
The magnitude of electrical impedance between two parallel conductive planes (e.g., adjacent power and ground planes) may vary widely within the frequency ranges of electronic systems with digital signals having short transition times. The parallel conductive planes may exhibit multiple electrical resonances, resulting in alternating high and low impedance values. High impedance values between power and ground planes are undesirable as transient switching currents flowing through the high electrical impedances cause relatively large power supply voltage perturbations.
It would thus be desirable to have a bypass capacitor method for achieving a desired value of electrical impedance between parallel conductive planes of an electrical power distribution structure, wherein variations in the electrical impedance are relatively small over a wide range of frequencies. It would also be advantageous if the desired method would provide for optional suppression of the electrical resonances of the planes in addition to achieving the desired value of electrical impedance over a wide range of frequencies. Magnitudes of power supply voltage perturbations resulting from transient switching currents would be significantly reduced in electrical power distribution structures resulting from applications of the above methods.
SUMMARY OF THE INVENTION
Several methods are presented for achieving a desired value of electrical impedance between conductors of an electrical power distribution structure by electrically coupling multiple bypass capacitors and corresponding electrical resistance elements in series between the conductors.
Chang Joseph
Kivlin B. Noäl
Meyertons Hood Kivlin Kowert & Goetzel P.C.
Pascal Robert
Sun Microsystems Inc.
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