Wave transmission lines and networks – Long line elements and components – Strip type
Reexamination Certificate
2001-01-16
2003-03-25
Pascal, Robert (Department: 2841)
Wave transmission lines and networks
Long line elements and components
Strip type
C333S185000
Reexamination Certificate
active
06538538
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a via for providing a signal path between conductors formed on separate layers of a printed circuit board, and in particular to a via that acts as a tuned filter to optimize characteristics of its frequency response.
2. Description of Related Art
FIGS. 1 and 2
are plan and sectional elevation views of a portion of a prior art printed circuit board (PCB) employing a conductive via
12
to link a microstrip conductor
14
formed on an upper surface
16
of the PCB to a microstrip conductor
18
formed on a lower surface
20
of the PCB. Via
12
includes an upper cap (annular ring
22
) contacting conductor
14
, a lower cap (annular ring
24
) contacting conductor
18
and a vertical conductor
26
extending between upper and lower annular rings
22
and
24
. PCB
10
also includes embedded power and ground planes
28
formed on PCB substrate layers below upper surface
16
and above lower surface
20
, and may also include additional embedded power, ground or signal planes
30
. Via conductor
26
passes though holes in power signal planes
28
and
30
sufficiently large to prevent conductor
26
from contacting planes
28
and
30
.
FIG. 3
is an impedance model of the path a signal follows through conductor
14
, via
12
and conductor
18
. Microstrip conductors
14
and
18
, modeled by their characteristic impedances Z1 and Z2 respectively, are often sized and spaced with respect nearby power or ground planes
28
so that they have a standard characteristic impedance such as 50 Ohms. Upper and lower via annular rings
22
and
24
add shunt capacitances C1 and C2 to the signal path provided by vertical conductor
26
. An inductor L1 models the vertical conductor
26
. The model of
FIG. 3
could also include some shunt resistance to account for leakage though the insulating substrate surrounding via
12
, but at higher frequencies capacitances C1 and C2 and inductance L1 are the predominant influences on the via's frequency response.
Via
12
, which behaves like a three-pole filter or passive network, can severely attenuate and distort a high frequency signal traveling between conductors
14
and
18
. The series inductance L1 provided by vertical conductor
26
depends primarily on its vertical dimension. Since vertical conductor
26
must extend through PCB
10
, its length is fixed by the thickness of PCB
10
, and there is generally little leeway in adjusting the value of L1 . Thus the conventional approach to reducing signal distortion and attenuation caused by via
12
in high frequency applications has been to minimize the via's shunt capacitance. The shunt capacitance C1 and C2 can be reduced by reducing the horizontal dimension of annular rings
22
and
24
and by maximizing the distance between annular rings
22
and
24
and nearby power and ground planes
28
. However there are practical limits to the amount by which capacitances C1 and C2 can be reduced. Therefore appreciable via capacitance and inductance will always be present and will always cause some level of signal distortion and attenuation, particularly in high frequency signals.
FIG. 4
includes a plot A of the frequency response of a typical via that has been designed to provide minimal shunt capacitance and series inductance. The bandwidth of a filter is normally defined as the lowest frequency at which it attenuation reaches −3db. Plot A of
FIG. 4
shows that the bandwidth of via
22
is approximately 3.2 GHz. Thus a circuit board designer would normally want to avoid using such a circuit board via to conduct a signal of frequency higher than about 3 GHz.
The conventional approach to the use of vias in high frequency applications is therefore quite often to avoid them entirely. However a restriction against using vias can make it difficult to route large numbers of high frequency signals on a circuit board. In some high frequency applications short “blind” vias which do not extend completely through a PCB are used to link embedded stripline conductors formed on PCB layers that are vertically close to one another. Since blind vias are short, they have relatively little series inductions, and therefore usually have larger bandwidths than through vias extending completely through a PCB. However blind vias are more expensive than through vias, and still do not have sufficient bandwidth to handle very high frequency signals.
Therefore what is needed is a way to substantially increase the bandwidth of PCB vias so that they can conduct very high frequency signals without unduly attenuating or distorting them.
SUMMARY OF THE INVENTION
A printed circuit board (PCB) via provides a vertical signal path between microstrip or stripline conductors formed on separate horizontal layers of a PCB. The via adds shunt capacitance and series inductance to the signal path that are functions of shape and size of the via and of spacing between the via and nearby power and ground planes implemented in the PCB.
In accordance with one aspect of the invention, the capacitances of the via are adjusted with respect to one another and to the via inductance to values above their minimum practically attainable values for which frequency response characteristics of the via such as bandwidth are optimized.
In accordance with another aspect of the invention, in particular embodiments thereof, the via capacitances are adjusted so that the via behaves as a multi-pole Chebyshev or Butterworth filter.
In accordance with a further aspect of the invention, the via includes a capacitive element embedded within the PCB in contact with the signal path provided by the via. The element's shunt capacitance and the magnitudes of capacitances of other portions of the via are adjusted relative to via's inherent series inductance and to the impedance of the stripline or microstrip conductors to tune the via for optimal frequency response characteristics.
It is accordingly an object of the invention to provide a PCB via conducting high frequency signals without unduly attenuating them.
The claims portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements.
REFERENCES:
patent: 4342013 (1982-07-01), Kallman
patent: 4472725 (1984-09-01), Blumenkranz
patent: 5270673 (1993-12-01), Fries et al.
patent: 5424693 (1995-06-01), Lin
patent: 5536906 (1996-07-01), Haas, Jr. et al.
patent: 5719750 (1998-02-01), Iwane
patent: 6208225 (2001-03-01), Miller
patent: 6211541 (2001-04-01), Carroll et al.
patent: 6218910 (2001-04-01), Miller
patent: 2303495 (1997-02-01), None
patent: 04-107940 (1992-04-01), None
patent: WO 00/50905 (2000-08-01), None
patent: WO 00/51012 (2000-08-01), None
patent: WO 00/51232 (2000-08-01), None
Hayward, “Introduction to Radio Frequency Design,” pp. 59-68 (Prentice Hall 1982).
Hreish Emad B.
Miller Charles A.
Chang Joseph
FormFactor Inc.
Pascal Robert
Smith-Hill and Bedell
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