Wave transmission lines and networks – Long line elements and components – Strip type
Reexamination Certificate
2001-09-21
2003-02-25
Tokar, Michael (Department: 2819)
Wave transmission lines and networks
Long line elements and components
Strip type
C333S034000, C333S128000, C333S238000, C333S206000, C333S02200F
Reexamination Certificate
active
06525631
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a microstrip termination. More particularly, the present invention relates to a microstrip termination using a grounded thin film resistor.
BACKGROUND
Terminations are common components in most microwave systems. Microstrip terminations are easy to process using thin film technology, but the performance drops off rapidly with increasing frequency. Thin film technology typically uses an alumina substrate, with gold and resistor material sputtered onto the substrate, which is then patterned with photolithography techniques to define transmission line traces and resistors. Thick films could also be used, but they typically do not go to high frequencies (above 20 Ghz).
FIG. 1
illustrates a standard microstrip termination, known as an edge ground circuit. In the microstrip
100
of
FIG. 1
, a microstrip transmission line
104
, typically a metal line, is formed on the microstrip substrate
102
, made of a dielectric such as alumina. An area of resistive material
106
is formed on the substrate
102
along the transmission line
104
near an edge ground. The edge ground is formed with a transmission line
110
connecting the resistive material
106
to the metal plated edge
108
which connects to a metal ground plane
112
deposited on the bottom surface of the substrate below the trace and resistive material. The resistive material
106
is used to terminate a signal propagating along the transmission line by matching the impedance of the transmission line and preventing reflection of the propagating signal.
FIG. 2
illustrates another standard microstrip termination used when a termination is required away from an edge. This termination
200
also includes a microstrip substrate
202
typically having a metal bottom layer
212
, a transmission line
204
, and an area of thin film resistive material
206
. The substrate
202
also has an area of metal
208
between the resistive material
206
and the edge of the substrate
202
. The substrate
202
typically contains Monolithic Microwave Integrated Circuits (MMICs) connected to the transmission line
204
, and the substrate
202
is mounted on a carrier. A carrier is typically a thin metal plate, on the order of ½ to 1 mm thick, and provides the ground for the microstrip substrate and the MMICs thereon in addition to the metal bottom layer
212
.
The termination of
FIG. 2
further uses a ground via
210
. The via
210
is formed from metal deposited in a hole in the substrate that extends from the area of metal
208
on the top surface of the substrate to the metal bottom layer
212
. The termination shown in
FIG. 2
can be placed anywhere in a subsystem circuit, but the performance is generally worse than the edge ground circuit of FIG.
1
. The poor performance is due to the increased inductance to ground resulting from the small via.
FIG. 3
shows the typical performance of an edge grounded microstrip termination. One reason for the poor performance illustrated in the figure is that the “environment” of the resistor is not correct.
It is desired that a DC to microwave termination be a reflectionless transition from the transmission line impedance of Zo to ground. The reactive part of the transition should, therefore, match the resistive part from the Zo ohm line to ground. For instance, if the midway resistance of a 50 ohm termination resistor is 25 ohms, the surrounding reactive environment is preferably also 25 ohms. In a coaxial termination, the outer conductor diameter over the resistor is tapered down from the 50 ohm diameter to the diameter of the resistor at the ground end, to provide a smooth impedance transition “environment.” It is therefore desirable to provide a microstrip termination that sufficiently prevents signal reflection by providing a smooth impedance transition.
BRIEF SUMMARY
The present invention provides a microstrip substrate that has a tapered edge used to form a tapered impedance ground plane, which provides a smooth impedance transition to ground for a termination. To form the termination, the substrate has a transmission line on the top surface and a metal ground layer on the bottom surface. An area of thin film resistive material is positioned on the top surface between the transmission line and the tapered ground plane. A layer of conductive material extends from the top surface across the tapered edge to the bottom metal ground layer. The tapered ground plane is at an angle with respect to the top surface of the substrate to provide a smooth change in impedance for a signal propagating through the thin film resistor.
REFERENCES:
patent: 3638148 (1972-01-01), Hallford et al.
patent: 3967218 (1976-06-01), McGowan
patent: 5160904 (1992-11-01), Babbitt et al.
patent: 6127908 (2000-10-01), Bozler et al.
U.S. patent application Ser. No. 09/960,891, Oldfield, filed Sep. 21, 2001.
Caldwell Barry and Don Getty, EDN Access-07.06.2000 Coping with SCSI at Gigahertz Speeds (Printed Apr. 25, 2001)http://www.ednmag.com.
CMD Products (Printed Apr. 25, 2001) http://www.calmicro.com.
Introduction to Transmission Lines (Experimental) (Printed Apr. 25, 2001) http://onlinetools.chipcenter.com.
Maloratsky G. Leo, Reviewing the Basics of Microstrip Lines (Printed Apr. 25, 2001) http://www.metcladinternationla.com.
Anritsu Company
Fliesler Dubb Meyer & Lovejoy LLP
Mai Lam T.
Tokar Michael
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