Microstrip dot termination usable with optical modulators

Details Microstrip dot termination usable with optical modulators Microstrip dot termination usable with optical modulators

2001-09-06

2003-07-15

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Dissipating terminations for long lines

Fluid-cooling

C359S327000

Reexamination Certificate

active

06593829

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to microstrip terminations, and to terminations used with optical modulators.
BACKGROUND
Terminations are common components in most microwave systems. Microstrip terminations are easy to manufacture using thin film technology, but the performance typically drops off rapidly with increasing frequency. Thin film technology typically uses an alumina substrate, with gold and resistor material sputtered onto it and then patterned with photolithography techniques to define microstrip transmission line traces and resistors. Thick films could also be used, but the thick film resistors do not function well at high frequencies (above 20 GHz).
FIG. 1
illustrates one 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 trace
110
connecting the resistive material
106
to the metal plated edge
108
which connects to a metal ground region
112
deposited on the bottom surface of the substrate. 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 microstrip termination typically used when grounding is desired away from a substrate 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 a via
210
between the ground side of the resistor
206
and the bottom metal of the substrate
202
. The substrate
202
often contains Monolithic Microwave Integrated Circuits (MMICs) connected to the transmission line
204
and the substrate
202
is often 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
.
This 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.
A microstrip termination that provides acceptable performance at high frequencies over a wide bandwidth, but not at low frequencies or to DC, is the dot termination. Dot terminations are high return loss terminations capable of performing adequately at high frequency and over a wide bandwidth. Dot terminations typically do not require a ground.
FIG. 3
shows a dot resistor
300
of the prior art. This dot resistor
300
typically includes a circular area of thin film resistive material
302
. The circular area of resistive material
302
typically has a protruding region of resistive material, or tongue
304
, which extends from the circular area and into contact with a metal trace
306
forming a transmission line. The thin film resistive material of the tongue
304
extends under the metal trace
306
, assuring an overlap or connection between the metal trace
306
and the resistive tongue
304
.
The resistance of the resistive material is typically about 50 ohms per square. Ohms per square is a unit of measure known and used in the art to describe the surface resistivity of a material, typically measured with a four point probe. With the four point probe, the resistance is measured by passing a fixed current though two points and measuring the voltage at the other two points. By controlling the input current, the surface resistance equals the voltage across the pair of test points, such that the units of distance drop out.
The size of the circular area, or “dot”, determines the low frequency limit of the termination. Dot diameters up to 15 times the trace width will typically perform to the upper frequency limit of a microstrip. Minimum dot diameters are typically at least three times the trace width. As an example, Table 1 shows the appropriate 20 dB and 15 dB low end frequencies of various dot sizes on a 10 mil alumina substrate.
TABLE 1
Frequency v. dot size
Dot diameter
20 dB Frequency GHz
15 dB Frequency GHz
1.2 mm
15
11 
2.0 mm
11
8
2.5 mm
10
7
4.0 mm
 9
3
The typical return loss performance of a dot termination at high frequencies, such as up to about 110 GHz, is better than 25 dB.
SUMMARY
In accordance with the present invention, a dot termination composed of a circular thin film resistive material connects a transmission line to a ground plane in a manner to provide a broadband high frequency performance that also goes to DC. The dot termination can use traces provided around the perimeter of the dot resistive material with vias connecting the traces to ground to provide multiple DC paths to ground. Each trace is formed with a metal portion connecting each ground via to a resistive trace portion which connects to the resistive dot material. A resistive tongue trace connects the dot material to a metal trace forming a transmission line providing a signal to the dot termination. The use of multiple DC ground paths allows the DC resistance to be approximately 50 &OHgr; without destroying the high frequency performance.
In accordance with the present invention, the dot termination can be used in a shunt configuration with an optical modulator to provide voltage biasing for the optical modulator. To maximize the biasing voltage, a DC blocking capacitor can be placed between the dot termination and ground. Biasing current can be applied at the connection of the dot termination and the optical modulator. Preferably to enhance performance, biasing current is applied between the dot termination and the blocking capacitor.


REFERENCES:
patent: 4291415 (1981-09-01), Buntschuh
patent: 5513390 (1996-04-01), Vice
patent: 5686872 (1997-11-01), Fried et al.
patent: 6044097 (2000-03-01), Kawamura et al.
patent: 0279601 (1989-11-01), None
Oldfield Bill, “Connector and Termination Construction above 50 Ghz,” Applied Microwave & Wireless Magazine, Apr. 18, 2001, pp. 56, 58, 60, 62, 64 and 66, Nobel Publishing.

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