Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2002-06-27
2004-05-04
Wimer, Michael C. (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C333S116000
Reexamination Certificate
active
06731244
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly for optimization of dielectric circuit board materials for improved performance in four port circuits such as directional couplers.
2. Description of the Related Art
RF circuits such as quarter-wave transformers and directional couplers are commonly manufactured on specially designed substrate boards. For the purposes of RF circuits, it is important to maintain careful control over impedance characteristics. If the impedance of different parts of the circuit does not match, this can result in inefficient power transfer, unnecessary heating of components, and other problems. A specific type of transmission line circuit often used to combine or divide two RF signals or obtain a low-level sample of a signal. Because the electrical length of the directional coupler must be a quarter wavelength at the center frequency, the performance of directional couplers in printed circuits can be a critical design factor.
A directional coupler is a four-port circuit formed by two parallel transmission lines in close proximity. When a signal passes through one of the transmission lines, a portion of the signal is coupled into the other line forming a signal in the opposite direction with a phase lead of 90 degrees. Being a linear device, a directional coupler can also add two signals in phase quadrature with no loss in total signal power except for ohmic loss. The level of the coupled signal is determined by the cross-section dimensions in the two transmission line region. The proximity of the two conductors creates a region of two characteristic impedances called the even and odd modes. The even mode is based on the two lines carrying currents in the same direction while the odd mode is based on the currents in opposite directions. Each mode has a different characteristic impedance, Zoe and Zoo respectively. The two conductor coupling region typically has a line length precisely &lgr;/4, where &lgr; is the signal wavelength in the circuit. The proper characteristic impedance of a quarter-wave transformer is given by the formula Zo=root (ZoeZoo), where Zo is the desired characteristic impedance of the coupler, Zoe is the even mode impedance in the coupled region, and Zoo is the odd mode impedance in the coupled region. In a similar manner to a quarter wave transformer, multiple quarter wavelength coupler regions can be connected in series to achieve increased bandwidth. In these circuits, the coupling values are adjusted like the transformer circuit thereby changing the two-line geometry from coupling region to coupling region. The root of the even-mode and odd-mode impedances in every section are all equal to the same characteristic impedance, Zo.
Printed directional couplers, and in particular edge coupled directional couplers, used in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places both edge-coupled directional coupler conductors on the same board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the edge-coupled directional coupler is covered with a dielectric substrate material. In a third configuration known as stripline, the edge-coupled directional coupler is sandwiched within substrate between two electrically conductive (ground) planes. Those familiar with the art know that the same principle can be attached to directional couplers whose transmission lines are not coplanar. In this instance, there is a third dielectric layer with the two coupled lines etched on opposite sides. This configuration is called an overlapped coupler. If the two transmission lines are completely overlapped, the device is called a broadside directional coupler. Two critical factors affecting the performance of a substrate material are permittivity (sometimes called the relative permittivity or &egr;
r
) and the loss tangent (sometimes referred to as the dissipation factor). The relative permittivity determines the speed of the signal, and therefore the electrical length of transmission lines and other components implemented on the substrate. The loss tangent characterizes the amount of loss that occurs for signals traversing the substrate material. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to {square root over (L
1
/C
1
)} where L
1
is the inductance per unit length and C
1
is the capacitance per unit length. Within the coupling region there two values of inductance and capacitance per unit length for the even and odd modes. Their values are generally determined by the physical geometry and spacing of the line structures and even or odd mode currents, as well as the permittivity of the dielectric material(s) used to separate the transmission line structures. Conventional substrate materials typically have a relative permitivity of approximately 1.0
In conventional RF design, a substrate material is selected that has a relative permittivity value suitable for the design. Once the substrate material is selected, the two characteristic impedance values are exclusively adjusted by controlling the line geometry and physical structure.
The permittivity of the chosen substrate material for a transmission line, passive RF device, or radiating element influences the physical wavelength of RF energy at a given frequency for that line structure. One problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is optimized for all of the various passive components, radiating elements and transmission line circuits to be formed on the board. In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. Similarly, the line widths required for exceptionally high or low values of coupling (up to and including splitting the power into two equal parts) can, in many instances, be too narrow or too wide respectively for practical implementation for a given substrate. Since the physical size of the microstrip or stripline is inversely related to the relative permittivity of the dielectric material, the dimensions of a transmission line can be affected greatly by the choice of substrate board material.
Still, an optimal board substrate material design choice for some components may be inconsistent with the optimal board substrate material for other components, such as antenna elements. Moreover, some design objectives for a circuit component may be inconsistent with one another. For example, it may be desirable to reduce the size of an antenna element. This could be accomplished by selecting a board material with a relatively high dielectric. However, the use of a dielectric with a higher relative permittivity will generally have the undesired effect of reducing the radiation efficiency of the antenna. Accordingly, the constraints of a circuit board substrate having selected relative substrate properties often results in design compromises that can negatively affect the electrical performance and/or physical characteristics of the overall circuit.
An inherent problem with the foregoing approach is that, at least with respect to the substrate material, the only control variable for line impedance is the relative permittivity, &egr;
r
. This limitation highlights an important problem with conventional substrate materials, i.e. they fail to take advantage of the other factor that determines characteristic impedance, namely L
1
, the inductance per unit length of the transmission line.
Yet another problem that is encountered in RF
Killen William D.
Pike Randy T.
Harris Corporation
Sacco & Associates PA
Wimer Michael C.
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