Apparatus for broadband matching

An apparatus is provided. The apparatus comprises a substrate and a circuit trace. The substrate includes a region that is adapted to receive a discrete component, a metal layer, a dielectric layer formed over the metal layer, a window formed in the metal layer that underlies the region, and a conductive strap that extends across the window. The circuit trace is formed on the dielectric layer and is discontinuous across the region.

TECHNICAL FIELD

The invention relates generally to a printed circuit board (PCB) and, more particularly, to a PCB having broadband matching structures.

BACKGROUND

For PCBs that operate at high frequencies (i.e. about 25 GHz), blocking capacitors are often employed. These discrete capacitors are usually placed in the signal flow path to block unwanted direct current (DC) signals. While these discrete capacitors (or other discrete components, like resistors) may (and usually do) represent a small impedance discontinuities due to the relatively small value (i.e., 1 μF represents 0.2 mΩ at 1 GHz), the discrete component is physically much larger that the trace or transmission line to which it is coupled; this physical size can cause a capacitive discontinuity in the transmission line or trace, which can limit the frequency performance.

One example of a conventional solution for this capacitive discontinuity is to make adjustments to the ground plane within the PCB, as shown inFIGS. 1 and 2. In this example, the discrete component106(which can, for example, be a capacitor) is coupled to trace108that is formed on PCB102. Within the PCB102, there is a metal layer that forms a ground plane104. In a region that underlies the discrete component106, there is a window110that is formed in the metal layer104. The hole or window110in the metal layer104decreases coupling to ground by trace108and increases the inductance to ground for the ground return current. This has the overall impact of raising the impedance of the discontinuity so as to achieve better matching.

There is, however, a drawback to this conventional solution. Namely, there is a frequency limit under which this solution will function. Thus, there is a need for an apparatus that allows matching at higher frequency ranges.

SUMMARY

A preferred embodiment of the present invention, accordingly, provides an apparatus. The apparatus comprises a substrate having: a region that is adapted to receive a discrete component; a metal layer; a dielectric layer formed over the metal layer; a window formed in the metal layer that underlies the region; a conductive strap that extends across the window; and a circuit trace formed on the dielectric layer and being discontinuous across the region.

In accordance with an embodiment of the present invention, the substrate further comprises a circuit board.

In accordance with an embodiment of the present invention, the dielectric layer further comprises a first dielectric layer, and wherein the metal layer is formed over a second dielectric layer.

In accordance with an embodiment of the present invention, the metal layer is formed of aluminum or copper.

In accordance with an embodiment of the present invention, the circuit trace is about 100 μm and has an impedance of about 50 Ω.

In accordance with an embodiment of the present invention, the window has a width that is greater than 500 μm.

In accordance with an embodiment of the present invention, an apparatus is provided. The apparatus comprises a printed circuit board (PCB) having: a first dielectric layer; a metal layer formed over the first dielectric layer; a second dielectric layer formed over the metal layer; a window formed in the metal layer; a conductive strap that extends across the window; a first portion of a circuit trace formed on the second dielectric layer; and a second portion of the circuit trace formed on the second dielectric layer, wherein the first and second portions of the circuit trace are separated from one another in a region that is substantially aligned with the window; a discrete component that is secured to the first and second portions of the circuit trace within a path that is substantially parallel to the conductive strap.

In accordance with an embodiment of the present invention, the discrete component is a resistor or a capacitor.

DETAILED DESCRIPTION

Turning toFIGS. 3-5, an example of a PCB202with a broadband matching structure in accordance with an embodiment of the present invention can be seen. As shown, the PCB202is generally comprised of several layers, and, there is a metal layer204(which can be formed of, for example, aluminum or copper) sandwiched between dielectric layers. There can also be several additional dielectric and metal layers. Formed on the PCB202is circuit trace208(which can, for example, be about 100 μm wide and have a nominal impedance of about 50 Ω or may have an impedance between 30 Ω and 70 Ω for single-ended and 60 Ω and 140 Ω for differential). The circuit trace208has two portions that extend to a region that is adapted to receive discrete component206(which can, for example, be a discrete resistor or discrete capacitor and which can, for example, have a width of about 500 μm). The circuit trace208(which can, for example, have a width of about 50 μm, but is typically a function of the thickness of the dielectric layer(s) between the metal layer204and trace208) is discontinuous in this region, and the discrete component208is coupled therebetween. In the metal layer204, a window or hole210(which can, for example, have a width greater than 500 μm and usually about 1.5 mm by about 1.5 mm) is formed therein such that the window210underlies or is substantially aligned with the region that is adapted to receive the discrete component206. Additionally, a strap212(which can be part of the metal layer204) extends across the window210. These windows210and straps212should also be included in any other plane (metal) layers within the PCB202.

With strap212, several advantages can be realized. This strap212provides a more direct return path for current within the ground plane (formed by metal layer204), and, because the strap212has a narrow width, a small amount of capacitance is added, while the inductance to ground is increased. The shape of the strap212can also be varied (i.e., curvilinear) to increase inductance to a desired level without significantly affecting current flow. As a result of employing this strap212, an impedance matching of 20 dB to 50 GHz and better than 12 dB up to 100 GHz, compared to 20 dB to 30 HGz and 12 dB to 40 GHz with the PCB ofFIGS. 1 and 2.

With the system ofFIGS. 1 and 2, having several traces (i.e.,108) located in proximity to one another may be problematic because the windows (i.e.,110) would disturb the ground plane104because of large discontinuities in the ground plane104. By using the straps201-1and206-2(as shown inFIG. 6), the problem can be avoided. Because these straps206-1and206-1-2for two adjacent traces208-1and208-2(depicted in this example) provide current paths for the ground plane204, the ground plane204would generally function normally (i.e., undisturbed). This allow for much higher density than would be achievable with the conventional system shown inFIGS. 1 and 2.