Abstract:
A signal distribution structure including: a dielectric material; an overlying conducting layer on a first level of the dielectric material; a first signal line on a second level of the dielectric material, the first signal line being physically separated from the overlying conducting layer by the dielectric material; wherein the overlying conducting layer includes a window running parallel to the first signal line, and further comprising within the window a first coupler electrode on the first level of the dielectric material, the first coupler electrode above, parallel to, and electrically isolated by the dielectric material from the first signal line, wherein the first coupler electrode is electrically isolated from the overlying conducting layer along at least most of its periphery.

Description:
This application claims the benefit under 35 U.S.C. §119(e) of Provisional Application Ser. No. 62/015,604 filed Jun. 23, 2014, entitled “Method for Transmission and Coupling of Signals on Multi-Layer Boards with Minimum Interference,” the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The described inventions generally relate to high frequency signal distribution on Printed Circuits Boards (PCBs) and other multi-layer substrates and to line tapping via couplers. 
     BACKGROUND OF THE INVENTION 
     High frequency signal transmission is most commonly point-to-point. Power is generated at a source (the transmitter), and delivered to a load (the receiver) via a transmission line. In such cases, the receiver usually includes a terminating resistance that is equal to the characteristic impedance of the transmission line. The transmitted signal power is dissipated in this resistance, and no signal reflection occurs from the receiver. 
     In some applications, it is desirable to transmit a signal from a single transmitter to multiple receivers arranged sequentially along the line. In such cases, each of the receivers necessarily receives only a fraction of the transmitted power, since it is shared among all of the receivers. Furthermore, the signal at each receiver arrives with a unique delay with respect to the transmitter, determined by the receiver&#39;s location. Impedance discontinuities in the signal transmission path are undesirable in this case, since they give rise to reflections that interfere with the original transmitted signal. In such arrangements, a simple terminating resistance alone is not sufficient to guarantee the integrity of the signal. In addition, a means is necessary to couple a fraction of the transmitted signal power to each of the receivers that are arranged along the line. A necessary requirement for these couplers is that they must not create local impedance discontinuities that would cause signal reflections. 
     A converse situation also occurs in which multiple transmitters are arranged serially along the transmission line, connected to a common receiver at the line&#39;s end. In this case, it is desired to aggregate the signal power of all of the transmitters at the receiver. Similar to the above case, the various signals each have a unique delay determined by the transmitter&#39;s location with respect to the receiver. Also as in the above case, signal reflections are a source of interference. The requirements for the structures that couple signal power between the transmitters and the transmission line are the same as in the above case. 
     More generally, distribution networks like the ones described above may be combined using active circuitry such that in some modes of operation the circuit at the end of the line transmits to multiple receivers arranged serially along the line, and in other modes the circuit at the end of the line receives signals from multiple transmitters arranged serially along the line. 
     SUMMARY 
     The embodiments described herein employ a method for distributing signals in planar technologies such as PCBs. One or more couplers is provided to electrically couple signals from the transmission line to transmitters or receivers arranged along the line without causing appreciable signal reflection to occur. 
     In general, in one aspect, the invention features a signal distribution structure including: a dielectric material; an overlying conducting layer on a first level of the dielectric material; a first signal line on a second level of the dielectric material, the first signal line being physically separated from the overlying conducting layer by the dielectric material; wherein the overlying conducting layer includes a window running parallel to the first signal line, and further including within the window a first coupler electrode on the first level of the dielectric material, the first coupler electrode above, parallel to, and electrically isolated by the dielectric material from the first signal line, wherein the first coupler electrode is electrically isolated from the overlying conducting layer along at least most of its periphery. 
     In general, in another aspect, the invention features a signal distribution structure including: a dielectric material; an overlying conducting layer on a first level of the dielectric material; first and second parallel signal lines on a second level of the dielectric material, the first and second signal lines being physically separated from the overlying conducting layer by the dielectric material; wherein the overlying conducting layer includes a window running parallel to the first and second signal lines, and further comprising within the window a first coupler electrode and a second coupler electrode on the first level of the dielectric material, the first coupler electrode above, parallel to, and electrically isolated by the dielectric material from the first signal line and the second coupler electrode above, parallel to, and electrically isolated by the dielectric material from the second signal line, wherein each of the first and second coupler electrodes is electrically isolated from the overlying conducting layer along at least most of its periphery. 
     Other embodiments have one or more of the following features. The signal distribution structure further includes an underlying conducting layer on a third level of the dielectric material, wherein the second level of the dielectric material is between the first and third levels of the dielectric material. Each of the first and second coupler electrodes are islands of metal physically separated from each other and physically separated from the overlying conducting layer. The signal distribution structure also includes a first resistive element electrically connecting one end of the first coupler electrode to the overlying conducting layer and a second resistive element electrically connecting one end of the second coupler electrode to the overlying conducting layer. Each of the first and second coupler electrodes physically contacts the overlying conducting layer at one end. The first coupler electrode includes a contact pad area for electrically connecting to the first coupler electrode and the second coupler electrode includes a contact pad area for electrically connecting to the second coupler electrode. The contact pad areas on the first and second coupler electrodes are located at one end of the first and second coupler electrodes, respectively. The first and second coupler electrodes are equidistant from the first and third levels on which the overlying and underlying conductive layers, respectively, are located. The signal distribution structure also includes a plurality of electrically conducting vias passing through the dielectric material and electrically connecting the overlying and underlying conducting layers together. The first signal line that is beneath the first coupler electrode is of a different width as compared to the portion of first signal line that is beneath the overlying conducting layer and the portion of the second signal line that is beneath the second coupler electrode is of a different width as compared to the portion of second signal line that is beneath the overlying conducting layer. The dielectric material and the overlying and underlying conducting layers are fabricated using printed circuit board technology. The first and second signal lines in combination with the overlying and underlying conducting layers form a shielded, differential transmission line. 
     In general, in yet another aspect the invention features a signal distribution system including a transmission line having a sequence of coupler structures of the type described above arranged sequentially along the transmission line from a first end to a second end. 
     Other embodiments may include one or more of the following features. The transmission line includes: a dielectric material; an overlying conducting layer on a first level of the dielectric material; first and second parallel signal lines on a second level within the dielectric material, the first and second signal lines being physically separated from the overlying conducting layer by the dielectric material. Each coupler structure includes: a window in the overlying conducting layer running parallel to the first and second signal lines; and within the window a first coupler electrode and a second coupler electrode on the first level of the dielectric material, the first coupler electrode parallel to and electrically isolated by the dielectric material from the first coupling line and the second coupler electrode parallel to and electrically isolated by the dielectric material from the second coupling line, wherein each of the first and second coupler electrodes is electrically isolated from the overlying conducting layer along at least most of its periphery. 
     Still other embodiments may include one or more of the following features. The signal distribution system also includes an underlying conducting layer on a third level of the dielectric material, wherein the second level of the dielectric material is between the first and third levels of the dielectric material. The coupler electrodes of each successive coupler structure of the plurality of coupler structures are longer than the coupler electrodes of the previous coupler structure in the sequence of coupler structures. Within each coupler structure of the sequence of coupler structures, each of the first and second coupler electrodes are islands of metal physically separated from each other and physically separated from the overlying conducting layer in which case each coupler structure of the sequence of coupler structures may further include a first resistive element electrically connecting one end of the first coupler electrode of that coupler structure to the overlying conducting layer and a second resistive element electrically connecting one end of the second coupler electrode of that coupler structure to the overlying conducting layer. Within each coupler structure of the sequence of coupler structures, each of the first and second coupler electrodes of that coupler structure physically contacts the overlying conducting layer at one end. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a simple point-to-point signal interconnection. 
         FIG. 2  shows a schematic diagram of signal distribution from a common transmitter to multiple receivers arranged serially along a transmission line. 
         FIG. 3  shows a schematic diagram of signal distribution from multiple transmitters arranged serially along a transmission line a common receiver. 
         FIG. 4  shows a schematic diagram of a general bi-modal serial signal distribution network. 
         FIG. 5  shows a cross-section of a typical structure used to implement a uniform shielded differential transmission line in a multi-layer planar technology. 
         FIG. 6  shows a coupler structure for use in connection with the transmission lines shown in  FIG. 8 . 
         FIG. 7  shows a plan view of a capacitive type coupler. 
         FIG. 8  shows a plan view of an inductive type coupler. 
         FIG. 9  shows a plan view of a coupler with resistive connections to the ground plane. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic diagram of a common circuit for point-to-point signal transmission. This simple arrangement includes a transmitter  11 , a receiver  12 , and a transmission line  1 . The physical transmission line typically consists of an electrically conducting pathway (for example, a wire) facilitating current flow from the transmitter&#39;s output to the receiver&#39;s input, and a second conducting pathway to allow for an opposing current to return from the receiver to the transmitter. In systems including more than one such pathway, the return pathway is sometimes shared by multiple transmission lines, in which case the transmission line is often referred to as “single ended”. In other such systems, each of the transmission line is provided with a unique return pathway, in which case the transmission line is often referred to as “differential”. 
     The various frequency elements of the transmitted signal propagate as waves along the transmission line. For the higher frequency elements in particular, the physical length of the transmission line may be an appreciable fraction of the wavelength of that element. In such cases, the integrity of the received signal may be degraded by the presence of reflected signals on the transmission line. To maintain signal integrity, it is common practice to provide a terminating resistance  20  at the receiver. If the value of this terminating resistance matches the characteristic impedance of the transmission line, then the signal power is dissipated in the resistance and no signal reflection occurs. 
     In some applications, it is desirable to transmit a signal to multiple receivers arranged along the transmission line, as shown in  FIG. 2 . In such an arrangement, the receivers share the total transmitted power, so each receives only a fraction of the transmitted power. To facilitate this arrangement, it is advantageous to provide a coupler  30  for coupling the signal from the transmission line to each receiver. In addition to controlling the signal coupling between the transmission line and the receiver, the coupler  30  should not cause an appreciable local change in the characteristic impedance of the transmission line, since this would cause an undesirable signal reflection that would degrade the integrity of the received signals. Also, in the usual event that some of the signal power is not distributed to the receivers, a resistance  20  is provided at the termination of the transmission line to avoid reflection of the power at that point. 
       FIG. 3  shows the converse situation in which multiple transmitters arranged along the line deliver signal power to a common receiver. A similar coupler structure may be used in this arrangement. As in the above case, it is similarly desirable for the coupler to introduce negligible local changes in the transmission line&#39;s characteristic impedance. 
     A generalization of these arrangements is shown in  FIG. 4 , in which bi-modal transmitter-receiver circuits  13  are used. With this type of arrangement, multiple receivers or transmitters may share common transmitters or receivers, respectively, depending on the chosen operating mode of the active circuitry. The same design considerations for the coupling structures apply in either mode. 
     Low-Reflection, Compact, Broadband Differential Couplers 
     In addition to the signal lines themselves, a critical structure in the serial distribution network shown in  FIG. 4  is the coupler  30 . To maintain good signal integrity, a desirable property of the couplers is that they introduce minimal signal reflections in the distributing transmission line. This requires the characteristic impedance of the main transmission line to be unperturbed by the presence of the coupler. Meeting this criterion is complicated by the manufacturing variations that typically exist in most planar technologies. These variations lead to variations in the characteristic impedance of the transmission lines. Consequently, it is desirable to devise coupling structures in which manufacturing variations affecting the transmission lines also affect the couplers in a similar fashion, so that the signal reflections are minimized regardless of the variations. 
     Directional couplers are commonly used in microwave systems to divert a portion of the signal energy from a primary signal transmission line onto another secondary line without causing appreciable signal reflection. In planar technologies, directional couplers are usually formed by a pair of transmission lines whose lengths are equal to one-quarter of the signal wavelength in the transmission line at the desired center operating frequency. The two lines are placed in proximity, resulting in electromagnetic coupling between them. With appropriate choice of structure dimensions, it is possible to realize couplers with desired properties of coupling strength and characteristic impedance. In addition, these structures exhibit the property of directivity, in which the signal that is coupled from one line to the other preferentially flows in one direction. Sophisticated couplers intended to exhibit coupling strength and directivity over a wide frequency range are made by cascading multiple quarter-wavelength sections of coupled line. 
     When implemented in the same planar structure as the lines to which they connect, these conventional directional couplers exhibit the desired property of insensitivity to manufacturing variations described above. The structure of the coupled transmission lines in the coupler is usually similar to the structure of the main transmission lines to which the coupler connects. Consequently, structural and material variations that affect the characteristic impedance of the transmission lines typically have a similar effect on the characteristic impedance of the coupler. A disadvantage of these types of coupler in some applications is their large physical size. 
     The embodiments described below can be used in applications requiring compact structures. In such cases, it is possible to obtain suitable coupling over a wide frequency range in a more compact structure that is substantially shorter than one quarter of the signal wavelength. 
       FIG. 5  shows a cross-section of a typical structure used to implement a uniform, shielded differential transmission line in a multilayer planar technology such as a PCB. In the structure, the differential pair of signal lines  60  and  61  are formed in a layer of metal with an overlying layer  62 , and an underlying layer  63  forming a surrounding shielding structure. These shielding layers are usually connected electrically by vertical conducting vias  70  placed at intervals along the length of the lines normal to the cross-section. The electrical potential of the surrounding shield structure is typically taken as a reference (“ground”) and the electrical potential on the differential signal lines varies symmetrically with respect to this reference, with equal amplitude and opposing sign. The region between the metal planes is filled by a dielectric  90 . 
     In a differential transmission line, the widths  81  of the two signal lines  60  and  61  are identical. The differential characteristic impedance is determined by the line width  81  and spacing  80 , as well as the vertical dimensions  82 ,  83  and  84  of the structure. 
     Line widths and spacings are usually comparable to the dielectric thickness. For example, in the described embodiment, the thickness of each dielectric layer is 250-micron (0.25 mm), the line width is 200 micron, and the spacing is 250. 
     The coupler structure is shown in cross-section in  FIG. 6 . The signal lines  64  and  65  within the coupler connect directly to the signal lines  60  and  61 , respectively, in the transmission line outside the coupler region, but their dimensions  85  and  86  may be different. The coupler electrodes  66  and  67 , having the same widths, are formed in the outer shield  62 . These electrodes extend for some desired length along the lines. In operation, a signal passing along the differential transmission line  64  and  65  will induce a differential voltage and/or current in the coupler electrodes  66  and  67 , respectively. The coupled signal strength is determined, in part, by the length of the electrodes. 
     It can be seen from inspection of  FIG. 5  and  FIG. 6  that the structure of the signal lines in the two is similar. Based on electromagnetic simulation and analysis, dimensions for the lines in the coupler,  85  and  86 , can be chosen such that the characteristic impedance of the lines in the coupler are a close match to the lines outside the coupler, thereby minimizing signal reflections at the junction between the two. If the induced voltage on the coupler electrodes is small relative to the voltage on the signal lines, then the field distribution surrounding the transmission line is not greatly perturbed by the presence of the electrodes, because their potential is not greatly different from that of the shield. In that case, the signal line widths within and external to the coupler,  86  and  81 , will be similar, as will the spaces  85  and  80 . 
     In the case that the dimensions of the lines within the coupler and external to the coupler are adjusted to result in similar characteristic impedances, and the dimensions of the lines are similar in the two cases, structural and material variations in manufacturing that affect the characteristic impedance of one have a similar effect on the other. In such cases, when the couplers are used in serial distribution networks like the ones described above, they will not significantly degrade the signal integrity through the introduction of signal reflections. Moreover, manufacturing variations that cause the characteristic impedance of the transmission lines to change will cause similar variations in the coupler structure so that the signal integrity is maintained. 
     Coupler structures of this type can be configured in three different ways.  FIG. 7  shows a plan view of a capacitive type coupler. In this figure, the coupler electrodes  66  and  67  are electrically isolated from the surrounding shield (ground)  62  as indicated by an open area (metal-free region) surrounding the islands of metal film representing the coupler electrodes. The signal lines  64  and  65  pass beneath the coupler electrodes. Contact areas  68  and  69  are provided off to the side to allow connection of the coupler to external circuits. Couplers of this type are predominantly capacitive, because voltages are induced in the coupler electrodes but little current can flow. When connected to high impedance sensing circuitry, these couplers have relatively constant voltage gain characteristics over a wide frequency range. Voltage gain is adjusted by changing the length of the coupler electrode. 
     A plan view of a similar inductive coupler is shown in  FIG. 8 . It has the same structural features as its capacitive counterpart. The only structural difference is that the electrodes at the right-hand side of the figure (i.e., the end opposite from the end with the contact pads) are connected to the ground plane  62 . This coupler type is predominantly inductive because currents are induced in the coupler electrodes, but little voltage is developed. When connected to low impedance active circuitry, this type of coupler has a relatively flat current gain over a wide frequency range. 
     A plan view of a resistively connected coupler is shown in  FIG. 9 . In this structure, added resistors,  100 , connect the electrodes to the ground plane. Because of the resistive connection, the signals induced on the coupler electrodes are a combination of current and voltage, so this coupler type is a hybrid of the types shown in  FIGS. 7 and 8 . Moreover, with suitable choice of structure dimensions and value of resistance used in the connections,  100 , these couplers exhibit the desirable property of directivity even if they are substantially shorter than the quarter-wavelength used in conventional directional couplers. Conventional quarter-wavelength couplers have a constant amplitude frequency response near the corresponding quarter-wavelength frequency. These short couplers generally show greater frequency dependence in the amplitude of their response. 
     As described above, these types of couplers are useful in applications requiring the serial distribution of a signal to many receivers, as shown in  FIG. 2 . In this case, it is often desirable to maintain nearly constant signal strength at all receivers  12 . However, because of signal losses inherent in the distribution transmission lines  1  and because each coupler in the chain extracts a small amount of power from the line, the signal strength in the line will decay with increased distance from the transmitter  11 . These losses can be compensated by modifying the length of each coupler in the chain so that couplers near the transmitter, where the signal is strongest, are shorter, giving them less gain, and couplers at the end of the chain, where the signal is weakest, are longer, giving them more gain. With suitable adjustments to the length of the couplers, the signal strength at each receiver can be made the same. 
     A similar situation occurs in applications requiring the serial aggregation of many signals, as shown in  FIG. 3 . In this case, the signals originating furthest from the receiver  20  will suffer greatest attenuation as they travel along the distribution line  1 . This attenuation can be compensated by adjusting the length of the couplers so that the gain from each transmitter  11  to the common receiver  20  is nearly equal (e.g. by making the length of the couplers farthest from the receiver longer). In this way, the aggregated signal that is received represents a nearly equal summation of all of the individual transmitted signals. 
     Conventional directional couplers are discrete components that are directly physically connected to the distribution lines. To insert them into a distribution line, it is necessary to break the line. So, the distribution line enters the coupler at one terminal port and emerges from another terminal port. Internal connections in the coupler provide the continuity of the electrical path between the entering and emerging lines. Because the main distribution line and the coupler are made in two distinct technological implementations, manufacturing variations will cause them to differ slightly in the transmission line characteristics, resulting in impedance discontinuities and signal reflections. These reflections may impair the circuit&#39;s function, especially in serial distribution networks where their effects may be cumulative. Integrated couplers of the type described above do not suffer from this. The composite structure consisting of two or more such couplers arranged in a serial configuration implemented in a common technology constitutes a novel structure with superior qualities compared with the conventional discrete implementation. 
     In the illustrated embodiments of  FIGS. 7, 8, and 9 , the contact or pad areas  68  and  69  are shown as being located off to the side so that they do not overly the coupler signal lines  64  and  65  on the lower layer. This was done because it was assumed that the three layers of metal described in this disclosure have other layers above these, i.e., they are part of a larger multi-layer stack. In other words, it was assumed that the two pads  68  and  69  would be buried in the middle of the stack. In that case, to make a vertical connection to them, drilled vias would need to be used. If the pads were on top of the lines, then the drill would pass through the signal lines and they would also be shorted to the via formed in the drill hole. So the pads are offset to avoid interference with the lines. 
     For some types of board construction this would be unnecessary. If the overlying metal layer is at the top of the stack, the concern about shorting to lower metal layers would not be an issue, in which case the contact areas could be directly above the coupler signal lines. 
     Also, note that for the capacitive coupler the pads  68  and  69  can be located anywhere along the length of the coupler. However, for the inductive type coupler, they need to be at the end opposite the point where they merge into the ground plane. 
     It is usually desirable for the couplers to be short compared with one-half wavelength in the line, but that&#39;s not a strict requirement. To keep the impedance nearly constant, a key requirement of the coupler is that it have voltage gain much less than unity. This ensures that the coupler electrodes are approximately similar to the surrounding ground planes, and don&#39;t perturb the potential distribution a lot. Longer couplers build up greater gain. At sufficiently large length, they start to behave strangely, and may exhibit undesirable resonances. 
     Conventional directional couplers are made of quarter-wavelength transmission line sections. Wideband couplers require a cascade of several such quarter-wave sections, so they may be quite large. For example, at 1 GHz a simple quarter-wave directional coupler would be 1.5 inches long. The intermediate-frequency couplers in a board that operates at a center frequency of 110 MHz would need to be 13.6 inches long. For comparison, these same couplers using the structure described herein are less than one inch long (with line widths and separations of 200 and 250 microns, respectively). 
     For PCBs a variety of well known fabrication methods may be used. Mainly, they are built up from sheets of dielectric (usually glass-reinforced resin) covered on two sides with copper foil. The dielectrics are usually a fiberglass matrix impregnated with an organic dielectric (epoxy, polyimide, lacquer, Teflon, etc). There are many proprietary resins and glass formulations. Conductor patterns are etched into the foil. Multiple layers of these sheets are aligned and stacked with intervening layers of more dielectric in an uncured state. They are pressed together and heated to laminate the multilayer structure. Then, holes are drilled and plated with metal to form the vertical layer-to-layer vias. 
     The technology described herein has particular applicability to multi-point signal generation networks and low cost antenna arrays such as those described in U.S. Pat. Nos. 8,259,884 and 8,611,959, respectively, the contents of which are incorporated herein in their entirety. For example, referring to  FIG. 5  in that &#39;884 patent, there is shown two tree-networks, one of which carries a first carrier signal and the second of which carries a second carrier signal. At various places the branch of one tree network runs alongside (e.g. “parallel”) to a corresponding branch of the other tree network. For that pair of branches, one carries the first carrier signal in one direction and the other branch carries the second carrier signal in the other direction. Along the length of the dual parallel transmission lines, there are number of Arrival-Time-Averaging Circuits (ATACs), each with one input connect to one of the transmission lines and a second input connected to the other transmission line. The connections to the ATAC circuits can be implemented using the couplers described herein to reduce the impact that those circuits have on the signals passing over the dual transmission lines. 
     Other embodiments are within the following claims.