Patent Publication Number: US-9433078-B2

Title: Printed circuit boards with embedded electro-optical passive element for higher bandwidth transmission

Description:
CLAIM OF PRIORITY 
     The present application for patent claims priority to U.S. Provisional Application No. 61/557,883 filed Nov. 9, 2011, which is hereby expressly incorporated by reference. 
    
    
     BACKGROUND 
     1. Field 
     Various features relate to multilayered printed circuit boards and more specifically to using embedded electro-optical passive elements to reduce electromagnetic interference, improve signal loss, and adjust, remove, or minimize frequency response notches at a frequency band of interest. 
     2. Background 
     High speed signal transmissions in printed circuit boards (PCBs) have an inherent impedance mismatch between the various components of the PCB, such as plated through holes or vias, signal traces and open propagation mediums. The impedance mismatches cause a significant impediment to obtaining lower loss and flat data transmission in high data rate (i.e. 8 to 25+ Giga bits per second) protocols such as Peripheral Component Interconnect Express (PCI-Ex), Gen 3, Institute of Electrical and Electronics Engineers (IEEE) 802.3ba, and Optical Interconnect Forum (OIF) Common Electrical Interface (CEI) 25G Long Reach (LR) standards. Some of these designs require high number of layers and a relatively thick PCB construction with long vias used to route signals between various layers in the PCB. These vias yield significant undesirable interference due to large electromagnetic reflections for the unused portion of an interconnection via back onto the signal line. Typically, to circumvent this undesirable interference, the unused stub of the via is back drilled up to near the signal layer. 
       FIG. 1  illustrates a via channel having an open-ended stub located within a PCB. The PCB  102  may include a plurality of non-conductive layers  104  (e.g., dielectric layers) with conductive layers  106  (e.g., reference/ground layers and/or signal layers) in between. Open-ended paired vias  108  and  110  may traverse the plurality of PCB layers. The open-ended paired vias may include a signal via  108  and a reference/ground via  110 . The signal via  108  may be coupled to a first signal trace  103  (on a top side of the PCB  102 ) and a second signal trace on the signal layer  106 . The reference/ground via  110  may be coupled to a reference/ground layer  107 . 
     A via channel  100  comprises primarily a dielectric medium bounded by current carrying rails (via barrels  108  and  110 ). The via channel  100  is a region across the thickness of the PCB  102  that is made up primarily of the non-conductive layers (e.g., dielectric material) of the PCB  102  but also includes thin signal layers and/or conductive layers (e.g., typically thin or foil signal layers and/or conductive layers). The current carrying rails may include the signal via  108  and the reference/return via  110 . A source current  120 ,  120 ′ and  120 ″ flowing through the signal via  108  and reference/return via  110  may provide a quasi-transverse electromagnetic (TEM) propagation mode for an electromagnetic wave, e.g. resulting from a source signal  105  (e.g., a high frequency signal, such as 5 GHz signal to 25 GHz or higher) flowing through the vias  108  and  110 . The bulk of the signal energy propagates inside the dielectric medium (e.g., across the thickness of the PCB non-conductive, signal, and conductive layers between vias  108  and  110 ) and through gaps (antipads) isolating signal vias from ground/reference layers and other signal layers. While  FIG. 1  illustrates a simplified case with one reference/ground via  110 , other designs may also include a plurality of reference/ground vias. 
     One negative effect of the forward electromagnetic wave  112  is that it is reflected off of the open-ended via stub in an uncontrolled manner, including dissipation/propagation from the end point  116  of the signal via  108  and/or reflecting back into to PCB  102  that causes interference with the signal  105 . For example, in a typical uncontrolled reflection, the total signal may be diminished up to 20 dB, for example, in a critical area of the 1st and 3rd harmonics. The transmission line dielectric medium (e.g., the via channel  100 ) and the conductive vias  108  and  110  may have significant losses in the multi-GHz frequency band, for example, and as a result it is not practical to use additional absorption and dissipation techniques as the additional loss uses too much of the signal noise budget. 
       FIG. 2  illustrates a typical S21 attenuation pattern  202  for a transmission line with an open ended plated through hole via. As shown, at the first and third harmonics for PCI Ex, Gen 3 and IEEE 802.3ba standards, significant interference notches  204  and  206  are present at critical frequencies (e.g., between 4.0 and 5.0 GHz, and between 12.0 and 15.0 GHz). The deep notch  204  near the first harmonic is primarily due to the open ended via stub reflections. A second deeper notch  206  is located in the region near the third harmonics with additional attenuation effects due to the higher dielectric and copper losses at these higher frequencies. 
     The current approach to circumvent this undesirable interference (e.g., notches at particular frequencies) is to back drill the vias.  FIG. 3  illustrates the via channel of  FIG. 1  where the vias have been back drilled within the PCB. However, this is an unsatisfactory solution. 
       FIG. 4  illustrates a S21 attenuation pattern  402  for a transmission line structure similar to  FIG. 3  with the stubs of the paired via back drilled. Back drilling involves removing an unused part of a via by drilling it so that conductive plating is removed. Although via back drilling removes the notch (caused by the reflected electromagnetic wave) in the region of the first harmonics, it may also create a new and more harmful notch  304  near the region of the third harmonics. The placement of the notch  304  may depend, at least partially, on the PCB laminate electrical properties and physical PCB design attributes. The back drilled vias relocate the interference notch along the frequency axis into a critical region of the third harmonics. Although the back drilled vias have a positive effect of transmission bandwidth improvements at the lower frequencies because the unused via section is removed, the improvement is limited at higher frequencies, or may actually move the interference notch to a higher frequency. 
     Consequently, a more effective way of reducing undesirable interference notches due to the use of vias is needed. At present time there are two general approaches to solve this issue. A first approach includes the placement some terminating element on an opened end via (circuit) stub. A second approach includes using optic acrylic waveguides embedded in regular PCB structure to increase bandwidth and avoid back drilling the via. Both of these approaches method. 
     A first prior art approach is presented in U.S. Pat. Nos. 5,161,086, 6,593,535, and 7,457,132. In this approach, absorption and dissipation of an incident electromagnetic wave is achieved by using a terminating element. However, with increasing of data rates, losses in non-conductive layers (e.g., dielectric material) and conductive/signal layers of the PCB also increase drastically, significantly restricting the bandwidth and length of PCB transmission lines. Consequently, this approach is not practical to use additional absorption and dissipation techniques as the additional loss uses too much of the signal noise budget. 
     In a second prior art approach, acrylic waveguides may be embedded or included in the PCB, but this is relatively expensive and suffers from unresolved problem, such as the optical connection of the backplane, middle plane with daughter cards. Additionally, this approach requires electrical-to-optical and optical-to-conversion interfaces/couplers with thousands times frequency scaling. The relatively high cost of implementing this approach and energy consumption make this approach undesirable. 
     Consequently, there is a need for a solution that addresses the shortcomings of the prior art while improving signal propagation within via structures of a laminate-copper PCB. 
     SUMMARY 
     A printed circuit board (PCB) is provided comprising a plurality of non-conductive layers with conductive or signal layers in between. The PCB includes a first conductive via traversing the plurality of non-conductive and conductive or signal layers as well as a second conductive via traversing the plurality of non-conductive layers and conductive or signal layers, the second conductive via located substantially parallel to the first conductive via. An embedded electro-optical passive element is also provided that extends perpendicular to and between the first conductive via and the second conductive via. The electro-optical passive element embedded is located within a selected layer at a first depth in the printed circuit board, wherein such first depth is selected to reflect an incident electromagnetic wave back into the printed circuit board to enhance or diminish an electrical signal in the first conductive via by creating a positive or negative electromagnetic interference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a via channel having an open-ended stub located within a PCB. 
         FIG. 2  illustrates a typical S21 attenuation pattern for a transmission line with an open ended plated through hole via. 
         FIG. 3  illustrates the via channel of  FIG. 1  where the vias have been back drilled within the PCB. 
         FIG. 4  illustrates a S21 attenuation pattern for a transmission line structure similar to  FIG. 3  with the stubs of the paired via back drilled. 
         FIG. 5  illustrates a current flow in a via channel when an EOP element or component is not used in a PCB. 
         FIG. 6  illustrates the flow of current through a via channel  600 , having an open-ended stub, but with an EOP element  601  coupled along a selected position between a via pair. 
         FIG. 7A  illustrates a side view of an electro-optical passive (EOP) structure according to one embodiment. 
         FIG. 7B  illustrates a top view of the electro-optical passive structure of  FIG. 7A . 
         FIG. 7C  illustrates one example of a differential signal via pair with a plurality of reference/ground vias arranged in regions where electromagnetic waves propagate. 
         FIG. 8  illustrates a S21 attenuation pattern response for a transmission line with an electro-optical passive (EOP) element embedded in a via channel. 
         FIG. 9  illustrates a simulation for the propagation of an electromagnetic wave through an air-laminate-air medium. 
         FIG. 10  illustrates simulation results of the via channel without the EOP element as shown in  FIG. 9 . 
         FIG. 11  illustrates a simulation for the propagation of an electromagnetic wave through an air-laminate-air medium having a built-in/embedded EOP element. 
         FIG. 12  illustrates a simulation of the via channel with the EOP element. 
         FIG. 13  illustrates a graph showing that electromagnetic field propagates through structure without EOP element as a transparent media with up to 15% attenuation only for the boundaries in  FIG. 9 . 
         FIG. 14  illustrates a graph that shows that for a structure having the embedded EOP element the electromagnetic field starts with only 40% of the incident field at 1 GHz and drastically drops to 0.05% at 15 GHz for the boundaries in  FIG. 11 . 
         FIG. 15  illustrates a graph showing the simulation results for visible characteristic impedance that is seen by transmitter in transmission link with plated trough hole via using simulation software. 
         FIG. 16  illustrates a graph showing the simulation of the visible characteristic impedance for transmission link with an EOP element embedded in a via channel. 
         FIG. 17  illustrates a method of forming a printed circuit board with an embedded electro-optical passive element. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, well-known operations, structures and techniques may not be shown in detail in order not to obscure the embodiments. 
     Overview 
     According to one aspect, an electro-optical passive (EOP) element is introduced as a component/device which works simultaneously in the electrical and optical domains. The electro-optical passive (EOP) element may operate in the electrical domain as an electrical element to re-direct a signal current flowing in a signal via to a reference/ground via and prevent a further excitement of an electromagnetic wave beneath a signal layer. The EOP element may also operate in the optical domain as a mirror to reflect the incident electromagnetic waves in a predefined manner to provide positive or negative interference for the total electromagnetic field in specified point of VIA channel where signal trace is connected. 
     According to one feature the location of the EOP element along a via pair is specifically selected to achieve a desired positive (constructive) or negative (destructive) interference. That is, the location of the EOP element along the via channel is not random or arbitrarily located along the thickness of the PCB. Rather, the location or distance (e.g., specific signal layer) for the EOP element may be selected, for example, to provide a desired value and/or direction of a geometrical sum of electromagnetic vectors (e.g., incident electromagnetic wave and reflected electromagnetic wave) to achieve a particular positive or negative electromagnetic interference. 
     Via Channel Current Flow without an Electro-Optical Passive Element 
       FIG. 5  illustrates a current flow in a via channel  502  when an EOP element or component is not used in a PCB  500 . The PCB  500  may include a plurality of non-conductive layers with conductive layers (e.g., signal layers, reference layers) in between. The electrical current  508  may flow from a signal source  507  through a signal via  504 , the open-ended stub of the signal via  504  and continues its way through the via channel dielectric  516  as a displacement current  522  and then returns to the signal source  507  through a reference/ground via  506  as a reference/ground current  509 . As can be appreciated here, an incident electromagnetic wave  512  is induced by the signal current  508 . The incident electromagnetic wave  512  may propagate beyond the end of the PCB  500  and/or may cause a reflected electromagnetic wave  514 . The incident electromagnetic wave  512  and/or the reflected electromagnetic wave  514  may cause unwanted interference which may result in signal loss and/or frequency notches. 
     Via Channel Current Flow with an Electro-Optical Passive Element 
       FIG. 6  illustrates the flow of current through a via channel  600 , having an open-ended stub, but with an EOP element  601  coupled along a selected position between a via pair. The PCB  600  may include a plurality of non-conductive layers with conductive layers (e.g., signal layers, reference layers) in between. The via channel  600  comprises a dielectric medium bounded by current carrying rails (e.g., a signal via  604  and one or more reference/return vias  606 ). The signal via  604  may traverse through a plurality of conductive and/or non-conductive layers of the PCB  600 . In a first implementation, the signal via  604  and/or reference/return via  606  may extend through the PCB layers (e.g., from a first surface of the PCB  600  to an opposite second surface). In a second implementation, the signal via  604  and/or reference/return via  606  may be blind vias where the hole extends only partially through the PCB (e.g., across a subset of the layers for the PCB  600 ). In a third implementation, the signal via  604  and/or reference/return via  606  may be through-hold vias (e.g., extending across the PCB layers) but has been back drilled so that the conductive material of the via only extends through a subset of the layers for the PCB  600 ). Note that in some implementation, both the signal via  604  and/or reference/return via  606  may be of the same via type (e.g., a through-hole via, a blind via, or a back drilled via, etc.) or they may be of different via types (e.g., combinations of a through-hole via, a blind via, and a back drilled via, etc.). 
     A signal source  607  may insert or provide a signal (e.g., a high frequency signal (e.g., 5 GHz or higher) to the signal via  604 . The electrical signal current  608  flows from the signal source  607  through the signal via  604 . From the signal via  604 , the signal current  608  then flows through a conductive body of an EOP element  601  and returns to the signal source  607  through its associated reference/ground via  606 . In one embodiment, the EOP element  601  may be partially realized as an element (e.g., low impedance resistor) that changes the direction of an electrical current with only a small energy loss to the signal. For instance, the EOP element  601  causes the electrical current  508  to be diverted through the EOP element  601  from a signal via  604  to the reference/return via  606 . As illustrated here, the signal current  608 ,  608 ′, and  608 ″ flows from the signal via  604 , across the EOP element  601 , and to the reference/return via  606 . The EOP element  601  thus prevents such signal current  608  from flowing into the via portions in layers below the EOP element  601 . 
     The signal current  608  through the signal via  604  induces an incident electromagnetic wave  612  while the signal current  608 ″ through the reference/return/ground via  606  induces a reflected electromagnetic wave  614 . The electromagnetic waves  612  and/or  614 ″ may be substantially excluded or cancelled by absorption or dissipation as well as by controlling the reflected electromagnetic wave  614  so that it interferes (e.g., cancels) the incident electromagnetic wave  612 . The electromagnetic waves  612  and  614  are excited by the source current  608  in the via barrels and propagates in the dielectric medium between the via barrels  604  and  606  and is reflected with the use of the electro-optical passive (EOP) element  601 . 
     According to one feature, reflection of the electromagnetic wave is specifically engineered to reduce signal loss and/or minimize or avoid attenuation notches within a frequency band. The EPO element  601  may be designed to provide an electrical solution by changing the direction of electrical current in the current rails/via barrels  604  and  606  with the smallest signal losses to prevent exciting/inducing an electromagnetic wave in the lower via portion  611  as well as preventing the propagation of the incident electromagnetic wave  612  into the lower via portion  611 . In this manner, the EOP element  601  inhibits a new portion of the electromagnetic wave in the dielectric material in the layers below the signal layer  609 . 
     The EPO element  601  is also arranged to provide an optical solution by selecting a location or position along the length of the via pairs  604  and  606  such that the EOP element  601  reflects the existing electromagnetic wave in the via channel so as to achieve positive (constructive) interference or negative (destructive) interference between the forward travelling (incident) electromagnetic wave  604  with backward travelling (reflected) electromagnetic wave  614 . In one example, constructive interference is used to increase a total signal strength and compensate for dielectric and copper losses at higher frequencies and/or suppress undesirable frequency notches. In another example, destructive interference is used, for instance, to attenuate glitches or spikes at particular frequency bands in the frequency response for the transmission link and/or via channel. 
     Due to these electrical and optical characteristics, the EOP element  601  operates simultaneously and/or concurrently in the electrical domain and optical domain to increase transmission bandwidth, control the characteristic impedance of the current path (not just the via section) without the need for back drilling. In some implementations, the EOP element  601  may be implemented on electrical paths for high frequency signals in a PCBs (e.g., 5 GHz and higher). By characterizing a signal in both the electrical domain and optical domain it has been observed that around 5-7 GHz, the signal behavior along the transmission path from (a) an electrical signal response loss to (b) an electrical and electromagnetic signal loss response. Above this 5-7 GHz boundary, the electromagnetic behavior (e.g., same as optical behavior) begins to be more and more pronounced as frequency increases. 
       FIG. 7A  illustrates a side view of an electro-optical passive (EOP) structure according to one embodiment.  FIG. 7B  illustrates a top view of the electro-optical passive structure of  FIG. 7A . The electro-optical passive structure  700  may comprise an EOP element  702  extending between a pair of current carrying rails (via barrels). The current rails may include a signal via  704  and a reference/return via  706 . In one embodiment, the EOP element  702  may be realized with any known material that meets the desired requirements to minimize signal loss and can be implemented within the structure without affecting the overall thickness of a printed circuit board. A conductor etched laminate  708  may surround the EOP element  702  and a conductive layer  710  may surround the conductor etched laminate  708 . The conductive layer  710  may be part of a conductive layer within a PCB in which the electro-optical passive structure  700  is embedded. 
       FIGS. 7A and 7B  illustrate a simplified EOP structure including one signal via and one reference/return via. However, it should be understood that other embodiments may comprise a plurality of signal vias and a plurality of reference/return vias. 
     The shape (e.g., size or area) of the EOP element  702  and distance or location of the EOP element relative to the signal source is specifically selected to minimize signal loss and a desired frequency response. In this example, the length and/or width of the EOP element  702  may be specifically selected to reflect the incident electromagnetic wave while also minimizing signal loss. The shape and/or location of the EOP element may be dependent on the operating frequency band, connector pin-field characteristics, via characteristics (e.g., via diameter, via length, separation between vias, etc.) PCB characteristics (e.g., laminate coefficients, layer thickness, PCB design, etc.). In one approach, the EOP shape, distance, location or displacement may be ascertained by obtain a three dimensional model/simulation of a particular PCB electrical and/or electromagnetic response using, for example, Computer Simulation Technology (CTS) Microwave Studio® or other modeling/simulation software. 
     While some of the examples herein illustrate a single signal via and a single reference via, it should be understood that there may be a plurality of signal vias and a plurality of reference vias. The number and/or position of the reference vias relative to a signal via and the shape, size, position, and/or location of the EOP element between a via and one or more reference vias will depend on the electromagnetic propagation for the signal via. Such electromagnetic wave propagation may be simulated and/or modeled for each type or arrangement of signal vias and may be dependent on many factors, including via diameter, PCB conductive layer and non-conductive layer characteristics, signal frequency, etc. That is, the electromagnetic wave propagation for a particular signal via may dictate how many reference vias will be used, the position of the vias, and the location, size, and/or position of the EOP element between the signal via and the one or more reference vias. 
       FIG. 7C  illustrates one example of a differential signal via pair  752  and  754  with a plurality of reference/ground vias  756   a - j  arranged in regions  758   a - d  where electromagnetic waves propagate. Once the regions  758   a - d  have been identified, the reference vias  756   a - j  may be positioned and/or arranged in these regions. The size and location of the EOP element(s)  760   a - d  is also dependent on the electromagnetic propagation characteristics of the differential via pairs  752  and  754 . In this example, a positive signal via  754  has two EOP elements  760   a  and  760   b  positioned in the regions of electromagnetic propagation  758   a  and  758   b . Note that the location of the reference vias  756   a - j  may actually be closer to the vias  752  and  754 , but have been shown further apart for purposes of illustration. 
       FIG. 8  illustrates a S21 attenuation pattern response  802  for a transmission line with an electro-optical passive (EOP) element embedded in a via channel. As shown, embedding an EOP element within the via channel removes the interference notches at the first harmonics and the interference and attenuation notches at the third harmonics. That is, the frequency notch  804  shows approximately a 27 dB loss or attenuation while the loss in  FIGS. 2 and 4  for the third harmonics is closer to 40 dB. Moreover, the signal insertion loss may be significantly reduced by approximately 20 dB in the critical area of the third harmonics, as compared to the S21 response for an open ended plated through-hole via and back drilled via illustrated in  FIGS. 2 and 4 . In one example, the EOP element may operate as a mirror that reflects forward traveling electromagnetic waves in a positive manner to provide compensation for the dielectric and copper losses with a targeted multi-GHz frequency region or band. 
     Simulation Model without EOP Element 
       FIG. 9  illustrates a simulation for the propagation of an electromagnetic wave through an air-laminate-air medium. To more fully understand the reflection and compensation phenomenon described herein, this simulation models the propagation of an electromagnetic wave  901  through an air-laminate-air medium  900  (e.g., air-Megtron 6-air medium) that matches the propagation of an electromagnetic wave in a via channel without the use of an EOP element. The electromagnetic simulation of this auxiliary structure illustrates an ideal case/upper evaluation for a real plated through hole and back drilled vias. In this example, the simulation  900  includes a Megtron 6 laminate  902  between a pair of air mediums  904  and  906 . First and second planes  908  and  910  illustrate infinitesimal electromagnetic boundaries located between different propagation mediums where boundaries O 1  &amp; O 3  indicate a top side of an electromagnetic boundary and O 2  &amp; O 4  indicate a bottom side of the same electromagnetic boundary. For example, boundary O 1  indicates the top side of an electromagnetic boundary with the first air medium  908 . 
       FIG. 10  illustrates simulation results of the via channel without the EOP element as shown in  FIG. 9 . The simulation illustrates the amplitude of incident, backward travelling waves and total electromagnetic field versus frequency at boundary O 1  surface in  FIG. 9 . As shown, a large and strong notch  1006  is present in the center of the total EM field plot  1004 . The notch  1006  is a result of up to a 50% reflection at the top of the Megtron 6 surface which changes up to 5 times in magnitude versus frequency. 
     Simulation Model with EOP Element 
       FIG. 11  illustrates a simulation for the propagation of an electromagnetic wave through an air-laminate-air medium having a built-in/embedded EOP element. The simulation model includes Air-Megtron 6-NiCr-Megtron 6-Air medium that matches propagation of an electromagnetic wave  1101  in the via channel with an EOP element implemented as shown, for example, in  FIG. 6 . More specifically, the simulation model  1100  comprises a first air medium  1102 , a first Megtron 6 laminate  1104  on bottom of the first air medium  1102 , an EOP element  1006  at bottom of the first Megtron 6 laminate  1104 , a second Megtron 6 slab  1108  on bottom of the EOP element  1006  and a second air medium  1110  on bottom of the second Megtron 6 laminate  1108 . In one embodiment, the EOP element  1106  may be a layer of, for example, Nickel Chromium (NiCr). However, the EOP element  1106  may be realized with any known material that meets the desired electrical characteristics, e.g., low impedance to minimize signal loss. For example, impedances in 0.05 ohms to 1 ohm may be used in a first instance, impedances, of 1 ohm to 5 ohms may be used in other instance, and impedances of 5 ohm to 100 ohms may be used in other cases. 
     As shown in  FIG. 11 , boundary O 2  indicates the bottom side of the electromagnetic boundary between the first air medium  1002  and the first Megtron 6 slab  1104  and pertains to the Megtron 6 medium. Boundary O 3  indicates the top side of the electromagnetic boundary between the first Megtron 6 laminate  1104  and the EOP element  1106  (NiCr layer/laminate) and pertains to the Megtron 6 medium. Boundary O 4  indicates the bottom side of the electromagnetic boundary between the first Megtron 6 laminate  1104  and the EOP element  1106  (NiCr laminate) and pertains to the NiCr medium. Boundary O 5  indicates the top side of the electromagnetic boundary between the EOP (NiCr laminate)  1006  and the second Megtron 6 laminate  1008  and pertains to the NiCr medium. 
     Boundaries O 1 -O 8  in  FIG. 11  indicate the planes for which the electromagnetic fields are calculated. Simulation data shows the intensity of the electrical field as a function of frequency. Note that a receiver will see only a total electromagnetic field which is a sum of a forward/incident electromagnetic wave  1101  and a backward/reflected electromagnetic wave  1103 . 
       FIG. 12  illustrates a simulation of the via channel with the EOP element. The simulation illustrates the amplitude of incident, backward travelling waves and total electromagnetic field vs. frequency at the O 1  boundary surface in  FIG. 11 . As shown, the presence of the EOP element (NiCr layer) causes large, up to 70% reflection (backward traveling wave  1202 ), with a small slope of reflection amplitude versus frequency as well as an increase of the total electromagnetic field  1204  versus frequency. As such, the EOP element acts as a mirror to increase the total electromagnetic field as a result of positive interference of a forward travelling wave  1200  and reflected/backward travelling wave  1202 . Also, at the same time, this phenomenon explains a growth up to 20 dB of S21 response in critical area of the 3rd harmonics that was shown in  FIG. 8 . 
       FIG. 13  illustrates a graph showing that electromagnetic field propagates through structure without EOP element as a transparent media with up to 15% attenuation only for the boundaries in  FIG. 9 .  FIG. 14  illustrates a graph that shows that for a structure having the embedded EOP element the electromagnetic field starts with only 40% of the incident field at 1 GHz and drastically drops to 0.05% at 15 GHz for the boundaries in  FIG. 11 . This shows that with higher frequencies the optical properties of the EOP element may be more dominant than the electrical effects and can provide significant improvement for higher bandwidth transmission in PCBs. Because of Air-Megtron 6-Air structure shown in  FIG. 9  matches plated through hole via or a back drilled via,  FIG. 13  shows also that both type of vias will strongly radiate and could induce noise in adjacent PCBs and stray currents in metal parts of a chassis which could bring problems to yield Electromagnetic Compliance (EMC) requirements. 
     The other side of the Air-Megtron 6-NiCr-Megtron 6-Air structure shown in  FIG. 11  matches a via having an embedded EOP element.  FIG. 14  illustrates a graph showing that output (i.e. radiation), drastically drops from 40% of the incident field at 1 GHz to 0.05% at 15 GHz due the reflection from EOP acting as a mirror. As such, the EOP element protects adjacent PCBs from noise and metal parts of chassis from stray currents and provides better conditions to meet EMC specifications. 
       FIG. 15  illustrates a graph showing the simulation results for visible characteristic impedance that is seen by transmitter in transmission link with plated trough hole via using Ansoft HFSS™ simulation software. As shown, the visible characteristic impedance has a huge oscillation up to 400 Ohms in the area of 1-2 GHz that makes impossible to tune ICs I/O buffer. 
       FIG. 16  illustrates a graph showing the simulation of the visible characteristic impedance for transmission link with an EOP element embedded in a via channel. As shown, the visible characteristic impedance variation may be significantly reduced compared with that of a plated through hole via making it possible and easy to tune ICs I/O transmitter buffers. Furthermore, the Ansoft HFSS™ simulation also shows same behavior for receiver ICs. 
       FIG. 17  illustrates a method of forming a printed circuit board with an embedded electro-optical passive element. A PCB design is obtained having a plurality of conductive and non-conductive layers and at least one signal via  1702 . The electromagnetic propagation characteristics for a signal through the signal via is obtained for the PCB  1704 . This may be done by modeling the signal current flow through the signal via (e.g., at one or more frequencies) to ascertain an electromagnetic propagation map. The number, position, and/or location of one or more reference vias is selected based on the ascertained electromagnetic propagation characteristics  1706 . Similarly, the distance between the reference via(s) and the signal via may be dictated by the electromagnetic propagation characteristics ascertained. The position, size, and/or location of an electro-optical element is then selected based on the electromagnetic propagation characteristics to reflect an incident electromagnetic wave back into the printed circuit board to enhance or diminish an electrical signal in the first conductive via by creating a positive or negative electromagnetic interference  1708 . 
     One example provides a printed circuit board, comprising a plurality of non-conductive layers with conductive or signal layers in between. A first conductive via is formed traversing the plurality of non-conductive and conductive or signal layers. A second conductive via is formed traversing the plurality of non-conductive layers and conductive or signal layers, the second conductive via located substantially parallel to the first conductive via. An electro-optical passive element is also formed extending perpendicular to and between the first conductive via and the second conductive via, the electro-optical passive element embedded within a selected layer at a first depth in the printed circuit board, wherein such first depth is selected to reflect an incident electromagnetic wave back into the printed circuit board to enhance or diminish an electrical signal in the first conductive via by creating a positive or negative electromagnetic interference. 
     In one implementation, the first conductive via may be coupled to a first signal trace in a first layer and a second signal trace in a second layer, wherein a source signal flows from the first signal trace into the first conductive via. The first conductive via may be coupled to a signal source at a first layer and the second conductive via is coupled to a second layer, a signal current from the signal source flows through the first conductive via to the second conductive via. 
     In various examples, at least one of the first conductive via and second conductive via is: (a) an open ended via, a blind via, and/or a back drilled via. 
     The electro-optical passive element serves to re-direct the signal current through the second conductive via and prevents its propagation to an open end of the first conductive via to inhibit further excitement of an electromagnetic wave beneath a signal layer. 
     In one instance, the depth at which the electro-optical element is embedded is selected to reflect the incident electromagnetic wave as a reflected electromagnetic wave that negatively interferes with the incident electromagnetic wave to substantially cancel the incident electromagnetic wave. 
     In another instance, the depth at which the electro-optical element is embedded is selected to reflect the incident electromagnetic wave as a reflected electromagnetic wave that positively interferes with the incident electromagnetic wave to augment the incident electromagnetic wave. 
     In yet another instance, the depth at which the electro-optical element is embedded is selected to reflect the incident electromagnetic wave as a reflected electromagnetic wave that reduces signal loss. 
     In still another instance, the depth at which the electro-optical element is embedded is selected to reflect the incident electromagnetic wave as a reflected electromagnetic wave that shifts or removes notches in a frequency response for a transmission path. 
     The electro-optical element may have a width that is between a diameter of the first conductive via and twice the diameter of the first conductive via. 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.