Traditional Connectors and Their Application Throughout the Industry are Changing for the Better
For many decades, RJ45 jack connectors have been used for low-cost, high volume applications throughout industrial, commercial, military and medical fields. The registered jack (RJ) is a standardized physical network interface for connecting telecommunications or data equipment to a service provided by a local exchange carrier or long distance carrier. It was introduced by the Bell System under a 1976 order by the Federal Communications Commission (FCC) that ended the use of protective couplers provided exclusively by the telephone company. The modular jack was then chosen as the main candidate for ISDN systems.
Historically, the biggest design problem for RJ45 jacks was to solve crosstalk coupled from adjacent lines. The problem (at least at lower frequency rates) was solved simply by isolation techniques within the connector, or split pair wiring of the Category Cable itself. Newly designed (Femto dielectric) flex core material incorporates a unique strip-line technology that allows data transmission paths to be differentially paired. This allows data packets to be easily driven over a copper line at ranges from 125 MHz all the way up to 20.0GHz.
The RJ45 jack has played a critical role in data transfer, from an integrated circuit (IC) all the way through to the receiver end. However, commercial and military applications require higher data rates, pushing RJ 45 signal rise times and clock speeds faster than any time in history.
Compliance requirements for radiated and conducted emissions now require broader measurement bandwidths. New bandwidth requirements now range from 10kHz ~ 26.5GHz, depending on whether the device is intended for use in military applications (MIL-STD-461), avionics (RTCA-DO-160), medical devices (IEEE802.11/IEC 60601) or commercial electronics (FCC part 15 and the EU’s EMC Directive 2004/108/EC). Since the transmission speeds going through an RJ45 jack have approached the effective radiating length of λ/4 (frequency in wavelength, GHz), its radiated emission characteristics become a primary point of interest for issues involving electromagnetic compatibility (EMC) and electromagnetic interference (EMI).
Crosstalk is usually described in the context of culprit versus victim. In high-current, low-impedance circuits, crosstalk is a direct result of mutual inductance between current loops of the connector and cable wiring/shielding practices. Further, crosstalk from mutual capacitance, associated with high-voltage and high-impedance networks, is usually negligible.
However, in the case of the standard RJ45 jack (especially in high-density connectors), the culprit and victim relationships are in very close proximity to each other, which raises mutual inductance and thus the susceptibility to crosstalk. The signal and return arrangement of a standard RJ45 jack causes two current loops to overlap. So, some amount of crosstalk will be experienced on all lines, and the mutual inductance and crosstalk from line to line becomes even greater. In a transmission line, impedance matching is necessary to minimize RF reflections and to allow the connector to deliver the amplitude signal required to maximize power at the load. The effect is a maximum amount of signal being transmitted and a minimum amount of data being reflected back as loss.
To simplify this last statement, the strip-line flex technology within RJ45 jacks in use today creates an extremely low impedance path, creating an insertion loss/isolation greater than 52.78dBm. This virtually eliminates the possibility for crosstalk within the connector and creates an edge-coupled line surrounded by a ground plane, reducing stray voltage and current expenditures. This can be expressed as:
Voltage V = 5Vrms
Impedance Z = 0.13180747 Ohms
thus Power Level L = 52.78dBm
This advantage is not directly due to differentially-paired signal lines. Rather, this design approach minimizes electronic crosstalk and electromagnetic interference. This results in both noise emission and noise acceptance, so it can achieve a constant, known characteristic impedance. Normally, single-ended signals in other types of RJ45 jacks are resistant to interference only when the lines are balanced and terminated by a differential amplifier of some type, wire-wound magnetics or a balun.
Crosstalk Analysis Using S-parameters
As a foundation for understanding how to characterize a linear passive physical layer device such as an RJ-45 jack, a brief discussion of multiport measurements is in order. The four port device shown in Figure 1 is an example of what a real-world structure might look like if we had two adjacent printed circuit board (PCB) traces operating in a single-ended fashion. Let’s assume that these two traces are located within relatively close proximity to each other on a backplane, and that some small amount of coupling might be present. Since this example involves two separate single-ended lines, this coupling creates an undesirable effect we call crosstalk.
The matrix on the left side of Figure 1 shows the 16 single-ended s-parameters that are associated with these two lines. The matrix on the right shows the 16 single-ended time domain parameters associated with these two lines. Each parameter on the left can be mapped directly into its corresponding parameter on the right through an inverse fast fourier transform (IFFT). Likewise, the right-hand parameters can be mapped into the left-hand parameters by a fast fourier transform (FFT).
If these two traces were routed close together as a differential pair, then the coupling would be a desirable effect and it would enable good common mode rejection that provides EMI benefits.
Once the single-ended s-parameters have been measured, it is desirable to transform these to balanced s-parameters to characterize differential devices. This mathematical transformation is possible because a special condition exists when the device under test is a linear and passive structure. Linear passive structures include PCB traces, backplanes, cables, connectors, IC packages and other interconnects. Utilizing linear superposition theory, all of the elements in the single-ended s-parameter matrix on the left are processed and mapped into the differential s-parameter matrix on the right. Much insight into the performance of the differential device can be achieved through the study of this differential s-parameter matrix, including EMI susceptibility and EMI emissions.
Interpreting the large amount of data in the 16-element differential s-parameter matrix is not trivial, so it is helpful to analyze one quadrant at a time. The first quadrant in the upper left of
Figure 2 is defined as the four parameters describing the differential stimulus and differential response characteristics of the device under test. This is the actual mode of operation for most high-speed differential interconnects, so it is typically the most useful quadrant that is analyzed first. It includes input differential return loss (SDD11), forward differential insertion loss (SDD21), output differential return loss (SDD22) and reverse differential insertion loss (SDD12).
Note the format of the parameter notation SXYab, where S stands for scattering parameter (or S-Parameter), X is the response mode (differential or common), Y is the stimulus mode (differential or common), A is the output port and B is the input port. This is typical nomenclature for frequency domain scattering parameters. The matrix representing the 16 time domain parameters will have similar notation, except the “S” will be replaced by a “T” (i.e. TDD11).
The fourth quadrant is located in the lower right and describes the performance characteristics of the common signal propagating through the device under test. If the device is designed properly, there should be minimal mode conversion, and the fourth quadrant data will be of little concern. However, if any mode conversion is present due to design flaws, then the fourth quadrant will describe how this common signal behaves.
The second and third quadrants are located in the upper right and lower left of Figure 3, respectively. These are also referred to as the mixed mode quadrants. This is because they fully characterize any mode conversion occurring in the device under test, whether it is common-to-differential conversion (EMI susceptibility) or differential-to-common conversion (EMI radiation). Understanding the magnitude and location of mode conversion is very helpful when trying to optimize the design of interconnects for gigabit data throughput.
Differential pairs mentioned earlier in this article technically include: 1) twisted-pair cables, shielded twisted-pair cables, and twin-ax; and 2) strip-line differential pair routing techniques onto “specialized” flex circuit boards.
Generally, a receiving device located at the end of any cable/harness connection reads the difference between the two signals. Since the receiver ignores the wires› voltages with respect to ground, small changes in the ground potential between the transmitter and receiver do not affect the receiver›s ability to detect the signal.
EMI/RFI interference tends to affect both TX and RX wires together. Because the data packet information is sent in the form of bit rates, utilizing differently paired wires, the technique improves the resistance to electromagnetic noise ratio compared with use of only one wire and an un-paired reference (ground). What is then needed is a high speed RJ45 jack which can be used for analog data, as well as digital data signaling, just as in any other Ethernet shield over twisted pair.
Designing the RJ45 for High Speed Data Transfer
A genuine high speed RJ45 jack and its corresponding interconnection system must have a well-designed base platform from which to start. To begin, it should utilize properly plated copper conductors to ensure a path of least resistance, thus lowering the induced currents and voltages expended dramatically. Utilizing the patent pending flex material, along with differentially paired strip-line components allows for higher transmission data rates the standard ceramic capacitors, inductors, or resistors soldered onto some form of FR4 flex material.
Many RJ45 Jack connectors produced today simply provide magnetically balanced, single-ended lines, in combination with common mode capacitive circuits. However, at much higher frequencies, this can diminish their transmission data rate capabilities. By implementing low pass, femto-dielectric constant materials, the strip-line flex circuit can be balanced differentially to provide the much needed insertion loss/isolation requirements.
Common Strip-line Design Models
Generally speaking, strip-line transmission lines are fully contained within a substrate, sandwiched between two chassis ground planes. In this implementation, it was performed by closely surrounding the strip-line circuit in a 360 degree manner with chassis ground, as shown in the strip-line cross section model depicted in Figure 4.
(It is important to note that special low loss dielectric flexible materials must be used for the strip-line flex development, especially since the dielectric material chosen will directly affect transmission line impedance.)
The Intuitive Explanation
There is an old physics truism that everyone seems to have forgotten when designing electronic circuitry and cables, that is, that electrons tend to flow down the path of least resistance.
When a conductor (in our case a plated copper wire) is filled with a voltage “charge” and then an external “potential” is applied across it, electrons distribute themselves across the length of the conductor. This forces all of the electrons to lose energy in all directions simultaneously across the conductor’s path. This same physics can be applied to multiple conductors that parallel to the current flow, the only difference being the different rates proportional to the conductivity of each conductor’s base material. (See Figure 5)
The biggest RJ45 jack design problem was to solve crosstalk coupled from adjacent lines and in the cable components themselves. The basic problem associated with coupled noise or crosstalk is that it increases as the signals for these components have higher and higher data transmission speeds. The historic approach was to just increase spacing between the lines or to add-in ferrites (also known as magnetics) to create needed signal isolation needed, but that alone does not protect the remaining transmission lines in the RJ45 jack from picking up unwanted noise within the jack itself.
However, the application of strip-line flex design techniques provide the important signal and data transmission advantages over conventional design approaches. Strip-line flex design works by incorporating a conductor sandwiched by dielectric material between a pair of ground planes. Traditionally, strip-line was usually made by etching circuitry onto a ceramic/copper substrate that had a ground plane on each opposite face, in order to achieve two opposing ground planes. Today, strip-line design techniques typically use “soft-board” flex technology.
Strip-line design is a transverse electromagnetic (TEM) transmission line media, just like coax, which means that it is non-dispersive. Further, strip-line filter and coupler lines, via shape and spacing, always offer better bandwidth than their counterparts using micro-strip or magnetics since, unlike other methods, the roll-off of strip-line is quite symmetrical. Another advantage of strip-line is the superior isolation between adjacent traces can be achieved with a “picket-fence” of grounds surrounding each transmit and receive line, keeping them spaced at less than 1/4 wavelength apart from each other.
Comparable Energy Use
Power saving tests were performed in real-time using a DC ammeter and BERT tester as a source. We took a traditional RJ45 jack with ferrites, measured its contribution to a known data transmission circuit, and compared the mA readings with those contributed by a high-speed data RJ45 jack featuring strip-line flex design. The traditional, magnetically-loaded RJ45 added 0.212mA to the PCB’s overall power consumption, compared with just 0.031mA for the high-speed RJ45 jack. This represents a power savings of 0.181ma with the high-speed jack.
TDD = Time domain differential
SDD = Frequency (signal) domain differential
RLCG = R=Ohms/m, L= H/m inductance, C = F/m capacitance, G = S/m conductance
S-parameters measurements are taken in magnitude and angle, because both the magnitude and phase of the input signal (angle) are changed by the network being measured.
(This is why they are sometimes referred to as complex scattering parameters).
The four S-parameters mentioned here actually contain eight separate numbers: the real and imaginary parts (or the modulus and the phase angle) of each of the four complex scattering parameters.
How much gain (or loss) you get is usually more important than how much the signal has been phase shifted.
S-parameters depend upon the network and the characteristic impedances of the source and load used to measure it, plus the frequency measured at (kHz, MHz, GHz).
S11 = b1 / a1, S12 = b1 / a2, S21 = b2 / a1, S22 = b2 / a2
The transmitted and the reflected wave will have changes in amplitude and phase from the incident wave. Generally, the transmitted and the reflected wave will be at the same frequency as the incident wave.
An RJ45 jack with integrated strip-line flex is backward compatible with older connector systems, so that upgrading or refurbishing of legacy data systems becomes much more affordable. In addition, the strip-line flex design allows for greater power savings compared with conventional connectors and PCBs. Strip-line flex technology integrated into the RJ45 jack allows the connector to be same size and format as original connector while enhancing the connector’s ability to perform throughput at higher data rates, without the need for magnetics. This approach also leaves more room on the PCB for additional components, since fewer components are required for higher speeds and signal integrity isolation.
Brett D. Robinson, Ph.D. is the principle of Robinson’s Enterprises, an engineering consulting firm based in Lake Elsinore, CA, and Chief Technical Officer for Sentinel Jack connector Systems (West). He can be reached at firstname.lastname@example.org.
Michael Resso is a signal integrity application scientist at Keysight Technologies (formerly known as Agilent Technologies) in Santa Rosa, CA. He can be reached at email@example.com.