With the advent of integrated circuits with balanced cable loss effects, system designers have the ability to transmit high-speed data over very long copper cables. In communication system design, it is important to control radiation from cables and connectors for optimum performance. The ability of an integrated circuit to balance out the effects of cable loss is a major cost impact of system interconnection. With fiber optic links just consider the distance and speed, can now be erected with copper cables. This avoids the high cost of fiber optic modules. Now, “copper modules” are available on the market as a direct replacement for fiber optic modules. These copper modules contain Maxim’s cable balancers. The equalizer device can receive 3.2Gbps data over long cables with very little signal distortion. Figure 1 shows a comparison of a signal through 115 feet, with and without an equalizer. The result: the signal improvement with the equalizer is astonishing. Maxim’s family of equalizers includes devices operating up to 12.5Gbps.
Figure 1 3.2Gbps signal before and after 115ft cable balancer
At high bit rates, “copper” cable interconnects must function well as transmission channels. They should have deterministic signal integrity and maximum power delivered to the receiver without leakage. Signal leakage from the cable interconnection becomes EMI (Electromagnetic Interference) to the outside world. Also, this becomes important when equipment is expected to comply with EMC from FCC and EC standards.
This article discusses simple ways to reduce radiation, while cable balancing can extend transmission length and reduce system cost.
Cables and connectors that connect different parts of equipment together prevent interference and reduce interactions that are sensitive to radiated energy. Some of the ways in which radiation from a cable system can be reduced are discussed below:
1. Balanced system
2. Ferrite beads
3. Twisted pair cable
4. Cable shielding
5. Cable/Backplane Termination Connector
A balanced cable system consists of two wires that carry positive and negative (bipolar) signals relative to a third wire, or ground. However, since differential line drivers are not ideal, there will be a certain amount of unwanted signals present, such as noise, hum from the power supply, and temperature effects on the signal lines. The ability of a differential receiver to reject these common signals is expressed by a performance factor called the common-mode rejection ratio (CMRR). CMRR is the ratio of differential mode gain to common mode gain, usually expressed in dB. Another factor that affects performance in balanced systems is the skew between the signals caused by the possible length difference between the two signal lines. This difference in length will cause the differential signal edges to misalign and produce small spikes in the ground system.
Systems that do not contain a balanced output can use a transformer at the output (see Figure 2) to produce a balanced output. Transformers are usually placed close to the output of the system. If the amplifier used is not ideal and there are mode switching problems, a small amount of primary current will appear at the output as a differential mode current. At high frequencies, this mode transition is most prevalent. Another factor of the transformer is the capacitance from the primary to the secondary. Capacitors will increase the common mode coupling of high frequency energy to the output, which increases radiation. Another benefit of using a transformer is that the transformer can provide DC isolation between systems that may have potential differences between system grounds.
Figure 2 Transformer conversion of the output
If a small ferrite bead or ring slides along the differential signal line, it acts as a vertical transformer, reducing common-mode currents. They behave like a reduced transformer. Ferrite beads or rings are useful for attenuating high frequency differentials such as switching transients and other high frequency signals. They are often found in power supplies and on cables used for video monitors, where they are used to reduce EMI in the system. Ferrite beads work best in low impedance circuits. At around 50MHz, the bead impedance is as high as 500Ω. Care should also be taken that the bead core cannot become saturated as this will reduce its effectiveness. When multiple pairs of cables are used, crosstalk between the cable pairs increases. Figure 3 shows the variation of bead insertion loss with load impedance and number of turns.
Fig.3 Insertion loss of ferrite bead versus frequency
twisted pair cable
Twisted pair leads to greatly reduced differential mode radiation, while common mode radiation is not affected. Compared with single-ended transmission, the radiation of differential signal transmission is reduced by 20~30dB. Whenever an electromagnetic field is radiated from the cable, the twisted pair cancels the magnetic field from the adjacent wires of the twisted pair. As a result, if the twisted pairs are in the same direction, the field coupled to adjacent pairs is close to zero. For most cables, there are multiple twisted pairs in a single bundle, and the twist rate varies between the twisted pairs. This decay will help eliminate the coupling effects caused by a little bit of asymmetry in the wire twisting process. As an example, look at the twist rate of a typical Category5 cable, which is used in most Ethernet networks, to see the variation in twist rate.
A more important factor in controlling radiation is cable shielding. This key parameter determines the radiated performance, which indicates the effectiveness of the shielding used by the cable. Transfer impedance represents the relationship between the current flowing through the shielding surface and the voltage across the surface. This voltage is caused by the diffusion current flowing through the thickness of the shield (Figure 4).
Figure 4. Transmission Impedance Diagram – Polarizability and Radiation
Leakage inductance in mesh shields can also play a role. The best shield for the best shielding effect is a thin layer surrounding a solid tube shield, such as a semi-rigid coaxial cable. Figure 5 shows the transmission impedance of several different coaxial cables.
Figure 5 Transmission impedance of different types of cables
Radiation can be further reduced by using shielded twisted pair or shielded balanced wire for shielded two-conductor feeders because the shield is no longer used as a return path. The only current flowing through the shield is caused by the balance wire asymmetry. Therefore, the radiation reduction is a percentage of the shield current to the total signal current on the signal line.
Cable/Backplane Termination Connectors
The final connection of the cable assembly to the load or source is as important as the cable performance. It is important that the connector provides good shielding and a low impedance connection to the system ground. The connector is in series with the signal path after all. The connector resistance versus frequency is shown in Table 1.
Table 1 Relationship between connector resistance and frequency
Connector type DC~10MHz 100MHz 1GHz
BNC connector 1~3mΩ 10 mΩ 100 mΩ
N connector < 0.1mΩ 1 mΩ 10 mΩ
Shielded multi-contact connector 10~50mΩ 10~50 mΩ 300 mΩ
Pigtail 5cm Z=“3” mΩ+j0.3Ω×FMHz <=same <=
In backplane applications, high-speed dense termination connectors are required. There are a number of manufacturers that produce connector systems that operate efficiently in the signal rate range up to 12Gbps. These connectors contain differential pair and ground systems that provide good controlled impedance and shielding. Teradyne and Molex manufacture a range of connectors, Model numbers VHDM, VHDM-HSD and Gbx. Their products integrate the floor into the connector housing, giving it controlled impedance and a high level of shielding.
IEC’s Tutorial has the performance of these connectors and is suitable for websites.
It is possible for an economical system with a cable balancer IC to combine the methods discussed here to meet performance and emissions requirements.