Choosing the cabling for data transmission

Transcription

1 Choosing the cabling for data transmission Jacob Ben Ary Senior Cable Design Engineer Cable Engineering & Design Dept. TELDOR Wires & Cables Ein-Dor, 19335, Israel KEYWORDS Characteristic impedance, Return Loss (RL), Cross talk (XT), Attenuation, Transmission parameters ABSTRACT In this paper, a family of graphical representations and equations has been developed to gain a solid basic understanding of twisted pair transmission lines used for digital signal transmission. The approach of graphical representation and some basic simple equations requires a little background in electromagnetic field theory. INTRODUCTION Digital data communication is of increasing importance in industrial automation, with no sign of slowing down. Applications such as industrial Buses, industrial Ethernet and industrial LANs are becoming part of plant and process implementations. A common component in all the applications is the communication channel. This may be a transmission line consisting of cable, optical fiber, wave-guide, `free space` propagation (wireless), etc. This paper is concerned with copper conductor twisted pair transmission line cables. A transmission line is a set of conductors used for transmitting electrical signals. The conductors are arranged in pairs where each pair is twisted and may have a shield. The twisted pair is called a cable element. In most cases the cable can be defined as uniform. Uniform cable is one whose geometry and materials are uniform. That is, the conductor shape, size, cable dimensions and electrical performance are uniform over the cable's length. An example of uniform transmission lines are twisted wires pairs and coaxial cables. When two wires are twisted to create a twisted pair transmission line, one can define a pair series resistance (R), inductance (L), capacitance (C) and conductance between the two conductors (G). These pair electrical parameters are called the primary parameters. Transmission parameters such as attenuation constant (a), characteristic impedance (Z 0 ), phase constant (ß) and propagation constant (?) rely on the calculation of the primary parameters. For a uniform transmission line the primary parameters are usually

2 specified in units per meter. The cable transmission parameters and the application transmission requirements can be used to choose the right cable for data transmission. PRIMARY PARAMETERS Practical wires and conductors in cables used for data transmission have a round shape. Each wire has an overall diameter (over insulation) D [mm], and conductor effective diameter d [mm]. The wire low frequency resistance R w in ohms [O] is defined as: l R w = ρ [O] (1) s Where l is the wire length in meters s is the wire cross section in mm 2? is the resistivity in O mm 2 /meter, for copper the value is 1/58 [O mm 2 /m]. The wire high frequency resistance is affected by the skin effect. The skin effect increases the resistance as a function of the square root of the frequency. The exact solution can be calculated by means of Bessel functions. For practical calculations there are some empirical formulas. The wire coaxial capacitance C w in pf/m can be calculated by the formula in equation (2). ε C w = 1000 [pf/m] (2) D 18lan d Where e is the effective relative permittivity of the wire insulation The wire inductance L w in mh/km can be calculated by equation (3). D L w = 0.2lan + L0 [mh/m] (3) d Where L 0 is the internal inductance of the conductor, L 0 can be calculated by means of Bessel functions or empirical formulas. For unshielded symmetrical twisted pair the primary parameters practical equations are (4), (5) and (6). R = 2R w [O] (4) Where R is the loop resistance, for high frequency resistance the proximity effect should be calculated in addition to the skin effect [2]. 1000ε C = [pf/m] (5) 1 D 36 cosh d Where C is the pair capacitance assuming that the wires are equal and in mechanical contact so that the distance between the wires centers is D. 1 D L = 0.4 cosh + L0 [mh/km] (6) d Here L is the pair inductance. The equations can be simplified by using the approximation for cosh -1 :

3 2 1 1 cosh ( ) ln{ ( ) x = x } (7) x The conductance between the conductors is the dissipative component of the capacitance in parallel to the insulation resistance of the insulation material. The insulation resistance value is very high and can be ignored. G = wctan δ [µs/km] (8) Here tand is the insulation material dissipation factor and is given in the manufacturer data-sheets. TRANSMISSION PARAMETERS In a simple transmission line, a source provides a signal that must reach a load (Figure 1). An ideal transmission line is defined as two parallel wires where the voltage at all points on the wires is exactly the same. In reality, this situation is never quite true. Any real wire and real pair has its primary parameters, thus the transmission line model consists of an infinite series of infinitesimal R, L, C and G elements (Figure 1). Notice several important points. First, a signal transmitted from the source charges and discharges the line's inductance and capacitance. Hence, the signal does not arrive instantly at the load but is delayed. Second, the signal level at the load is lower than in the source because of the attenuation in the elements. Last, the impedance at any point along the line depends not just on the source and load, but also on the primary parameters L, C, R and G. The transmission line can be described as a guide for electromagnetic waves. With this description the source sends the electromagnetic signal, which consists of a voltage wave and a current wave, to the load (Figure 2). The transmission line acts as transmission medium which guides the signal waves along the line. The signal travels at the speed of light within the medium,?. The speed? can be calculated from the permittivity e, and the permeability µ of the dielectric between the conductors. 1 ν = c [m/sec] (9) µε Where c is the speed of light in a vacuum. For practical calculations the speed? is defined as percentage of the speed of light.

4 100 ν p = [%] (10) µε For most of the insulation materials the permeability µ has the value 1. As the signal travels along the transmission line, the voltage and current waves define the voltage and current at each point. At each point the ratio of the voltage to the current is defined as the characteristic impedance, Z 0. Since the line is assumed to be uniform, the characteristic impedance is constant. The characteristic impedance is defined by the line primary parameters and the geometry of the wires. R + jwl Z0 = [O] (11) G + jwc Where j = 1, and w=2pf, f is the signal frequency. For data transmission cables the common standard values of characteristic impedance are: 120 O, for RS-485, RS-232, RS O for Ethernet. 150 O for Profi-Bus. Whenever an electromagnetic wave encounters a change in impedance, some of the signal is reflected (Figure 3). The difference between the impedances determines the amplitude of the reflected and the transmitted waves. The reflection coefficient? is defined as: Vreflected Z2 Z1 ρ = = (12) V Z Z incident 2 1 Thus the transmission coefficient? equation will be: V Γ = transmitte d = 1+ ρ V incident Notice that higher reflections mean a lower transmitting signal or higher signal attenuation. In summary: Signal reflections occur at impedance boundaries Reflections means higher signal attenuation A signal traveling down a line has delay associated with it A traveling signal can be defined as traveling voltage and current waves related by the characteristic impedance of the line The practical Transmission Parameters equations are: Characteristic Impedance R + jwl Z0 = [O] G + jwc Notice that approximation for low frequency (less than 100 KHz) and high frequency is suitable. Propagation constant γ = R + jwl G + jwc (14) ( )( ) Line Attenuation (13)

5 α ( p γ ) = 8.686γ cos [db] (15) Return Loss (Reflections) This paragraph graphically describe the effect of reflections occur at the load end of a line. The following lattice diagram shows the propagation of 1V step wave and its subsequent reflections. The diagram shows the first five reflections.

6 Here T is time and t is the time the signal travels from one end to the other. In summery Reflection can cause over-shots Reflections occur at any point within the line where the impedance has mismatch values The reflected wave can be sensed as signal at the source CROSS TALK Coupling between circuits which enable signal on one circuit to be picked up on a neighboring circuit is called cross talk. In data transmission cables cross talk can occur between cables or between twisted pairs within the cable. The known couplings between pairs are electrostatic coupling, magnetic coupling and resistive coupling. Electrostatic coupling between terminated pairs are shown in Figure 3. The difference between the coupling capacitor's values is used to define the capacitance unbalance. Smaller electrostatic coupling is achieved by a small capacitance unbalance value. Screening each pair practically reduces the capacitance unbalance to a value of zero. The magnetic coupling between terminated pairs is shown in Figure 4.

7 The value of the coupling constant M can be reduced by twisting the pairs and by optimization of the twisting scheme of the pairs. Notice that individual screening of each pair does not remove the necessity of twisting the pair. The resistive coupling between terminated pairs is shown in Figure 5. Notice that resistance coupling G is a parallel connection of the insulation resistance and the dissipative component of the capacitance. To reduce this coupling the insulation material should have a very high insulation resistance value and very small dissipative value. For comparison, PVC has dissipative factor and insulation resistance ten times higher than PE. To reduce the cross talk between pairs in a data transmission cable the user should consider Low capacitance unbalance pairs which can be achieved by the use of PE insulation material or alike Individual screening of each pair (recommended) Using a data transmission cable which contains twisted pairs with different lays Insulation materials with high insulation resistance values Insulation materials with a stable performance over frequency and temperature ranges in use (PVC is not recommended) RFI/EMI (Radio frequency and Electromagnetic Interference) A common source of electrical noise which can interfere with the data signals is the EMI/RFI. The source of this noise can be any source outside the cable (e.g. cellular phones, electric machines, switching power supplies) or a cable which contains power lines or high frequency signals. To reduce the interference effect shields should be used. The common shields types are foil tape and braid. The foil shield can provide 100% coverage, but because of manufacturing limitations the foils for data cable are made from thin aluminum or copper which are not effective over the entire frequency range, and require a drain wire which cannot provide optimal electrical contact with the foil. The braid shield can provide up to 95% coverage, but flexibility is low while price is high. The optimal solution is a double shield consisting of foil and braid. The double shield effectiveness is very high, and provides good drain connection by using the braid. The recommended shielding structure

8 is to combine the advantages of an individual pair screen and overall shield by using foil screens over each pair and an overall braid. In this way the internal cross talk is reduced almost to zero. In any case of using screens and shields the user must avoid ground loops. A lot of interference can occur if current is flowing in ground loops. CONCLUSION The preferred cable for data transmission in the industrial environmental should have: A twisted pair construction if the application can handle it Low capacitance Stable and matched characteristic impedance, to the load and to the source Low impedance mismatch along the pair (cable) length Individual screened pairs Overall braid shield Stable insulation material performance over the entire frequency and temperature range Available transmission test results (e.g. attenuation, crosstalk, impedance) REFERENCES 1. Telecommunication cables, Harold Hoghes, John Wiley & Sons Ltd. 2. High speed signal propagation, Howard Johnson, Practice Hall. 3. Twisted pair cable design analysis and simulation, Mohammed M Al-Asadi, IWCS 49 th. 4. Analyze transmission lines, Ron Schitt, EDN March 1999.