Common Impedance Shield Coupling

Transcription

1 Common Impedance Shield Coupling When a coaxial cable is used at low frequencies and the shield is grounded at both ends, V R I IN S S The shield serves two functions: 1. the return conductor for the signal; 2. a shield and carries the induced noise current. JHLin, EMC; Cabling 1

2 Common Impedance Shield Coupling A remedy: using a three-conductor cable (e.g., a shielded twisted pair). CISC is often a problem in consumer audio systems that use unbalanced interconnection, which usually consists of a cable with a center conductor and a shield terminated in a phono plug. Remedies: R S or a balanced interconnection and a shielded twist pair. JHLin, EMC; Cabling 2

3 Common Impedance Shield Coupling Even if the shield is grounded at only one end, noise currents may still flow in the shield because of EM field coupling --- SCIN (shield current induced noise) This problem does not occur at high frequencies, because as the result of skin effect, a coaxial cable actually contains three isolated conductors. JHLin, EMC; Cabling 3

4 Experimental Data Test setup 50 khz > 5 times the shield cutoff frequency JHLin, EMC; Cabling 4

5 Experimental Data Circuits are grounded at both ends. JHLin, EMC; Cabling 5

6 Experimental Data Circuits are grounded at one end only. It is almost always better to connect the shield and the signal conductors together at just one point. That point should be such that noise current from the shield does not have to flow down the signal conductor to get to the ground. JHLin, EMC; Cabling 6

7 Example of Selective Shielding The shielded loop antenna is an example where E is selectively shielded, whereas H is unaffected. Such an antenna is useful in radio direction finders and as a magnetic field probe for precompliance EMC measurements. It can also decrease the antenna noise pickup in broadcast receivers. The latter effect is significant because most local noise sources generate a predominantly electric field. Vm 2 f BAcos Vm 0 V m 2 f BAcos V c 2 AE cos Vc 0 Vc 0 JHLin, EMC; Cabling 7

8 Shield Transfer Impedance A means of measuring the shielding effectiveness of cable shields. 1 dv ZT, where V is the open circuit voltage IS d developed between the center conductor and the shield. The smaller the transfer impedance, the more effective the shielding. At low frequencies, the transfer impedance is equal to the dc resistance of the shield. At higher frequencies (above 1 MHz for typical cables), because of the skin effect. Z T JHLin, EMC; Cabling 8

9 Shield Transfer Impedance JHLin, EMC; Cabling 9

10 Coaxial Cable vs. Twisted Pair Ethernet and HDMI: Less C and more tightly and uniformly twisted Twisted pair cables do not have as uniform as coaxial cables. Bonded twisted pair: more uniform, immune to noise, much less radiation. A twisted pair cable is inherently a balanced structure and effectively rejects noise. For example, Cat 5 and 6 Ethernet cables. JHLin, EMC; Cabling 10 Z 0 Z 0

11 Coaxial Cable vs. Twisted Pair Cat 5e (125 MHz), Cat 6 (250 MHz), Cat 7 (600 MHz). Coaxial cables have more uniform, lower losses. A double-shielded, or triaxial, cable can eliminate the noise voltage produced by the shield resistance. But it is expensive and awkward to use. Z 0 A coaxial cable behaves better at high frequencies. A STP has characteristics similar to a triaxial cable. A UTP, unless its terminations are balanced, not good at capacitive pickup protection, but very good for protection against H pickup. UTP JHLin, EMC; Cabling 11

12 Coaxial Cable vs. Twisted Pair When terminating a twisted pair, do not untwist the conductors any more than necessary to make the termination. Twisted pair effective in reducing H coupling when 1. the signal must flow equally and in opposite directions on the two conductors; 2. the pitch of the twist < 1/20 of at the frequency of concern. (500 MHz a twist/inch) JHLin, EMC; Cabling 12

13 Braided Shields The advantage of braid is flexibility, durability, strength, and long flex life. Braided shields usually provide just slightly reduced E shielding (except at UHF frequencies) but greatly reduced H shielding. The reason is that braid distorts the uniformity of the longitudinal shield current. JHLin, EMC; Cabling 13

14 Braided Shields Loose-weave braid provides more flexibility. A tighter weave braid provides better shielding and less flexibility. For the best shielding, the braid should provide at least 95% coverage. JHLin, EMC; Cabling 14

15 Spiral Shields A spiral shield is used on cables for one of 3 reasons: 1. reduced manufacturing cost; 2. ease of termination; 3. increased flexibility. The shield current follows the spiral JHLin, EMC; Cabling 15

16 Spiral Shields I L behaves the same as the shield current does on a solid, homogeneous, shielded cable and produces a circular H external to the cable. The longitudinal H increases the shield inductance. Braided shield cables have better high-f performance than spiral shield cables. Z T that results from the circular component of the shield current increases with f. as f Z T larger, the larger. Z T above about 100 khz. The JHLin, EMC; Cabling 16

17 Spiral Shields Spiral shields are used at audio frequencies. JHLin, EMC; Cabling 17

18 Shield Terminations Most shielded cable problems are the result of improper shield termination. Requirements of a proper shield termination: 1. Termination at the proper end, or ends, and to the proper point or points. 2. A very low impedance termination connection. 3. A 360 contact with the shield. Pigtails The shield should be terminated uniformly. BNC connectors. 18

19 Shield Terminations UHF connectors Type-N connectors JHLin, EMC; Cabling 19

20 Shield Terminations A female XLR connector with spring fingers. Without a connector, still providing a 360 contact of the shield. The use of a pigtail termination whose length is only a small fraction of the total cable length can have a significant effect on the noise coupling to the cable at frequencies above 100 khz. Coupling to a 3.7-m shielded cable with an 8-cm pigtail termination. JHLin, EMC; Cabling 20

21 Shield Terminations JHLin, EMC; Cabling 21

22 Shield Terminations Grounding of Cable Shields Where should the shield be terminated? It depends! 1. Low-frequency cable shield grounding 2. High-frequency cable shield grounding 3. Hybrid-frequency cable shield grounding 4. Double shielded cable grounding JHLin, EMC; Cabling 22