{"id":41,"date":"2019-08-20T21:15:38","date_gmt":"2019-08-20T21:15:38","guid":{"rendered":"http:\/\/iconoclastcable.com\/blog\/?p=41"},"modified":"2019-08-20T21:15:38","modified_gmt":"2019-08-20T21:15:38","slug":"shields-and-noise-cable-dynamics","status":"publish","type":"post","link":"https:\/\/iconoclastcable.com\/blog\/shields-and-noise-cable-dynamics\/","title":{"rendered":"Shields and Noise Cable Dynamics"},"content":{"rendered":"\n

We tend to believe\nthat SHIELDED cables are superior to UNSHIELDED cables but the opposite is true\nfrom a signal transmission evaluation. Why we feel SHIELDED is better is\nbecause we overestimate NOISE ingress (outside the cable into the cable)\nenvironmental issues.<\/p>\n\n\n\n

Cable electrical is determined by the primarily REACTIVE\nvariables that change signal shape arrival times. SHIELDING is to be considered\na necessity if, and only if, the ingress noise is more damaging than the time\nbased errors and physical size shielding imposes on cables. Why even have\nshields if it doesn\u2019t HELP improve the signal integrity from one end of the\ncable to the other?<\/p>\n\n\n\n

Capacitance is derived by the relationship of the shield to\nthe signal conductors in cable. The shield is usually at GROUND potential to be\na low impedance path for noise, so far so good. The bad news is that the CLOSER\na shield is to the signal wires, the more the cable varies per unit length in\nmeasured electrical values of capacitance. It isn\u2019t the same cable all along\nits length from shield geometry variation, and the variation is much more\naggressive the closer the shield is to the signal wires. Capacitance, and thus\nalso inductance, change with smaller physical changes in the cable.<\/p>\n\n\n\n

If you want to keep cable size SMALL, a shield means much\nhigher CAPACITANCE. And, a smaller size WITH that higher capacitance means a\nlarger per unit length variation in measured electrical.  Even with AIR as a dielectric, we will see\nmuch higher capacitance, and have a harder time controlling it with shields, so\nwe better need one for the function of the cable, and where it is used. <\/p>\n\n\n\n

The following calculated table shows that the DIELECTRIC\nin-between the shield and the signal wire can REDUCE capacitance, but only to a\npoint. It cannot remove the shield to conductor physical variation, which is\nbuilt into the DESIGN, good or bad.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

How bad is the actual variation between the shield and\nsignal wire? This exact question was discussed when ultra high-speed\ncommunications cables were being developed. Do we control the center-to-center\nspacing in a BONDED PAIR over all else, or do we control the shield spacing and\ngeometry AROUND that bonded pair? BOTH will influence the final impedance, and\nits variation. Which is really the bigger problem? Can we make better cables\nmanaging what really makes the biggest difference, and reserve the less\naggressive physical attribute for higher performance requirements? This can\nmake the AVERAGE level of performance much higher at a much lower cost than\nblindly trying to manage every variable all the time without a firm reference\nto the cable\u2019s final electrical values and variations. <\/p>\n\n\n\n

Here is that exact analysis;<\/p>\n\n\n\n

To demonstrate the\neffectiveness of conductor center to center (C-C) in an ISTP cable, the example\nbelow shows a change of C-C from 0.055\u201d to 0.072\u201d, holding a constant 0.061\u201d\ninsulation diameter. This simulates a conductor with poor concentricity within\na well-controlled and constant insulation diameter. The impedance is nominally\n102 ohms with a 0.061\u201d C-C spacing and changes ever so slightly as the\nconductors are spaced closer, or farther apart. \nThe shield inside dimension is a constant 0.122\u201d. Under these\ncircumstances, the impedance goes from just over 101 ohms to just over 97 ohms.\nA total impedance spread of about 4 ohms.<\/em><\/p>\n\n\n\n

The significance of\nthe calculations is the relative insensitivity of impedance value with changing\nC-C spacing compared to the variation in diameter of the shield, both of which\naffect impedance variation with frequency. The impedance versus shield spacing\ngraph shows how severe the impedance change is with ISTP shield inside diameter\n(I.D.) changes. Just a 20 mil change (0.120\u201d-0.140\u201d) moves the impedance almost\n14 ohms. Our specifications allow only a 15-ohm swing.  <\/em><\/p>\n\n\n\n

The control of the effective shield diameter is three and\none-half times more sensitive than the C-C spacing of the conductors in ISTP\ncables. Or, shield tape control is much more important than insulation\ncentering or backtwisting to compensate for off-center conductors. Also notice\nthat the closer the conductors move towards the shield in the IMPEDANCE VS\nCONDUCTOR SPACING chart, the more Zo changes. When the conductors are 0.055\u201d to\n0.065\u201d C-C, the impedance varies by less than one ohm. In contrast, when the\nconductors are near the shield in the 0.065\u201d to 0.072\u201d C-C range, the impedance\nchanges 4 ohms. Unless your C-C is well out of spec (we have a 0.01\u201d variation\nwith little change in impedance in this example) good shield dimensions are\nmuch more important.<\/p>\n\n\n\n
\"\"<\/td>\"\"<\/td><\/tr><\/tbody><\/table>\n\n\n\n

In contrast to ISTP cables, the UTP cable example shows\nhow profound the impedance impact is when the C-C changes just 11 mils compared\nto 17 mils in the ISTP example above. \nWhere the ISTP cable had about a 4-ohm swing, the UTP cable has a 60-ohm\nswing! In UTP cable, ground plane consistency is inherently stable because it’s\nthe metallic area around the cable which, under normal circumstances, is\nperceived to be infinitely far away by the cable, too far to effect the\nelectrical to any significant degree. So the crucial variable in UTP cable for\nconsistent impedance is the strict control of C-C. This is why Belden\u2019s\npatented bonded pair technology is so important in UTP cable designs.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Impedance is, after all, a function of the Inductance,\ncapacitance and dielectric values. The impedance variation, and even at each\nfrequency in the audio band, changes with the dielectric and the spacing.<\/p>\n\n\n\n

A cable with NO SHIELD, sometimes called UTP, does have a\nreference ground \u201caround\u201d the cable, the environment. But, the capacitive and \/\nor inductive coupling are so far away that changes in the \u201creference\u201d are\nessentially zero. <\/p>\n\n\n\n

Shields have to pull their weight in signal integrity\nimprovements compared to cables used without a shield. If we have no external\nnoise, SHIELDS ARE WORSE than no shields! The math of cable electrical\nstability firmly squares that up, per the data shown above.<\/p>\n\n\n\n

This forces the consideration of NOISE. It even considers\nHOW noise is transferred into (ingress) a cable, and even if the cable itself\nis the source of NOISE for other external devices (EGRESS).<\/p>\n\n\n\n

First, let\u2019s be super straightforward about this from a\n25,000-foot view. The closer a shield is, the capacitance value is high, and it\nvaries the most around the average value. Knowing that the proximity a shield\nhas around the signal wire can really upset the cable\u2019s uniformity of\nelectrical, and how uniform we can engineer them, would we not want to use\ndesigns that NATURALLY calculate an advantage to use with shield? Yes, we\nwould. <\/p>\n\n\n\n

To keep this easy, look at coaxial cables. This technology\nHAS TO HAVE a shield to work. A signal wire surrounded by a shield. The signal\nwaveform travels along the wire surface, and under the shield surface and\ninside the dielectric as a TEM (Transverse Electromagnetic Wave) wave. The more\nperfectly round the inner surface of the shield and the outer surface of the\nsignal wire, the lower the capacitive and inductive variation and thus a lower\nimpedance variation.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

\nFor signal transmission, we use 75-ohm cable (77-ohm is the ideal) and for\npower 50-ohm Cables  (30-ohm is the\nideal). Approximately 53.5-ohm military RG cables came about because it is the\nmean between 33 and 77. If we freeze the materials we use to make the cable\n(same plastics and metals) we will see that a 75-ohm cable has a larger\ndielectric layer (lower capacitance) than the 50-ohm cable. \n\n\n\n\n\n\n\n<\/p>\n\n\n\n

This is nice, since the farther away the shield is, the less\na given VARIATION of the shield changes the electrical stability. Reactive\nvariation impacts small voltage signals far more than larger 50-ohm power cable\napplications with much more robust signal levels. <\/p>\n\n\n\n

In a 50-ohm power type cable, we have a shield that is far\ncloser to the signal wire. This seems like a problem and it is, but the SIZE of\nthe signal is vastly larger than the NOISE. We can overcome the noise with a\nlarger signal, and even the return loss caused by more variable impedance can\nalso be mitigated with the size of the signal on power type coaxial cable.<\/p>\n\n\n\n

This is simply the signal to the noise reference working in\nour advantage in each design. <\/p>\n\n\n\n

  • Voltage signal cables, dB or dBm, need shields farther\naway (higher impedance) and it so happens this is the case with 75-ohm cables,\nreducing capacitive coupling of noise.<\/li>
  • Power signal cables, often in WATTS, need closer\nshields for energy transfer (lower impedance) but this allows more capacitive\nnoise coupling. 50-ohm cables use more robust signals to overcome the noise.\nThis is like a low impedance speaker cable\u2019s signal WAY over the terrestrial\nnoise floor.<\/li><\/ul>\n\n\n\n

    There is NO EXCEPTION, lower impedance cables are much more\nsubjective to NOISE than higher impedance cables with the same<\/em> noise ingress. We must fit the signal levels to the\nimpedance for ideal overall performance. 75-ohm cables are far better for\nlow-level signals as they capacitively couple less noise, as the DISTANCE to\nthe shield is larger. <\/p>\n\n\n\n

    \"\"<\/figure>\n\n\n\n

    \nTo put the signal in perspective to NOISE, look at the table below.\n\n\n\n\n\n\n\n<\/p>\n\n\n\n

    Digital data cable go 100 meters \/ 328 feet with\nover 23 dB of attenuation at 100 MHz and with ZERO errors due to external\nnoise, with UTP designs. Audio cables go mere feet, and yes we seem to want to\nbe the underdogs of signal integrity but we aren\u2019t, and that\u2019s a really good\nthing, too. <\/p>\n\n\n\n

    The integrity that even a MC phono cartridge\u2019s\n0.35mV signal represents to the noise is in our favor.  The robust signal even covers up POORLY made\nSHIELDED cables. Do the shield really right, and it can help some RF, but\nusually in a good unbalanced RCA system a RF bleed capacitor routes RF to\nground through the cap somewhere in the ground.<\/p>\n\n\n\n

    Coaxial cables need shields to work, and they need\nshields to be super low DCR to prevent ground loop differential currents\nbetween devices. The GROUND is shared in coaxial cables at uneven ground\nreference points. RCA grounds have resistive differences. This can cause signal\nbleed between channels. A BIG part of an audio coaxial cable shield is to\nmitigate ground potential differences, and not to \u201cshield\u201d ingress.<\/p>\n\n\n\n

    \"\"<\/figure>\n\n\n\n

    \nA balanced XLR uses a SHIELD, yes, but it is NOT a part of the signal path, and\neach right and left signal doesn\u2019t share the virtual ground between the\ndifferential voltage signals. Each amp has its isolated virtual SIGNAL ONLY\nground reference. There can be no inductive or capacitive coupling of right and\nleft channels. Unhook the GROUND on a XLR and it will work, with MAYBE a\nslightly higher SN ratio. The outer shield simply knocks down the noise ingress\nat RF, if any is there, so UNBALANCE in the pairs mitigates to a lower residual\nvalue. One-percent unbalance of a small signal is better than one-percent of a\nlarger signal.\n\n<\/p>\n\n\n\n

    This is the true advantage of XLR cables over RCA.\nBoth have good RF noise immunity with the XLR having far superior signal\nchannel isolation and\u2026low frequency noise isolation.<\/p>\n\n\n\n

    Since an XLR FLOATS the virtual ground independent\nfrom any other signal, noise is the same on each leg, so it cancels. We see the\n\u201cdifference\u201d of each leg as the signal, which doesn\u2019t change potential. This\nincludes magnetic and electric fields. Coaxial cables can\u2019t shield magnetic\nfields since copper is \u201cinvisible\u201d to 60 Hz magnetic interference.<\/p>\n\n\n\n
    \"\"<\/td>\"\"<\/td><\/tr><\/tbody><\/table>\n\n\n\n

    ICONOCLAST™ uses SHIELDS, but the WAY we use shields\ninsures geometric consistency to the signal wires. Care was taken to insure a\ngood BALANCE within the XLR signal wires so even if a shield is broken, little\nperformance impact will be measured;<\/p>\n\n\n\n

    Capacitance @ 1 kHz<\/strong> per ELP 423, Agilent E4980 Precision LCR Meter,\nBelden 4TP Cap\/Ind Test Fixture, all tests performed on a 20ft specimen.<\/p>\n\n\n\n

    Pr to Pr(star quad) \u2013 10.4113 pF\/ft<\/p>\n\n\n\n

    UnBalanced:  Pr 1 to Shld \u2013 401.9868 pF\/20ft<\/p>\n\n\n\n

                           Pr 2 to Shld \u2013 405.9738<\/p>\n\n\n\n

    Cap UnBal ((diff\/max) * 100) \u2013 0.98%<\/p>\n\n\n\n

    Requirement \u2013 3% maximum<\/p>\n\n\n\n

    \"\"<\/figure>\n\n\n\n

    SHIELD TRANSFER IMPEDANCE \u2013 <\/strong>This is\na measure of the cable\u2019s shield impedance in milli-ohm\/meter. The lower the\ntransfer impedance at a specification frequency the better the shield at that\nfrequency. It is frequency and design dependent. The current traveling in the\nshield times the transfer impedance produces a interference voltage product to\nground in the shield, E=I*R where R is the transfer impedance. \n\n\n\n\n\n\n\n<\/p>\n\n\n\n

    SUMMARY<\/strong> \u2013 Shields have to be considered relative to noise and the\nresulting S\/N ratio since the application of a shield is ALWAYS a negative\nvariable to signal integrity that has to be over weighed by true noise\nmitigation. If noise is paramount over the signal, then shields are a necessary\nrequirement. If shields are a part of the signal path, then the noise they\ngenerate has to be mitigated with shield DCR. <\/p>\n\n\n\n

    ICONOCLAST uses shields properly,\nand insures that the negative influences are geometrically stabilized, and\nmeasured for performance in both RCA (DCR) and XLR (unbalance percentage). <\/p>\n","protected":false},"excerpt":{"rendered":"

    We tend to believe that SHIELDED cables are superior to UNSHIELDED cables but the opposite is true from a signal transmission evaluation. Why we feel SHIELDED is better is because we overestimate NOISE ingress (outside the cable into the cable) environmental issues. Cable electrical is determined by the primarily REACTIVE…<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"class_list":["post-41","post","type-post","status-publish","format-standard","hentry","category-cable-design-principles"],"_links":{"self":[{"href":"https:\/\/iconoclastcable.com\/blog\/wp-json\/wp\/v2\/posts\/41","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/iconoclastcable.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/iconoclastcable.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/iconoclastcable.com\/blog\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/iconoclastcable.com\/blog\/wp-json\/wp\/v2\/comments?post=41"}],"version-history":[{"count":0,"href":"https:\/\/iconoclastcable.com\/blog\/wp-json\/wp\/v2\/posts\/41\/revisions"}],"wp:attachment":[{"href":"https:\/\/iconoclastcable.com\/blog\/wp-json\/wp\/v2\/media?parent=41"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/iconoclastcable.com\/blog\/wp-json\/wp\/v2\/categories?post=41"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/iconoclastcable.com\/blog\/wp-json\/wp\/v2\/tags?post=41"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}