Shields and Noise Cable Dynamics

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 variables that change signal shape arrival times. SHIELDING is to be considered a necessity if, and only if, the ingress noise is more damaging than the time based errors and physical size shielding imposes on cables. Why even have shields if it doesn’t HELP improve the signal integrity from one end of the cable to the other?

Capacitance is derived by the relationship of the shield to the signal conductors in cable. The shield is usually at GROUND potential to be a low impedance path for noise, so far so good. The bad news is that the CLOSER a shield is to the signal wires, the more the cable varies per unit length in measured electrical values of capacitance. It isn’t the same cable all along its length from shield geometry variation, and the variation is much more aggressive the closer the shield is to the signal wires. Capacitance, and thus also inductance, change with smaller physical changes in the cable.

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

The following calculated table shows that the DIELECTRIC in-between the shield and the signal wire can REDUCE capacitance, but only to a point. It cannot remove the shield to conductor physical variation, which is built into the DESIGN, good or bad.

How bad is the actual variation between the shield and signal wire? This exact question was discussed when ultra high-speed communications cables were being developed. Do we control the center-to-center spacing in a BONDED PAIR over all else, or do we control the shield spacing and geometry AROUND that bonded pair? BOTH will influence the final impedance, and its variation. Which is really the bigger problem? Can we make better cables managing what really makes the biggest difference, and reserve the less aggressive physical attribute for higher performance requirements? This can make the AVERAGE level of performance much higher at a much lower cost than blindly trying to manage every variable all the time without a firm reference to the cable’s final electrical values and variations.

Here is that exact analysis;

To demonstrate the effectiveness of conductor center to center (C-C) in an ISTP cable, the example below shows a change of C-C from 0.055” to 0.072”, holding a constant 0.061” insulation diameter. This simulates a conductor with poor concentricity within a well-controlled and constant insulation diameter. The impedance is nominally 102 ohms with a 0.061” C-C spacing and changes ever so slightly as the conductors are spaced closer, or farther apart.  The shield inside dimension is a constant 0.122”. Under these circumstances, the impedance goes from just over 101 ohms to just over 97 ohms. A total impedance spread of about 4 ohms.

The significance of the calculations is the relative insensitivity of impedance value with changing C-C spacing compared to the variation in diameter of the shield, both of which affect impedance variation with frequency. The impedance versus shield spacing graph shows how severe the impedance change is with ISTP shield inside diameter (I.D.) changes. Just a 20 mil change (0.120”-0.140”) moves the impedance almost 14 ohms. Our specifications allow only a 15-ohm swing. 

The control of the effective shield diameter is three and one-half times more sensitive than the C-C spacing of the conductors in ISTP cables. Or, shield tape control is much more important than insulation centering or backtwisting to compensate for off-center conductors. Also notice that the closer the conductors move towards the shield in the IMPEDANCE VS CONDUCTOR SPACING chart, the more Zo changes. When the conductors are 0.055” to 0.065” C-C, the impedance varies by less than one ohm. In contrast, when the conductors are near the shield in the 0.065” to 0.072” C-C range, the impedance changes 4 ohms. Unless your C-C is well out of spec (we have a 0.01” variation with little change in impedance in this example) good shield dimensions are much more important.

In contrast to ISTP cables, the UTP cable example shows how profound the impedance impact is when the C-C changes just 11 mils compared to 17 mils in the ISTP example above.  Where the ISTP cable had about a 4-ohm swing, the UTP cable has a 60-ohm swing! In UTP cable, ground plane consistency is inherently stable because it’s the metallic area around the cable which, under normal circumstances, is perceived to be infinitely far away by the cable, too far to effect the electrical to any significant degree. So the crucial variable in UTP cable for consistent impedance is the strict control of C-C. This is why Belden’s patented bonded pair technology is so important in UTP cable designs.

Impedance is, after all, a function of the Inductance, capacitance and dielectric values. The impedance variation, and even at each frequency in the audio band, changes with the dielectric and the spacing.

A cable with NO SHIELD, sometimes called UTP, does have a reference ground “around” the cable, the environment. But, the capacitive and / or inductive coupling are so far away that changes in the “reference” are essentially zero.

Shields have to pull their weight in signal integrity improvements compared to cables used without a shield. If we have no external noise, SHIELDS ARE WORSE than no shields! The math of cable electrical stability firmly squares that up, per the data shown above.

This forces the consideration of NOISE. It even considers HOW noise is transferred into (ingress) a cable, and even if the cable itself is the source of NOISE for other external devices (EGRESS).

First, let’s be super straightforward about this from a 25,000-foot view. The closer a shield is, the capacitance value is high, and it varies the most around the average value. Knowing that the proximity a shield has around the signal wire can really upset the cable’s uniformity of electrical, and how uniform we can engineer them, would we not want to use designs that NATURALLY calculate an advantage to use with shield? Yes, we would.

To keep this easy, look at coaxial cables. This technology HAS TO HAVE a shield to work. A signal wire surrounded by a shield. The signal waveform travels along the wire surface, and under the shield surface and inside the dielectric as a TEM (Transverse Electromagnetic Wave) wave. The more perfectly round the inner surface of the shield and the outer surface of the signal wire, the lower the capacitive and inductive variation and thus a lower impedance variation.

For signal transmission, we use 75-ohm cable (77-ohm is the ideal) and for power 50-ohm Cables  (30-ohm is the ideal). Approximately 53.5-ohm military RG cables came about because it is the mean between 33 and 77. If we freeze the materials we use to make the cable (same plastics and metals) we will see that a 75-ohm cable has a larger dielectric layer (lower capacitance) than the 50-ohm cable.

This is nice, since the farther away the shield is, the less a given VARIATION of the shield changes the electrical stability. Reactive variation impacts small voltage signals far more than larger 50-ohm power cable applications with much more robust signal levels.

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

This is simply the signal to the noise reference working in our advantage in each design.

• Voltage signal cables, dB or dBm, need shields farther away (higher impedance) and it so happens this is the case with 75-ohm cables, reducing capacitive coupling of noise.
• Power signal cables, often in WATTS, need closer shields for energy transfer (lower impedance) but this allows more capacitive noise coupling. 50-ohm cables use more robust signals to overcome the noise. This is like a low impedance speaker cable’s signal WAY over the terrestrial noise floor.

There is NO EXCEPTION, lower impedance cables are much more subjective to NOISE than higher impedance cables with the same noise ingress. We must fit the signal levels to the impedance for ideal overall performance. 75-ohm cables are far better for low-level signals as they capacitively couple less noise, as the DISTANCE to the shield is larger.

To put the signal in perspective to NOISE, look at the table below.

Digital data cable go 100 meters / 328 feet with over 23 dB of attenuation at 100 MHz and with ZERO errors due to external noise, with UTP designs. Audio cables go mere feet, and yes we seem to want to be the underdogs of signal integrity but we aren’t, and that’s a really good thing, too.

The integrity that even a MC phono cartridge’s 0.35mV signal represents to the noise is in our favor.  The robust signal even covers up POORLY made SHIELDED cables. Do the shield really right, and it can help some RF, but usually in a good unbalanced RCA system a RF bleed capacitor routes RF to ground through the cap somewhere in the ground.

Coaxial cables need shields to work, and they need shields to be super low DCR to prevent ground loop differential currents between devices. The GROUND is shared in coaxial cables at uneven ground reference points. RCA grounds have resistive differences. This can cause signal bleed between channels. A BIG part of an audio coaxial cable shield is to mitigate ground potential differences, and not to “shield” ingress.

A balanced XLR uses a SHIELD, yes, but it is NOT a part of the signal path, and each right and left signal doesn’t share the virtual ground between the differential voltage signals. Each amp has its isolated virtual SIGNAL ONLY ground reference. There can be no inductive or capacitive coupling of right and left channels. Unhook the GROUND on a XLR and it will work, with MAYBE a slightly higher SN ratio. The outer shield simply knocks down the noise ingress at RF, if any is there, so UNBALANCE in the pairs mitigates to a lower residual value. One-percent unbalance of a small signal is better than one-percent of a larger signal.

This is the true advantage of XLR cables over RCA. Both have good RF noise immunity with the XLR having far superior signal channel isolation and…low frequency noise isolation.

Since an XLR FLOATS the virtual ground independent from any other signal, noise is the same on each leg, so it cancels. We see the “difference” of each leg as the signal, which doesn’t change potential. This includes magnetic and electric fields. Coaxial cables can’t shield magnetic fields since copper is “invisible” to 60 Hz magnetic interference.

ICONOCLAST™ uses SHIELDS, but the WAY we use shields insures geometric consistency to the signal wires. Care was taken to insure a good BALANCE within the XLR signal wires so even if a shield is broken, little performance impact will be measured;

Capacitance @ 1 kHz per ELP 423, Agilent E4980 Precision LCR Meter, Belden 4TP Cap/Ind Test Fixture, all tests performed on a 20ft specimen.

Pr to Pr(star quad) – 10.4113 pF/ft

UnBalanced:  Pr 1 to Shld – 401.9868 pF/20ft

                       Pr 2 to Shld – 405.9738

Cap UnBal ((diff/max) * 100) – 0.98%

Requirement – 3% maximum

SHIELD TRANSFER IMPEDANCE – This is a measure of the cable’s shield impedance in milli-ohm/meter. The lower the transfer impedance at a specification frequency the better the shield at that frequency. It is frequency and design dependent. The current traveling in the shield times the transfer impedance produces a interference voltage product to ground in the shield, E=I*R where R is the transfer impedance.

SUMMARY – Shields have to be considered relative to noise and the resulting S/N ratio since the application of a shield is ALWAYS a negative variable to signal integrity that has to be over weighed by true noise mitigation. If noise is paramount over the signal, then shields are a necessary requirement. If shields are a part of the signal path, then the noise they generate has to be mitigated with shield DCR.

ICONOCLAST uses shields properly, and insures that the negative influences are geometrically stabilized, and measured for performance in both RCA (DCR) and XLR (unbalance percentage).