Getting the Best EMC from Shielded Cables Up to 2.8 GHz, Part 2

In Part 1 of this article, I shared with you the origins of my journey to assess the shielding effectiveness (SE) of screened1 cables and discussed some basic rules for terminating cable shields. In Part 2, I’ll summarize the testing I recently conducted on various approaches to improving the shielding effectiveness of screened cables used in high-frequency applications and the results from that testing.

1 In the context of this article, the words: screened; screen, or screening may be replaced by shielded; shield, or shielding respectively, and vice-versa, without any changes in meanings.

Note: all these cables’ overbraids, whether single or double layers, used the same type of braid clamped to the backshells in the same way at both ends.

A single overbraid on its own, to check that the noise floor of the test is low enough.

Figure 4: The cable assemblies for the reference measurements, and for the single-braid-shielded TP cables with single overbraids (i.e., two braid shield layers in total)

An unshielded twisted-pair (TP) cable on its own (actually, the single-braid-shielded TP cable used to assemble cables 3 to 6, with its outer plastic jacket and shield removed).

The measured results on this cable were used as the reference that was subtracted from the measured results of each of the other cable tests (i.e., cables 3 to 12) to determine their relative SE versus frequency.

Careful control of the entire test set-up tried to ensure that the RF coupling from the antenna to the cable and the room resonance effects were identical on every test so that they canceled out. The results showed that we were reasonably successful in this.

Note: these cables, and Cables 6, 10, 11, and 12 below, all used the same type of single-shielded TP cable.

Note: these four cables, and Cables 3, 4, and 5 above, all used the same type of single-shielded TP cable.

Figure 5: The cable assemblies having single-braid-shielded TP cables with double overbraids (i.e., three braided shield layers in total)

Note: these two cables both used the same type of double-braid-shielded TP cable.

Figure 6: The cable assemblies having double-braid-shielded TP cables with double overbraids (i.e., four braided shield layers in all)

There are many ways of testing the SE of cable assemblies (i.e., cables plus their connectors), and each should be expected to give different results even with identical cable assemblies. So, I chose a test method that best represented the situation I was most interested in and that was also the easiest and quickest to do with the facilities and resources I had available at the time (see Figures 7, 8, and 9).

Figure 7: Sketch of the test set-up

Figure 8: Example of measuring a cable, showing the connections to bulkhead-mounted connectors on the bulkhead connector panel in the wall of the test chamber

Figure 9: Example of measuring a cable, showing injecting RF into a cable

The worst of the imperfections in this method were canceled out by careful control of consistency and repeatability, and by subtracting the measured results for each cable assembly from the measurements of the Reference unshielded TP cable, Cable 2 (see above, and Figure 4).

The test chamber had once been a large TEMPEST chamber for secure communications, but for a long time had been used as a storeroom.

With a spectrum analyzer, near-field RF probe effective up to 6GHz, and a Tek box TBCG1 radiating comb generator, 100MHz – 6GHz, it did not take long to identify the RF leakages and fix them (corroded spring fingers around the door, and a telephone wire that had been brought in without RF suppression). A connector panel (visible in Figure 8) was designed, fabricated, and affixed to a hole cut in the chamber wall and also checked for RF leaks up to 6GHz.

I would have preferred either an anechoic chamber or a mode-stirred chamber, but at least the metal racking and the stored equipment in the room broke up most of its major resonant modes! And a few scraps of left-over ferrite tiles from an anechoic EMC test chamber were enough to deal with the worst remaining standing waves.

I was not interested in absolute values of SE, only in which cable design/assembly methods were the best for SE. In other words, their relative SE performances. I hoped to extract some general guidance rules for overbraid-shielded cables or cable bundles containing at least one individually shielded TP cable.

To help achieve this, with the imperfect test set-up briefly described above, a null cable (Cable 1, see Figure 4) was first measured. Being just an empty overbraid, the measurement identified any leakages from the antenna to the CM measurement pins of the bulkhead-mounted shielded connector, which included all chamber and panel leakages, and also the leakages inherent in the overbraid and its shield-bonding to the cable connectors, and from the cable connector to the bulkhead-mounted shielded connectors. This measurement showed that leakages were at or below the measurement noise floor for both frequency ranges.

Next, the Reference cable, Cable 2, was measured. This was an unshielded twisted-pair (TP) cable on its own, as shown in Figure 4, and previously described in detail.

Two different RF power amplifiers, one operating at 100MHz – 1GHz and a second at 800MHz – 2.8GHz, were used to cover the two frequency ranges reported in this article, with the above null and reference tests repeated for each amplifier.

To help achieve consistency between the different RF power amplifiers, a triaxial field probe with a fiber-optic cable passed through a waveguide-below-cutoff in the bulkhead connector panel was used to measure the field strengths around the antenna and the measured cables.

External low-noise preamplifiers with good, flat frequency responses over the measured frequency ranges were used before the spectrum analyzer's input in cases where they would help reduce the noise floor.

All the other measured cables covered by this article consisted of the same null cable assembly used for Cable 1. Additional internal conductors and cables were made by the same very skilled cable assembler, in the same ways, with the same materials, and within a limited time span (a few days) so that we could assume consistency between them.

Given all the above and with the results from each amplifier, subtracting each cable's results from the reference result should have substantially reduced the effects of:

This subtraction/cancellation approach was successful enough to draw conclusions on how best to terminate the shields of multiple shielded cables in an overall cable or bundle with overbraids, up to 2.8GHz. However, there were still some small errors that were deemed insignificant (see if you can spot them in the following figures!).

These are shown in Figure 10 for 100MHz – 1GHz, and Figure 11 for 800MHz – 2.8GHz.

Figure 10: Results for internal single-braid-shielded TP cable, plus a single overbraid 360° clamped to the backshells at both ends — 0.1 to 1GHz

Figure 11: Results for internal single-braid-shielded TP cable, plus a single overbraid 360° clamped to the backshells at both ends — 0.8 to 2.8GHz

These are shown in Figure 12 for 100MHz – 1GHz, and Figure 13 for 800MHz – 2.8GHz.

Figure 12: Results for internal single-braid-shielded TP cable, plus double overbraids both 360° clamped to the backshells at both ends — 0.1 to 1GHz

Figure 13: Results for internal single-braid-shielded TP cable, plus double overbraids both 360° clamped to the backshells at both ends — 0.8 to 2.8GHz

These are shown in Figure 14 for 100MHz – 1GHz, and Figure 15 for 800MHz – 2.8GHz.

Figure 14: Results for internal double-braid-shielded TP cable, plus double overbraids both 360° clamped to the backshells at both ends — 0.1 to 1GHz

Figure 15: Results for internal double-braid-shielded TP cable, plus double overbraids both 360° clamped to the backshells at both ends — 0.8 to 2.8GHz

Note: both of these cables use an internal TP cable with a double shield that is pigtailed at both ends.

I would expect double-braid-shielded TP cables in an overall cable or bundle with double overbraids, with all shield layers 360° terminated to the overbraids and/or backshells at both ends (and no pigtails at all), to give better results than any of the cables measured above. But we did not assemble or measure such a design.

But How to Terminate the Shields of Internal Cables Without Using Pigtails?

Few publications in the public domain (including mine) address how to terminate the shields of individually shielded cables within overbraided cables or cable bundles (ignoring those recommending pigtailing through connector pins!).

This is perhaps because it tends to be an issue for high-spec military or aerospace companies, whose internal design/assembly guides often seem to me to be specifying outdated or non-cost-effective practices, such as pigtailing via connector pins, or requiring a great deal of (costly!) manual assembly by skilled personnel (e.g., 360° soldering an internal braid to an overbraid).

How to cost-effectively terminate cable shields could, on its own, easily fill a whole article, but rather than extend this article by a few thousand words I’ve added Figures 16 to 18, taken from my training course on cable EMC [25], and hope they are sufficiently self-explanatory.

Figure 16: Slide 2.7.23 from [25]

Figure 17: Slide 2.7.24 from [25]

Figure 18: Slide 2.7.25 from [25]

I would like to thank Lockheed Martin (UK) Ltd, near Ampthill, for the use of their facilities and for providing the test equipment used.

I would also like to thank the many people at LM(UK) who helped with these tests, particularly the following:

(Note that 1 and 3 through 8 are available as free downloads from official websites)

emckeith armstrongshielded cableshielding

After working as an electronic designer, then project manager and design department manager, Keith started Cherry Clough Consultants in 1990 to help companies reduce financial risks and project timescales through the use of proven good EMC engineering practices. Over the last 20 years, Keith has presented many papers, demonstrations, and training courses on good EMC engineering techniques and on EMC for Functional Safety, worldwide, and also written very many articles on these topics. He chairs the IET's Working Group on EMC for Functional Safety, and is the UK Government's appointed expert to the IEC committees working on 61000-1-2 (EMC & Functional Safety), 60601-1-2 (EMC for Medical Devices), and 61000-6-7 (Generic standard on EMC & Functional Safety).

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