ABOUT

A few words.

TECHNOLOGY

How we make them

WHERE TO BUY

Where to find our products

REVIEWS

What they wrote

NEWS & EVENTS

All our news

TECHNOLOGY

Shielding

Since the early days of radio communications, all engineers and circuit designers of the industry inevitably engaged on a battle against the unwanted effects of radiated interferences. This battle continues even in our days, where the significantly increased number of external sources of radiation, the complexity of today's electronic circuits and the increased number of connections, are causing consistent problems to the engineers of the audio and video industries. All these problems are encapsulated in two main forms of interferences, namely Electromagnetic Interferences (EMI) and Radio Frequency Interferences (RFI).

EMI and RFI

An Electromagnetic Interference may appear either as an unwanted induction, or as an electrical or electronic disturbance which is emitted either from external or from internal sources, affecting negatively the response and consequently the overall performance of circuits and cables.

A Radio Frequency Interference, as the abbreviated words describe it, is any undesirable electrical energy with negative effects in the range of radio frequency transmissions. There are two main categories of RFI. The radiated, which is mostly found in the frequency range between 30MHz and 10GHz, and the conducted which is oscillating inside the audible frequency range (usually on the upper bands) and up to 30MHz. In our field, both Electromagnetic and Radio Frequency interferences can become equally harmful to the total performance of cables, since they can use them either as a source of interference by conducting noise to the connected equipment, or as a receiver by using cables to pick up the emitted interferences from other sources.

This is the reason why shielding is one of the main factors to determine the performance of both signal-carrying and power-carrying cables. The effectiveness of any given shielding configuration can be evaluated by the transfer impedance test. This method is the most widely accepted for evaluating the shielding performance of a cable against ESD and radiated emissions coupling at frequencies from DC to 1000 MHz and is recommended by the International Electrotechnical Commission as well as the military.

EMI and RFI can become equally harmful to the total performance of cables.

In measurement terms, the lower the transfer impedance value is, the more effective the shielding configuration of the cable is. In practice, there are various shielding configurations and each of them offers different levels of efficiency.

An Electromagnetic Interference may appear either as an unwanted induction, or as an electrical or electronic disturbance which is emitted either from external or from internal sources, affecting negatively the response and consequently the overall performance of circuits and cables.

A Radio Frequency Interference, as the abbreviated words describe it, is any undesirable electrical energy with negative effects in the range of radio frequency transmissions. There are two main categories of RFI. The radiated, which is mostly found in the frequency range between 30MHz and 10GHz, and the conducted which is oscillating inside the audible frequency range (usually on the upper bands) and up to 30MHz. In our field, both Electromagnetic and Radio Frequency interferences can become equally harmful to the total performance of cables, since they can use them either as a source of interference by conducting noise to the connected equipment, or as a receiver by using cables to pick up the emitted interferences from other sources.

This is the reason why shielding is one of the main factors to determine the performance of both signal-carrying and power-carrying cables. The effectiveness of any given shielding configuration can be evaluated by the transfer impedance test. This method is the most widely accepted for evaluating the shielding performance of a cable against ESD and radiated emissions coupling at frequencies from DC to 1000 MHz and is recommended by the International Electrotechnical Commission as well as the military.

In measurement terms, the lower the transfer impedance value is, the more effective the shielding configuration of the cable is. In practice, there are various shielding configurations and each of them offers different levels of efficiency. The most common shielding types of today are:

Foil shield

This type of shielding consists of an aluminum foil and a film layer of polypropylene which is used to add strength and prevent foil from shredding. The main advantage of foil shielding is the increased level of protection from the emitted interferences due to the fact that it provides 100% coverage of the conductor’s insulation surface. The disadvantage of foil shielding is the relatively increased resistance compared to a braid shield, and this is the reason why aluminium foils cannot provide equally effective grounding.

Braid shield

The braid shielding is a woven mesh of bare or tinned copper wires, which offers a greater structural integrity and increased flexibility compared to aluminium foil shielding. Besides that, the main advantage of braid shielding is the very low DC resistance compared to aluminium shielding, a fact that makes it ideal for grounding. Another advantage of a braided shield is its ability to minimize interference at low frequencies. On the other hand, braided shields cannot cover the complete surface of a cable, even when the woven mesh is very tightly braided. Due to this specific structural characteristic, braided shields cannot offer 100% protection from the emitted interference.

Foil / Braid shield

Combination shields consist of more than one layer of shielding and provide maximum shield efficiency across the frequency spectrum. The foil/braid shield combines the advantages of 100 percent foil coverage with the strength, the flexibility and the very low DC resistance of a braid.

French Braid shield

This type of shielding was developed especially for audio and RF cable applications. It is a very flexible double spiral design, consisting of two bare or tinned copper wires in a spiral form, tied together by one weave across the full length of the cable. The French BraidTM shielding offers a longer flex life than the standard spiral shields, and it is a lot more flexible than the conventional braid shields. It produces a much lower level (approx. 50% less) of microphonic and triboelectric noises than the conventional braid shields and an even better performance than the single spiral, due to the relatively lower DC loop resistance.

Problems of poor shielding

There are many ways that Radio Frequencies can be induced on shielded cables. However, the most common situations of RF induction into signal conductors are:

- When the shields of cables are based only on the braid and not on the foil/braid configuration, where there is a great possibility for coupling of electric fields through tiny openings in the braided shield. The same action can occur when cables are shielded with poor quality aluminium foil that is usually torn when cables are bent at extreme angles.

- In situations where there is an imbalance of the capacitances between the signal conductors and the shield, then voltage gradients may be caused affecting signal conductors and consequently the overall performance of the system.

- The situation of an imbalance in the magnetic coupling between the shields of cables and signal conductors increases significantly the chances for noise induction. This mechanism was named Shield Current Induced Noise and was widely analysed by a few great researchers like Neil Muncy, the Eastern Vice President of AES, Jim Brown of Audio Systems Group and Bill Whitlock of Jensen Transformers, with some very interesting conclusions.

More specifically, Neil Muncy performed a number of tests on many cables with different shielding configurations and by driving their shields with a specific current he firstly proved that the audio frequency current that flows on the shield of twisted-pair cables will be converted into differential mode voltage on the signal pair, and secondly he measured a relatively higher shield current induced noise on the signal conductors of cables with poorer shield configuration.

Based on the results of Neil Muncy's research, the foil/drain-shielded cables had the worst SCIN performance, the braid-shielded cables were about 30 dB better and the foil/braid-shielded cables with no drain wires had by far the best SCIN performance. In addition to the above, Jim Brown and Bill Whitlock reassured the conclusions of Neil Muncy’s research by proving that the current which flows on the shield of twisted-pair cables produces a corresponding differential mode voltage on the twisted pair that is proportional both to the frequency of the current as well as to the length of the cable (and therefore to its inductance), for all cables that are shorter than the 1/20 of the wavelength of the interfering signal. They also drew a number of other crucial assumptions, and below we make a reference to those who are closely related to our field, as they were printed on their research:

- Between 1/20 and 1/10 wavelengths, the SCIN will continue to increase approximately in proportion to frequency and cable length. However, when a cable is longer than 1/10 wavelength at the frequency of the interfering current, then some frequency-dependent terms will dominate and it will be very difficult to predict SCIN.

- The SCIN appears to include an additive term whose magnitude is approximately proportional to the fraction of the shield current that flows through a drain wire to the total shield current. The data suggests that the maximum degradation caused by a drain wire is approximately 25-30 dB.

- The SCIN appears to include algebraically additive reactive terms that result from imbalances in the lengths of the signal conductors, imbalances in the inductance of the signal conductors, and imbalances in the capacitances between the signal conductors and the shield. These reactive components have both positive and negative signs, because a) magnitude and phase of voltages and currents vary along the length of a cable longer than 1/20 wavelength, and b) the mechanism that produces current is so variable from one installation to another, making the prediction of these terms very complex.

- For cables less than 1/10 in wavelength at the frequency of the shield current, reactive factors are insignificant for foil/drain-shielded cables, and relatively low in influence with braid/drain shielded cables. For foil/drain-shielded cables, SCIN attributable to the drain wire is in the region of 20-30 dB greater than SCIN attributable to other factors. And for most braid-shielded cables having a drain wire, SCIN attributable to the drain wire is more or less 10 dB greater than SCIN attributable to other factors.

- Any asymmetry in the termination of a cable can degrade SCIN performance. This is most significant with cables having relatively good SCIN characteristics. In addition to that, manufacturing tolerances should be expected to cause variations to the SCIN performance from one sample to another of braid-shielded cable.

The shielding techniques of Signal Projects

Based on the combination of Foil/Braid shields and of Foil/French BraidTM shields, we have designed specific shielding configurations for each type of cable that we produce, achieving the maximum protection from all kinds of emitted interferences. Additional protection against magnetically coupled interferences is also provided by the twisted pair structure of our conductors.

The appropriate number of twists per unit length and the accurate uniform continuation is helping our cables to achieve a very effective noise rejection. The efficiency of our shielding configurations and of our twisted pair structures can be proved by the impressively low transferred impedance values of our cables and of course by the accurate response across the audible frequency range, which is absolutely free of hiss and hum noises.