Frayed ends of various cables, which can cause EMC challenges if managed incorrectly.

Cables: The Bane of EMC

During a recent discussion at Embedded World, the subject of the effect of cables on electromagnetic compatibility (EMC) was broached. A product’s cabling type is driven by many factors including its intended environment, cost, electrical characteristics, looks, robustness, safety, and length. With so many factors influencing the choice of ideal product cabling, the EMC effects of these choices can be a secondary concern, until failing day.

Cables act as antennae. A long cable can potentially create a path for lightning to hit directly or be coupled on. Why would you attach antennae to a product you don’t want to radiate or receive electromagnetic interference? Well, often you have no choice. Cables are power leads, mains connections, wired communications forms and sensor wires emanating from a product.  Such cabling is also referred to as a harness in automotive circles.

Using cables in your design may not be avoidable, but they don’t have to throw a wrench in your EMC performance, either. Here are some things to consider when designing with cables.

How Are Cables Tested at the EMC Lab?

Many EMC standards refer to cable length to decide how a product should be tested. Is it less than 3 meters or will it run around the factory floor and out through the wall?  The length, type and function of product cabling directly dictates the test coupling mechanism an EMC lab will use to measure and expose your cabling.

Industrial products that have long cables that could potentially give a path for lightning to hit directly or be coupled onto are classified differently to say a sensor cable 1m long, it’s all down to the likelihood and severity of potential EMC exposure.

In the automotive industry, there are specific requirements for cable length during testing. For example, in a Bulk Current Injection Test, energy is coupled directly onto a product's harness using a current clamp (injection probe) that encompasses the harness. The general cable length range for this test is 1.7m to 2m. However, this range is often redefined in each carmaker's own specifications or based on specific needs.


The length, type, and quality of the cable are crucial factors for electromagnetic compatibility (EMC).The length directly relates to its wavelength tuning as antennae, essentially the efficiency at which unwanted energy can be received into the product (Immunity) or radiated out of the product (emissions). Line up a clock signal with quarter wave antenna (cable) and you can easily exceed the emission requirements AND allow the clock to be easily interfered with. It’s type is critical to the best functional performance of product as the primary requirement, remembering an optimized communication channel’s performance has a better chance than a reasonably performing channel, when stressed with EMC. The cable type is often the cheapest that just meets product functional requirements with the hope of meeting EMC requirements a little later. If it works out then great, but do consider pre-compliance testing even earlier if that is the approach. Get an early read on performance to manage expectations, because your customer might need to budget for a coaxial cable as opposed to a twisted pair when nothing can be done on the PCB side!  The quality can simply refer to the performance variation between a number of the same cables.  

Best is simulating product and test system on the initial simulated design, but many companies don’t have the tools or resources for this upfront work, unfortunately.

A few steps to consider when testing EMC on cabled product:

  • Define your cabling early and be clear as to your absolute cable requirements verses just preferred, this informs you clearly as to what is up for consideration and what is untouchable.
  • Discuss your design with an EMC Test Lab to agree a test plan, this will show you how your product will be stressed and how cable emissions will be recorded.
  • Review this test plan with regards to potential Common Mode, Differential Mode, and Mode Conversion design exposure.
    • Common Mode: The interference is coupled on all cable elements in the same direction, possibly taking a ground return path directly or through capacitive coupling.
    • Differential Mode: the test exerts a difference signal between say power positive and negative or across the two ports of a sensor.
    • Mode Conversion: where a common mode test produces a small but disturbing differential signal you were not counting on, or vice versa.
  • If possible, test even on a prototype, issues don’t come and go, there is often a specific reason and worth knowing at the very earliest. Having multiple design version results is also a powerful tool for identifying the root causes.
  • Simulation or “early read” test results will very likely save you in the long run as you simply have to resolve these issues, before you can qualify product.  

So, as far as cable types are concerned:

Twisted pair - should have a repeatable twist per inch and be very carefully considered when you untwist at connectors etc, a very vulnerable section of the cable’s performance, from an EMC perspective.

Coaxial - provides great shielding performance when ground appropriately and ideally on both ends to enclosed DUT and Auxiliary circuitry, costly though.

Flex or ribbon cables - handy, but carefully consider trace pairings and proximity, there’s often no ground reference plane so try keep as short as possible.

Power leads - if separate leads, keep them as close to each other as possible help with flux cancellation, even tie wrapping if possible, again short as possible too.

Communications cabling - on a case by case basis, but take every effort to design plenty of margin for just functional performance, to start with. The weakness is generally around the connectors and the impedance discontinuity they can introduce.


  • A simple start is to consider cable wavelength not lining up your clock/data rates, if possible.
  • Having an additional extremely short cable or even multiple length of cables available during testing is an extremely valuable tool for identifying the cable’s role in failing product.
  • Characterization of the cable is very useful too, even beyond the manufacturer’s datasheets. You can see how the impedance changes with frequency and how well its characteristic impedance matches with the DUT and Auxiliary circuitry, in and out of communications bands.

We will go into more detail on cable type performance, in future blogs