An Overview of Conducted Emissions

Each product on the market is required to meet the EMC regulations that define the upper limits for conducted and radiated emissions.

Conducted emissions are the noise components that are generated by a device or subcircuit and transferred to another device or subcircuit via cabling, PCB traces, power/ground planes, or parasitic capacitance. The conducted emissions that appear on the interface and power cables must be kept low or they can propagate through cables and reach other devices, causing problems to them.

Radiated emissions are the noise components that the whole system generates as an electromagnetic field, so it can propagate through the air and reach other devices.

Engineers know that signal propagation through cables occurs without reflection when the cable length is much shorter than the signal wavelength (low frequency). This is why conducted emissions are considered an issue for low frequencies, where a lumped model can be considered for the signal propagation medium.

On the contrary, when the cable length is much longer than the signal wavelength (high frequency), the signal propagation along the cable will occur without reflection only if impedance matching is provided along the signal path. In this case, the propagation through the conductors can be analyzed as a distributed model (transmission line theory), but in the real world, systems often present conductors (cables and circuit traces) that are not designed as transmission lines for high frequencies. Such conductors can easily radiate signals as an electromagnetic field because they behave in an antenna-like fashion instead of a transmission line. This is why radiated emissions are considered an issue for high frequencies.

The common rule of thumb for designers is to consider the critical conductor length that separates the lumped behavior from the distributed behavior as follows:

L_CRITICAL= λ/6

 

Figure 1. Lumped systems vs distributed systems.

 

The following table shows the wavelength for different frequencies, the associated critical length, the structures whose typical dimensions are comparable to the critical length and the type of emissions typically generated:

 

Table 1. Typical emissions propagation methods for different frequencies.

 

Conventionally, the breakpoint between the conducted and radiated emissions is set at 30MHz, where the wavelength (in free air) is about 10m and the critical length is about 1.7m.

The EMC regulations are targeted to measure the emission from devices and their cables (the so-called EUT - Equipment Under Test). Considering that the typical dimensions of devices and associated cables are up to 1.5m, the above table shows that radiated emissions from them can occur only for frequencies over 30MHz, where the dimensions of conductors that are part of the EUT can be approximately longer than the critical length. For frequencies below 30MHz, the radiated emissions don’t occur significantly and generally, it is requested to test the EUT only for conducted emissions.

 

Testing Conducted Emissions

The typical setup for testing conducted emission from the EUT requires the following:

• EMI receiver or spectrum analyzer (suitable for pre-compliance)
• LISN (Line Impedance Stabilization Network)
• Ground Plane - The EUT, the LISN and the receiver are placed on and connected to the ground plane.

 

Figure 2. Basic conducted emissions test setup.

 

The LISN is a three-port device that is connected to the EUT, the receiver, and the power supply.
The purpose of the LISN is to provide a standardized impedance at RF across the EUT measurement point. The LISN couples the measurement point of the EUT to the receiver and attenuates (virtually eliminates) unwanted interference signals coming from the power supply, in order to prevent such signals from influencing the test execution.

There are different types of LISN for analyzing DC, single-phase or three-phase AC. The most common type is defined in CISPR 16-1-2 and presents to the EUT an equivalent impedance of 50Ω in parallel with 50uH + 5Ω across each line to earth. It is named a “V-network” type for single-phase supply, since the stabilized impedance appears across each arm of the “V,” between line or neutral and earth connection.

 

Figure3.  LISN circuit for each line of the “V-network”

 

Figure 4.  Impedance vs frequency at EUT terminals. Image courtesy of Tekbox – TBLC08 user manual.

 

The noise measured with the receiver must be compared to the noise limits provided by EMC regulations. In the case of single-phase devices, the noise measurements must be repeated for each line (phase and neutral).

 

Figure 5. Single phase LISN 9KHz-30MHz from different manufacturers. Image courtesy of NARDA and Tekbox.

 

In the basic test setup presented above, the ground plane makes a fundamental role to standardize the testing environment, since usually part of the conducted noise can be influenced by parasitic parameters, as better explained later.

The EMI receiver is a specific device designed for EMI testing, like the LISN. It differs from a generic spectrum analyzer for various aspects not covered in this article, but as first idea, it can be considered as a spectrum analyzer with specific EMI testing features:

• scan parameters accordingly to the global EMC regulations (i.e. hold time, resolution bandwidth (RBW), detector, etc)
• automatic execution of the test with control of the LISN and switch between line phases in case of AC supply (single-phase or three-phase)
• software interface for scan view, configuration and saving of test results

 

Figure 6.  An EMI receiver. Image courtesy of NARDA

 

A spectrum analyzer can be a less expensive replacement for the EMI receiver when performing pre-compliance testing of products during the design and verification process. A complete pre-compliance test setup (LISN + spectrum analyzer + ground plane) can be bought for less than $2000, a reasonable price also for small companies. For such limited cost it is possible to make a scan of conducted emissions from a product, discover excessive noise emissions and make corrections before going to an accredited test lab for the final compliance test.

 

There are numerous governing bodies that regulate the permissible levels of emissions generated biproducts. The global standardization body is the IEC (International Electrotechnical Commission).

At a regional level, there are different bodies to provide local implementation of regulations based on EMC standards released by the IEC: in the United States, the most common regulations about emissions are issued by FCC (Federal Communications Commission), while in Europe they are issued by CEN/CENELEC organizations.

The following table shows the main product standards for conducted and radiated emissions:

 

Product sector	EN standard	US standard
Multimedia equipment (MME)	EN 55032	FCC Part 15
Industrial, scientific and medical equipment (ISM)	EN 55011	FCC Part 18
Lighting equipment	EN 55015	FCC Part 15/18

Table 2.  Main product standards for conducted and radiated emissions

 

Each standard is developed to regulate terms, test methods and limits for conducted and radiated emissions. As an example we can examine the standards EN 55032 and FCC Part 15.

The standard EN 55032 regulates in Europe the conducted and radiated emissions of signals in the frequency range of 9kHz to 400GHz for multimedia): equipment having rated RMS supply voltage not exceeding 600V. The devices are divided in two classes:

 

• Class-B (commercial): this is applicable to devices that are used in residential and domestic environments. They must have emissions below the lower emission limits defined for Class-B.
• Class-A (industrial): this is applicable to all devices that exceed the Class-B limits. In this case the device must have emissions below the limits defined for Class-A and may cause interference in the residential areas, so the device manual must have a warning notice.

   

  

  Figure 7.  EN 55032 Class A and Class B conducted emission limits. Image courtesy of Texas Instruments.

   

  Similarly, products designed for the US market must be compliant with equivalent limits established by the standard FCC Part 15, subpart B (unintentional radiators), section 15.107 (conducted limits), where conducted emissions limits are equivalent to those given in EN 55032. Also in FCC Part 15 the devices are divided in two classes:

  • Class-B: digital devices marketed for use in a residential environment notwithstanding use in commercial, business and industrial environments.
  • Class-A: digital devices marketed for use in a commercial, industrial or business environment; exclusive of devices marketed for use by the general public, or intended to be used in the home.