Test EMI Emissions on Telecom Test Ports

If you perform emissions tests based on the measurement methods described in FCC Part 15 or if you test to earlier versions of CISPR 22 (which references methods described in CISPR 16), you won’t detect these common-mode emissions.^2,3 So, products that currently comply with regulations may no longer comply after the August 1, 2001 date of withdrawal, the date after which all noncompliant products must be pulled from the market. Fortunately, you still have time to perform tests and make design changes.

EN 50022:1998 defines a telecom port as any port, analog or digital, that connects to the telecom network. The standard also includes LAN ports. EN 55022 regulates the limits of common-mode emissions between 150 kHz and 30 MHz. Common-mode means that the noise signals on each conductor are in phase with each other; any out-of-phase signals won’t contribute to a port’s emissions.

You can assess the common-mode noise in telecom cables by measuring either voltage or current. Depending on frequency, emissions limits reach down to 1.6 mV or 10 mA for Class B (residential) products and 5 mV or 32 mA for Class A (commercial or industrial) products. These limits are below the levels that typically cause functional problems in most high-speed LANs or telecom systems. Still, your equipment must comply with the regulations.

At Intertek, we’ve found testing telecom ports to the requirements of EN 55022:1998 cumbersome. For example, you need a different impedance stabilization network (ISN) or coupling network for each type of cable—twisted-pair or coaxial—that your equipment uses. With the ISN or coupling network in Figure 1, you can measure the common-mode emissions of one-pair and two-pair twisted-pair cables with an EMI receiver or spectrum analyzer. The ISNs for twisted-pair cables have specific common-mode isolation values, depending on the cable type. Table 1 lists some functional parameters for ISNs.

Longitudinal conversion loss (LCL) describes the ISN’s imperfect impedance balance to earth, where a small fraction of the differential signal entering the ISN becomes a spurious common-mode signal. Expressed mathematically, LCL = 20 log (applied differential signal voltage/resulting common mode or longitudinal signal voltage).

Break the Flow

In some cases, an impedance mismatch at the ISN will degrade a data signal’s integrity to the point where communication fails. If that happens, you must substitute a capacitive probe for the ISN (Fig. 2). The capacitive probe is also used for multipair cables.

Figure 1. One-pair and two-pair twisted-pair telecom cables require an ISN to measure their common-mode EMI emissions.

Coaxial cables require yet another test setup (Fig. 3). Here, you connect a 150-V resistor between the cable shield and your ground plane. The test procedures require you to measure and control the common-mode impedance of the cable. To control the impedance, you must move a pair of ferrites along the cable and possibly change the types of ferrites you use. The common-mode impedance of the ferrite combines with the common-mode impedance of the cable, which changes with the distance to the ferrites, to produce a controllable value. You can select ferrite materials that have higher or lower impedance characteristics as a function of signal frequency. The effective impedance of the ferrite rings or beads also depends on the frequencies of any standing waves set up on the signal cable under test. You can experiment with the locations of ferrites to find the point at which they best reduce common-mode emissions.

Once you set up the tests, you must apply either the voltage limits or the current limits specified in tables 3 and 4 of CISPR 22:1997 or EN 55022:1998. If you use the capacitive probe method shown in Figure 2, then your EUT must comply with the limits for both voltage and current measurements. 

Figure 2. Multipair twisted cables require a capacitive probe for measuring voltage and current.

 If you use the methods in Figure 1 or Figure 3, however, you have the option of applying either the voltage limits or the current limits. With this option, if your first measurement—voltage or current—falls within its limits, you’re done. If it does not, then you must perform the other measurement. If that’s within limits, your EUT is in compliance.

To increase the chances that your telecom equipment will meet these new requirements, equipment designers should:

• Increase the common-mode filtering at twisted-pair telecom ports by redesigning the common-mode filters to provide greater attenuation. You also can add a filter in series with the existing one.

• Lay out components on all PCBs to minimize differential impedances and coupling to ground planes or to the chassis. Remove conductive planes from the vicinity of twisted-pair ports or lay out conductive planes symmetrically to all exiting conductors.

Figure 3. Coaxial cables require a 150-V resistor and ferrites for adjusting the cable’s impedance.

 • Provide grounding of the connector shield to the chassis at coaxial cable ports. Doing this will provide robust grounding of the coaxial cable. Use wider or shorter PC traces. Use a high-quality braided mating-cable shield terminated 3608 around all mating connectors.

 Even with all of its shortcomings, EN 55022:1998 has been adopted as a mandatory compliance standard in the European Union. Many other countries have adopted CISPR 22:1997 as their standard for emissions measurements. Design and test engineers need to become familiar with these standards to understand their ramifications for product design and qualification. T&MW

FOOTNOTES

1. CISPR 22:1997 (EN55022:1998), Limits and Methods of Measurement of Radio Disturbance Characteristics of Information Technology Equipment, CENELEC, Brussels, Belgium. www.cenelec.be.  

2. Title 47 of the Code of Federal Regulations, Chapter I, Subchapter A (General), Part 15, Radio Frequency Devices, Federal Communications Commission, Washington, DC. www.fcc.gov/oet/info/rules/.

3. CISPR 16-1:1993, Specification for radio disturbance and immunity measuring apparatus and methods - Part 1: Radio disturbance and immunity measuring apparatus, CENELEC, Brussels, Belgium. www.cenelec.be.

Roland Gubisch is chief engineer for EMC and telecom at Intertek Testing Services NA, Boxborough, MA. E-mail: rwg@itsqs.com.