As of Jan. 1, all electrical products sold in the European Union must comply with the EMC Directive 89/336/EEC. As a result, manufacturers are facing, possibly for the first time, the prospects of testing their products not only for RF emissions but also for RF immunity–which is a new commercial requirement that can be very complex as well as expensive.
Routes to EMC Compliance
The simplest approach is to contract with a qualified test facility. However, because of the current push for CE certification, most test houses are booked solid for months. So in the interest of minimizing time to market, reducing testing costs and providing testing flexibility, many manufacturers are developing in-house test capabilities.
Regardless of whether an outside test facility or in-house testing is used, a test report must be produced and the manufacturer must complete a Declaration of Conformity (DOC) prior to physically attaching a CE Marking to a product. The test report is kept on file and the DOC is included in the product manual or attached to the warranty statement.
The EN50082-1 and EN50082-2 Generic Immunity standards provide specific test levels depending on the operational environment of the product. These standards call out the IEC 1000-4-3 EMC basic standard, which describes the test setup in detail.
The E-field uniformity required by this spec is quite stringent and is best served by testing the product in an anechoic chamber. Given the large investment involved in fully implementing this spec, Amplifier Research has found that many of its customers conduct precompliance testing in shielded enclosures or semi-anechoic chambers. After appropriate engineering changes are made, a visit to a certified test house capable of conducting IEC 1000-4-3 tests is required prior to applying the CE Marking.
Precompliance Suggestions
Depending on the environment the DUT will operate in, you must test to either 3 or 10 V/m. For products that operate in a residential, commercial or light industrial environment, 3 V/m is sufficient. For products used in heavy industry, 10 V/m is required. In both cases, the test object is placed at a distance of 3 meters from the tip of the antenna and the field is 80% amplitude modulated.
The fundamental question at this point is, “How much power is required to generate the specified field?” Of the many factors involved in determining the required amplifier power, the single most important one is the characteristics of the transducer used to generate the electric field. While there are many devices available, a log periodic antenna covering the frequency range of 80 MHz to 1 GHz is sufficient to cover the radiated immunity tests required by the European Union.
Given that a log periodic antenna is appropriate, let’s first consider the 3-V/m case.
Step 1. Determine the power required to generate a given E-field. If available, use vendor-supplied data relating E-field at a distance of 3 meters as a function of frequency for a wide range of power applied to the antenna (Figure 1). This figure suggests that 2 W are required to provide the 3 V/m CW.
If an E-field vs power graph is not available, fall back on the standard equation used to predict far-field voltage levels for log-periodic antennas.
_________
E=Ö {(30)(P)(G)}/d (1)
where (P) = net forward power supplied to the antenna
(G) = numeric gain of the antenna
(d) = distance from the tip of the antenna to the DUT in meters
In the situation where antenna data is supplied for a distance of only 1 meter, you must make adjustments for 3 meters. Equation 1 shows that the E-field is inversely proportional to the square root of the distance. To correct for 3 meters, merely multiply the desired field by 3 and proceed to use the 1-meter curves. In other words, 3 V/m at a distance of 3 meters is equivalent to 9 V/m at a distance of 1 meter.
Step 2. Consider the impact amplitude modulation has on the power amplifier. Since the E-field data provided by vendors is for CW conditions, make allowances for the effect of 80% amplitude modulation. As seen in Figure 2, amplitude modulation results in an increase in instantaneous peak E-field levels. The degree of amplitude modulation in Figure 2 is given by the formula
m = (Emax-Ec)/Ec (2)
where Ec is the peak value of the carrier or CW signal
Assuming 80% AM and allowing for no AM envelope distortion, Equation 2 reduces to
Emax = 1.8Ec (3)
As a result, the RF amplifier must supply a peak RF voltage 1.8 times that required for CW operation. The additional power required to provide this increase in peak RF voltage can be determined by applying the standard logarithmic voltage ratio formula
dB = 20 log (E2/E1) (4)
where E2 can be replaced by Emax and E1 by EC.
Remember that Emax is the peak voltage when 80% AM is applied and EC is the peak unmodulated voltage. Substituting into Equation 4, dB = 20 log (Emax/EC). Further substitution results in dB = 20 log (1.8/1)= 5.1 dB.
When 80% amplitude modulation is applied to a CW signal, the peak RF power increases by 5.1 dB. Accordingly, when specifying an RF power amplifier, first determine the power required to generate a given CW E-field; and if 80% AM modulation is required, increase the power level by 5.1 dB or a factor of 3.24
{the log-1 (5.1/10)}. Applying this to the 3 V/m sample yields a power of 6.5 W.
Step 3. Consider typical system losses. To this point, ideal conditions were assumed. Many factors impact the power required to generate a given RF field. Depending on the actual test configuration, consider cable and connector losses, insertion loss through the directional coupler, variations in antenna characteristics such as gain and VSWR and, finally, field fluctuations due to reflections from the DUT and various room surfaces.
A general rule of thumb that takes losses into consideration doubles the output of the power amplifier. Factoring this into the 3 V/m example, the RF power required is somewhere between 6.5 W (no loss, ideal conditions) and 13 W (assuming a 3-dB system loss).
Specific Suggestions
For products covered by the EN50082-1 Generic Immunity standard,
an amplifier capable of providing more than 6 W is required. Depending on how significant the system losses are, a 10- to 15-W power amplifier driving a log periodic antenna with a minimum gain of at least 6.5 dBi (gain reference to isotropic) over the range of 80 to 1,000 MHz should prove sufficient for precompliance testing.
An isotropic field-monitoring system of sufficient sensitivity covering the same frequency range is also required. Applying the same procedure, a 100- to 150-W amplifier must be substituted for 10 V/m testing for products addressed by EN50082-2.
To self-certify without the aid of a qualified test house, you should use an anechoic chamber. The software also should be specifically designed to accommodate IEC 1000-4-3 testing.
Cautions
Regardless of whether you are conducting precompliance testing or testing to the full requirements of IEC 1000-4-3, take special care when selecting the power amplifier. While salient amplifier specifications such as gain, frequency response, output power, noise figure and distortion are generally understood and accepted without question, load tolerance is often overlooked. This characteristic is vital in applications such as immunity testing, where the load impedance is rarely an ideal 50 W .
Power amplifiers encounter widely varying load impedances due to variations in antenna characteristics, room reflections and resonances, imperfect cables and connectors, and reflections from the DUTs. Starting with a typical antenna VSWR of 2.5:1 and factoring in room and signal path effects, it is not uncommon to experience a VSWR in excess of 5.0:1. Since most amplifiers are not capable of providing full rated output power to loads that vary considerably from an ideal 50 W , it is essential that you specify products with load tolerance expressed as a percentage of rated power (Figure 3). According to this figure, a properly designed Class A power amplifier is required when the application involves varying load impedance.
Summary
Many manufacturers have opted to conduct precompliance testing to identify EMC problem areas. The good news is that relatively little hardware is required to prescreen products for compliance with the radiated immunity specs called out by the EMC Directive 89/336/EEC.
However, you must know the techniques available to determine the RF power required to generate the specified E-fields and understand the impact EMC testing imposes on power amplifiers. Fortunately, rugged Class A power amplifiers have been developed to withstand the drastic load variations that are typically encountered in EMC testing applications.
About the Author
Pat Malloy has been a Sales Application Engineer at Amplifier Research since 1989. Previous work experience included four years with the U.S. Navy as a Guided Missile Electronic Technician, followed by seven years in an engineering group at AT&T Bell Laboratories and 16 years as a Senior Sales Engineer at Tektronix. Mr. Malloy graduated from Lafayette College in 1972 with a bachelor’s degree in electrical engineering. Amplifier Research, 160 School House Rd., Souderton, PA 18964-9990, (215) 723-8181.
Copyright 1996 Nelson Publishing Inc.
February 1996
