Emissions testing is the common requirement that runs through the EMI test standards from around the globe. The purpose of emissions testing, especially for radiated emissions, is to verify that the equipment-under-test (EUT) emits a less-than-specified electromagnetic field level during operation.
The test setup consists of a receive antenna, interconnecting cables, a preamplifier, and a noise meter such as a receiver or spectrum analyzer. Figure 1 shows a block diagram of an emissions test system that could be used for ANSI C63.4 testing.
The receiving antenna scans the EUT for worst-case emissions at a distance of 3 m or 10 m and at a height from 1 m to 4 m. The performance parameter of the antenna is the antenna factor, and it relates the value of the incident electric field to the voltage output of the antenna. The units are volts output per volt/meter in the incident field. Usually, it is provided by the manufacturer in decibels with units of inverse meters.
There are two interconnecting cables. The first connects the antenna output to the preamplifier input. The second cable connects the preamplifier output to the noise meter. Losses in each cable cause a reduction in the measured signal amplitude. To increase measurement accuracy, these losses must be added to the voltage-out value of the antenna.
The noise meter may be either a receiver or a spectrum analyzer. It is essentially a 120-kHz-bandwidth, tunable, RF microvolt meter calibrated in dBµV.
The preamplifier is used with a spectrum analyzer to compensate for the high input noise figure typical of the analyzer. A receiver, however, may not need the preamplifier to boost the signal. The preamplifier gain must be subtracted to obtain the correct E-field signal level.
The E-field signal level is calculated by adding the RF noise value to the cable-loss values and the antenna factor and then subtracting the gain of the preamplifier:
E(dBµV/m) = V(dBµV) + CL1(dB) – PAG(dB) + CL2 + AF(dBm-1)
where: E(dBµV/m) = measure of E-field
V(dBµV) = noise meter value
CL1(dB) = loss in cable 1
PAG(dB) = preamplifier gain
CL2 = loss in cable 2
AF(dBm-1) = antenna factor
The EUT complies with the noise emissions requirement if the calculated value is less than the specification limit.1
The test system should perform fast and accurate emissions evaluations that are corrected for antenna and cable loss and amplifier gain, said Dennis Handlon, product manager at Hewlett-Packard. The system needs a display screen that shows the specification limits and allows you to perform a variety of measurements including automated peak, quasipeak, and average measurements.
For emissions tests on PCBs, Amplifier Research recommends the use of scanners to measure the RF current and put the data into visual presentations with spectral- and spatial-scan information. Frequency charts and color maps of RF emissions serve this purpose and are easily understood.
For test receivers, said Cliff Morgan, product marketing manager at Tektronix, a list of features useful for measuring conducted and radiated emissions is:
Parallel-peak, quasipeak, and average detectors—they reduce the time required to perform the testing.
Bar graph of peak, quasipeak, and average detectors—they show the relative indications of all three detectors and information about the type of interfering signal.
Color display with the capability to show limit lines—it helps you easily distinguish between test data and specification limits such as FCC and CISPR.
Automatic overload detection—it warns you if there is an overload condition in the front end of the receiver, which can put it in a state of gain compression and affect the accuracy of the test data.
Subrange maxima reduction— typically it is more useful to divide data into a user-specified number of subranges with a peak in each frequency subrange than it is to test all data points. A quasipeak or an average detector will check these highest amplitude points.
AM/FM demodulator—if there are ambient signals mixed with the emissions from the device-under-test, you may want to tune into a questionable signal, demodulate it, and listen to it so you can determine if it is an ambient or test signal.
Tracking generator—it creates a continuous wave signal that matches the frequency of the test receiver. This is useful for stimulus/response testing. Applications include measuring site attenuation and the frequency response of system components such as cables.
Resolution bandwidths—commercial and military standards require different filters so you must check the frequency ranges and filters needed for the test receiver or spectrum analyzer. For example, commercial testing requires 9-kHz and 120-kHz filters and possibly a 200-Hz filter, which differs from the military requirements.
Interfacing and Calibrating
Interface and calibration features also are important when considering emissions test equipment. But what are the necessary as well desirable features to look for?
The EMI receiver should have an RS-232 and a GPIB interface, a digital I/O port, and a printer interface, said Shu-Li Wang of Antenna Research. The RS-232 and GPIB ports may be used for remote control of the receiver. The I/O port drives any external device such as the EUT, an LISN, an antenna mast, or a turntable.
A keyboard input is useful on a receiver if you need to enter labels for limit lines and disk filenames, said Tektronix’s Mr. Morgan. Any other interface that may be helpful is a VGA adapter for displaying a larger image of the graph and data.
Built-in software macros also are useful. Stored in the receiver, these macros can help tune the receiver between start and stop frequencies, measure the amplitude at each frequency, correct each amplitude for antenna or other correction factors, store the data in a test array, perform the final test with a quasipeak or an average adapter, and present a graphic or tabular report.
A calibration button provides a quick way to check all of the primary modules within the receiver, said Mr. Morgan. It also verifies that the modules are performing according to the specified parameters.
Battery-operated receivers and spectrum analyzers also are useful for taking measurements in the field such as open-area test sites. Mobile radio monitoring applications also find battery operation useful.
As part of a complete test system, the antenna is a critical component that must be properly calibrated. An incorrectly calibrated antenna can lead to mistaken rejection of valid data or acceptance of invalid data. The antenna factor, a ratio of measured E- or H-field strength to the induced voltage delivered at the output of the antenna, should be very accurate, and the equipment used to make the measurements needs to be traceable to a national standard.
The antennas should be calibrated to the Standard Site Method recommended in ANSI C63.5, a three-antenna calibration process. Although time-consuming, this method is preferred by ANSI because it gives exact antenna factors, not averages, and is not compromised by a reference standard that may be inaccurate.
Some antennas such as the loop and rod types cannot be calibrated with the Standard Site Method. These antennas are calibrated using methods described in the IEEE standards.
EMCO recommends that you calibrate loop antennas in accordance with the Standard Field Method described in IEEE Std 291-1991, using a standard field-generating loop with a vacuum thermocouple in the current path. To calibrate rod antennas, EMCO uses the Equivalent Capacitance Substitution Method outlined in IEEE Std 291-1991. The telescopic element is removed from the base of the antenna, and a test fixture is attached. The fixture contains a capacitor of the same value as the rod. A signal is injected into the fixture and capacitively coupled to the base. The calibration fixture is designed to allow measurement of the input voltage without relying on the accuracy of the generator.
The importance of antenna calibration cannot be overemphasized. However, it is only part of the emissions test system. If you want useful data, all the instruments in the system—from the antenna to the receiver—must be properly calibrated.
Reference
1. “Essential Equipment for EMC Testing,” Antenna Catalog, EMC Test Systems, p. 67, 1997.
Emissions Test Products
Emissions Scanner Offers
Real-Time Spectral Scanning
The EMSCAN/Q is a PCB emissions scanning system with real-time spectral-spatial detection and display capabilities. It provides complete profiles of RF emissions and shows frequency charts of spectral scans and color maps of spatial scans for RF emissions. The system also synchronizes with an EUT to create and display spatial scans at a single frequency. System components include a pedestal controller module and a 9″ × 12″ scanner. Amplifier Research, (215) 723-8181.
Portable Analyzer
Measures to 1,000 MHz
The SCOPE 4600 is a portable spectrum analyzer for use in the field or laboratory. It performs broadband measurements with selectable resolution bandwidths to 1,000 MHz. The instrument has a lighted graphical LCD, built-in batteries and a charger, a PC interface for remote control, and a noise generator. The software keeps track of the calibrated test components attached to the SCOPE 4600 and provides the appropriate display for each test. Antenna Research, (301) 937-8888.
LISN Provides Interchangeable
Power Adapters on Front Panel
The EMCO Model 4825/2 Line Impedance Stabilization Network (LISN) provides interchangeable power adapters that mount on the front panel. The units let you switch between NEMA®, SCHUKO®, British, or Australian receptacle types. The LISN is used for conducted emissions measurements per standards such as FCC Part 15, EN55022, VDE 0871 and 0876, and CISPR 22. This two-line, low-pass filter network covers the frequency spectra from 9 kHz to 30 MHz. It features a line choke, an artificial hand for testing hand-held equipment, a high-pass filter, and an integral 25-A circuit breaker. EMC Test Systems, (800) 253-3761.
EMI Receivers Offer
Four Test Modes
The HP 8542E and HP 8546A EMI Test Receivers consist of two parts: the receiver RF section and the RF filter section. The HP 8542E covers the frequency range of 9 kHz to 2.9 GHz and the HP 8546A from 9 kHz to 6.5 GHz. Both comply with CISPR 16-1 and ANSI C63.4 requirements. The setup key on the front panel accesses the internal software that lets you define all parameters for making measurements. The test key provides access to the continuous scan, signal list, stepped measurement and manual tuning modes. Built-in quasipeak and average detectors are selectable from the front panel. Hewlett-Packard, (800) 452-4844.
Scan System Helps Locate
EMI on PCBs
The EPS-3000 EMC Precision Scan System helps locate electromagnetic
interference at the circuit-board level. It consists of a scanner, a PC, a spectrum analyzer, and an optional printer. The scanner enables the antenna probe to move to any point on two axes. A CCD camera captures circuit-board images. The spectrum analyzer displays the level of detected noise in the frequency domain to 1.5 GHz. Watahan Nohara International, (800) 366-3515.
Receiver Used for Compliance
Testing to CISPR, EN, FCC
The ESCS 30 EMI Test Receiver is a portable instrument used for compliance testing to CISPR, EN, and FCC EMI standards. The instrument has a 9-kHz to 2.75-GHz frequency range and an amplitude accuracy of £ 1.0 dB @ 1 GHz and £ 1.5 dB @ 2.75 GHz. The 6.5″ color display uses a bar graph to show the peak, quasipeak, and average signal levels simultaneously. The unit includes a time-domain oscilloscope mode for click testing and measures radiation from mechanical switching with a 100-µs resolution. Tektronix, (800) 426-2200, press 3, code 1037.
Copyright 1998 Nelson Publishing Inc.
January 1998