Pre-compliance tests run in your own lab can avoid bad news when products go into formal conformity checks.
DYLAN STINSON, TEKTRONIX INC.
EMI regulations are in place to ensure reliability and safety for users of electrical and electronic equipment. With few exceptions, any product emitting or susceptible to electromagnetic interference must be certified for EMI compliance. Those who’ve taken a product through EMI certification know that a great deal of time and effort goes into minimizing both EMI signatures and the possibility of failing EMC certification.
To ensure compliance with regulations, many companies have historically employed a specialized test facility, or test house, to perform EMC pre-compliance and compliance testing. The downside to this approach is that third-party test houses can add significant development cost and time to a project. As an alternative, a growing number of design teams now perform affordable pre-compliance testing themselves, in their workspace, throughout the design process.
Pre-compliance testing in-house offers an efficient way to identify and address potential EMI issues before full compliance testing. It also lets developers methodically isolate problem areas and apply appropriate corrections, saving time and money.
RADIATED AND EMISSIONS TESTING
The term radiated emissions refers to both the intentional and unintentional release of electromagnetic energy from an electronic device. A radiated test ensures emissions emanating from the device under test (DUT) or equipment under test (EUT) comply with the applicable limits.
The term conducted emission refers to electromagnetic energy created in an electronic device and coupled to its power cord. As with radiated emissions, regulatory agencies dictate the allowable conducted emissions from electronic devices.
A full compliance test in a certificated lab can be expensive. Costs range from $1,000 to greater than $20,000 per submission depending on the device and the number of countries to be covered. Full compliance testing can also be time-consuming. Emissions and immunity testing generally takes two to six days and another three-to-ten days for generating the final test report. And, of course, this doesn’t include the time spent waiting in the test queue.
To prepare for testing, many companies have historically rented time in full-compliance test facilities. This, too, can be expensive with costs ranging from $700 for a half day to $10,000 per day depending on the services needed. As an alternative, some firms set up fully compliant test facilities in-house. Certainly, this is a luxurious option – and one few companies can afford with costs ranging from hundreds of thousands of dollars on up.
Fortunately, there is a happy middle ground that significantly reduces the risk of failing EMC compliance testing. For a fraction of the cost of a full test facility, design teams can acquire everything necessary for pre-compliance checks. Modern software tools greatly simplify test set-up and analysis so even non-RF experts can gain meaningful insights.
During development, periodic diagnostics help produce designs that emit little electromagnetic energy and aren’t susceptible to interference. In later development stages, pre-compliance testing can catch compliance problems and improve the chances of passing EMC compliance testing on the first try.
The goal of pre-compliance testing is to mimic the compliance-test set-up well enough to uncover potential problems. Pre-compliance testing does not require test equipment identical to that for compliance testing. It typically involves a spectrum analyzer with a quasi-peak detector, a preamplifier (optional), an antenna with a non-metallic stand for radiated emissions, a line impedance stabilization network (LISN) for conducted tests, a power limiter for conducted tests, near-field probes for diagnostics (optional), and an oscilloscope with frequency and time correlation capabilities to assist in debugging (optional). The latest USB-based spectrum analyzers offer general-purpose filters and detectors to support pre-compliance testing and for troubleshooting any EMI issues.
Pre-compliance testing can employ fast measurement techniques intended to give a quick look at problem areas. The test site is usually temporary. When selecting a site, it is best to pick a location with an electromagnetic environment as free as possible from external signal sources. Rural areas, conference rooms or basements are good because they tend to see a minimum of signals that might mask the DUT/EUT emission levels you are trying to measure. Other considerations that improve measurement accuracy include having a good ground plane and minimizing the number of metal objects near the test area.
Standards documents spell out the setups for radiated compliance tests. Developers running their own tests try to mimic these setups as closely as possible. In setting up radiated testing, there are several challenges. For one, electromagnetic waves don’t extend out from your product in a nice spherical pattern. The emissions tend to be directional. To address this, a test lab will typically vary the height of the receiving antenna between one and four meters, and it will rotate the EUT on a turntable. The receiving antenna picks up both the signal direct from the EUT, as well as reflections from the ground. Additionally, the floor should be covered with an electromagnetically reflective surface (aluminum, steel, wire mesh, etc.) to create a ground plane for measurement accuracy. And the ground-plane area should be relatively flat. RF receivers then scan EUT in the frequency band of interest while technicians look for emissions close to the limits.
A radiated emission pre-compliance test setup includes your EUT, ground plane, an antenna on a tripod, a pre-amplifier, and a spectrum analyzer. The pre-amp amplifies the output for better sensitivity. The output of the pre-amplifier goes to the input of the spectrum analyzer for analysis, generally with the help of EMC test software.
Conducted emission tests consider electromagnetic energy the DUT conducts onto the power supply cord. The goal is to minimize this conducted interference. These emissions (usually from 150 kHz to 30 MHz) must stay within specified limits.
For this testing, it helps to ensure the local power source is relatively clean, and your DUT won’t affect nearby devices. Conducted emissions testing usually takes place on devices that connect to an ac power supply. For some standards, there are also limits placed on devices that operate from dc supplies.
Conducted measurements use a LISN (line impedance stabilization network), a low-pass filter placed between an ac or dc power source and the EUT. The LISN creates a known impedance and provides an RF noise measurement port. It also isolates unwanted RF signals from the power source. Adding a pre-amplifier is a good way to boost the relative EUT signal levels.
In some cases, the interference conducted on a 60 or 50-Hz power supply can be an issue. While most of conducted EMI tests specify a measured frequency range of 9 kHz to 1 GHz, it can be useful to measure the signals at lower frequencies when the need arises.
The bandwidth of an EMC measurement is defined by a receiver bandwidth shape or a resolution bandwidth (RBW) filter in the case of a spectrum analyzer. The bandwidths used are representative of the perceived threats within the spectrum, and the bandwidths vary with frequency. The reading of any non-continuous signal by a receiver or spectrum analyzer will depend on the measurement bandwidth used. For consistent results, regulatory agencies define the bandwidth and shape of the filters used in compliance measurements.
The standards also specify how EMI noise must be detected. Most commercial standards use three major detection methods defined by CISPR16-1: positive or negative peak, the RMS or average value of voltage, or the quasi-peak (QP) value. Compliance labs use QP detection for full compliance tests. The tests usually begin with a scan using simple peak detection to find problem areas that exceed or are near specified limits. Signals that approach or exceed the limits are then quantified with QP measurements. QP detection serves to detect the weighted peak value (quasi-peak) of the envelope of a signal. It weights signals depending on their duration and repetition rate. Signals that arise more frequently or last longer will result in a higher QP measurement than infrequent, short impulses.
Thus, the first scan for conducted emission (after determining the ambient electrical noise in the environment) should be via peak detection. It is fast and gives a quick overall perspective of both the noise amplitude margin from the limit and which frequencies are concerning.
To account for inaccuracies in the pre-compliance test setup, it is best to have a sizeable signal margin — ideally, at least 6 dB.
For EMC/EMI pre-compliance testing, it is important to select a spectrum analyzer with software supporting quasi-peak, peak and average detection. During product design and testing, there are situations where all three of these tests are necessary for an accurate picture of EMC/EMI compliance status.
While most modern spectrum analyzers are up to the task, one of the more economical and efficient options may be a real-time USB-based spectrum analyzer. In addition, comparisons with swept-spectrum analyzers have shown pre-compliance tests such as quasi-peak can process faster on a USB real-time spectrum analyzer with a laptop, tablet or desktop computer.
A real-time spectrum analyzer can reveal failures hidden under broadband noise that swept spectrum analyzers are not able to see.
All in all, tests run with a real-time spectrum analyzer and EMC analysis software help ensure that EMI emissions don’t leave your products short of the finish line.
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