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Dear Colleague,
Designers of electronics products know all too well that boards can emit unwanted electromagnetic radiation. An engineer at a small or medium size enterprise usually has to rely on his experience and on best practice methods in order to design an EMC-compliant product. Nevertheless, it is estimated that > 50% of products fail testing first time around. Anytime an engineer sends a new product for compliance testing, it is a ‘shot in the dark’. Failing is very expensive. Not only that, re-testing costs are high, and the project schedule and market introduction is delayed. What is needed is an affordable lab set-up to measure radiated emissions to be used prior to formal compliance testing.
Regulatory agencies, such as the FCC in North America, create standards that define the allowable limits of EMI over specific frequency ranges. Testing designs and products for compliance to these standards can be difficult and expensive (but ultimately necessary) at professional testing houses, but there are tools and techniques available that can help to minimize the cost of testing and help to ensure that designs pass compliance testing first time.
The benefits of pre-compliance EMC testing include: detecting problems early, fixing potential issues, reducing testing and design costs, as well as avoiding future warranty issues.
Here are some of our most popular products to help with these challenges.
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EMC sniffer probes connected to a spectrum analyzer can be used to measure electromagnetic radiation from a device-under-test using magnetic (H) field and electric (E) field probes. Board-level emission testing can be performed using a spectrum analyzer with near field electric (E) and magnetic (H) probes like those from TekBox. (H) field probes can be tried in different probe orientations to help isolate problem areas e.g. seams, grounding, and shielding, etc. Ribbon cables and inputs with bad shielding and grounds are common causes of radiated emissions. EMC probes and in combination with TEM cells and a spectrum analyzer make excellent basic tools to identify sources of radiated emission issues. See
Radiated Noise measurements
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Analyzers like the Rigol DSA-815 (9kHz to 1.5GHz) or the Siglent SS3021X have made frequency spectrum analysis much more affordable. A spectrum analyzer measures the magnitude of an input signal versus frequency within the full specified frequency range of the instrument. Some come with built-in tracking signal generators to allow a sweep to be performed and for feeding susceptibility/immunity signals into nearfield probes.
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The
Aim-TTi PSA2702
(1MHz to 2.7GHz) is a high quality battery-powered handheld analyzer that provides settings for resolution bandwidth, frequency setting, averaging, etc. You can connect a near field probe directly to the RF input to check even a large system for EMI problems.
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Real-time spectrum analyzers in general are ideal for sensing signals that vary dynamically in amplitude or are agile in frequency, such as pulsed radar systems, frequency-hopping spread spectrum radios, and pulse modulated radios, multi-signal environments, or unwanted signals such as unintentional self-interference, intentional interference (jammers), and listening devices (bugs). For products that feature wireless or fast serial data streams, a real-time spectrum analyzer can detect and capture short, intermittent signals. RSAs are also helpful for capturing modulated wireless and digital signals. Capturing all wavelengths at once instead of scanning through the range makes this possible. Standalone desktop products from Rigol include the
RSA3000
and
RSA5000
series, while portable versions from
ThinkRF
and
Aaronia
include comprehensive PC software. The Rigol RSAs also have HDMI output, which is useful for large displays in teaching situations.
Monitoring dynamic and agile signals and sporadic EMI usually requires a large budget instrument, but RSA instruments like ThinkRF’s R5x50 offer advanced real-time spectrum analysis capability that is no more expensive than an ordinary spectrum analyzer. The real-time analysis of the R5x50 design is based on patented, low-cost digital software-defined radio technology, open source software, standard APIs, and a PC-controlled architecture to provide unparalleled performance in an economical, portable instrument. A PC’s widescreen capability also provides enhanced display options not available on benchtop instruments.
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A very low-cost spectrum analyzer is the
Triarchy Technologies USB-connected TSAxG1
. About the size of a large memory stick, there are models that go up to 4.15GHz. 6.15GHz, and 12GHz. They come with Windows PC software that works well for troubleshooting and providing indicators for pre-compliance testing.
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RF / EMI interference during testing can be an enormous problem for electronic and wireless parts manufacturers. During the production process, wireless devices often need to be activated and tested without external interference for quality and process control checks. To insure that radio frequency signals are solely from the individual unit being tested, RF isolation is required on assembly lines or in test areas. And when multiple adjacent manufacturing cells are building electronic and wireless products, RF isolation is essential to allow quality control or activation testing of the similar components on adjacent lines. RF isolation prevents RF/EMI noise from equipment in other parts of the factory interfering with production testing. The best way to accomplish this is with an EMI enclosure – either
desktop size metal box
with filtered I/O panel, or a l
arger EMI tent
, depending on the situation.
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Professional radiated emission tests are typically carried out in large, expensive anechoic chambers, using antennas to pick up the radiated signals. But a TEM cell (Transverse Electro-Magnetic transmission-line cell is an affordable piece of equipment for initial desktop testing of radiated emissions. TEM cells are devices used for establishing standard electromagnetic (EM) fields in a shielded environment. The cell consists of a section of rectangular coaxial transmission line tapered at each end to adapt to standard coaxial connectors. The cell has its limitations, among which is that the upper useful frequency is bound by its physical dimensions which, in turn, constrains the size of test object with the cell.
Tekbox developed open TEM cells to cover the complete frequency range up to 2GHz and with usability even at frequencies beyond. Combined with a spectrum analyzer, products can be tested before and after EMC related design modifications. While a set-up with a TEM cell will not deliver exactly the same quantitative results as a measurement in a certified test house. However, it will give an excellent indication of whether the design suffers from excessive radiated noise or not – and at far less cost. The engineer will clearly see whether his changes improved or deteriorated the EMC performance or whether it remained unchanged. Using TEM cells eliminates the guesswork.
See Immunity testing example using Tekbox TEM Cells
and
EMC Analysis Using TEM Cells
.
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EMI antennas can be quite expensive, but alternatives, like simple rabbit ears or bowtie TV antennas, can be used for troubleshooting purposes. The small, active
Aaronia BicoLOG 30100X
broadband antenna from Aaronia AG covers 30 to 1000 MHz and includes a battery-operated broadband preamplifier.
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Placing a current probe around a wire or cable will measure common-mode RF currents. Measuring cable current is extremely valuable when investigating a product’s RF emissions. Detecting and reducing RF currents in cables can help in finding other radiating culprits on the device being tested. Current probes can also be used to inject RF energy into a cable when doing conducted RF immunity tests.
TekBox
makes three economical current probes.
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EMC pre-compliance testing equipment is now quite affordable and even self-funding. It offers a fast, affordable method for evaluating a design and any resulting modifications. The goal of EMC pre-compliance testing is to approximate the compliance test set up to reduce risk of failure before reaching the expensive compliance test stage.
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