![]() ![]() Compare the feedback signal to a previously calibrated level and then adjust the signal source to maintain the test level inside this margin.This forward power must be maintained so that the test level always is obtained but not exceeded by a certain amount for example, power levels to produce the test level or up to 2 dB over the test level. Obtain a feedback signal, such as provided by a directional coupler, that monitors how much power is being sent to the injection device.The signal is sent to an antenna, current probe, coupling decoupling network (CDN), or other injection device. Control a signal source, which then is amplified.The basic control software to perform conducted and radiated susceptibility or immunity testing must: ![]() Even if a basic 20-dB/decade modification is made to the data, this will yield results close enough for assurance when you take the product to an open area test site and measure at the proper distance ( Table 1). Most likely, the shielded enclosure will not be this large, and some type of allowance must be made for the change in test distance. FCC Class A and most CISPR testing require a 10-m distance from the antenna to the equipment under test. In the commercial electronics world, this will include tests based on CISPR 22 and FCC Part 15, often performed in a shielded environment such as a screen room or shielded enclosure. Power supply manufacturers will be very interested in conducted emissions while control and display manufacturers often are more interested in radiated emissions. For military and aerospace work, it can be either radiated or conducted depending on the product you manufacture. It is important to be able to repeat a test several times, test over a limited frequency range, change sweep speeds, track signals, maximize a time-changing signal, measure the bandwidth of emissions, change the bandwidth of the analyzer, and determine harmonic frequencies.įor commercial work, the most common testing is radiated emissions. conducted emissions will greatly simplify the EMC software selection process. Distinguishing between development work vs. When obtaining an EMC control software package, first consider which tests you need to perform. Each of these areas should be studied when purchasing or creating an EMC test control software package. I also have not touched on other areas, such as susceptibility or immunity testing, in what we now call electromagnetic compatibility (EMC) testing. Up to this point, I have only discussed military and aerospace emissions testing and not commercial testing. I could perform a broadband conducted emissions scan from 20 kHz to 50 MHz in about one minute. Our Eaton Series VII Receiver System was controlled with an HP 9836 Computer using HP Basic. Now we had the ability to control equipment from computers, download the data, correct the data for gains and losses, and plot the result.įigure 2. Scans were performed in one-decade bands, and limit lines were drawn by hand.īy the 1980s, the IEEE 488 Instrument Control Bus, also known as the Hewlett-Packard Interface Bus (HPIB) or General-Purpose Interface Bus (GPIB), was standard on most equipment. This was great for doing broadband measurements however, the slew rate on the chart recorder was so slow that narrowband measurements had to be performed by storing the signal on the screen and taking a picture with a Polaroid camera. In the 1970s, the company acquired a used HP 141T Spectrum Analyzer and connected a chart recorder to it. When I operated an EMC testing laboratory for my first employer, an aerospace company, the standard operating procedure was to schedule a 40-hour week to perform conducted emissions testing and evaluation. After all this, we had our first data point. Other known gains or losses had to be corrected as well. Okay, we had one reading, which we corrected for antenna factors, cable losses, preamplifier gain, and, if it was a broadband signal, the broadband correction factor. If the reading dropped 3 dB when moved two bandwidths above and below, we considered this a narrowband signal otherwise, it was a broadband signal.Īnother method was to measure the pulse repetition rate of the signal, and if it exceeded the bandwidth, it was narrowband. However, to determine if it really was broadband, you could tune two bandwidths above and below the peak of the signal. If it was noisy, there was a possibility of it being broadband. When the signal was quiet, it likely was a CW signal and usually narrowband. Measurements were made by slowly tuning the receiver over a frequency range, watching the needle for movement, and listening to the headphones for the type of signal. NM-40A Radio Frequency Field Intensity Meter ![]()
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