Designing EMC Test Systems FAQ
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- What do you recommend when performing calibration of a radiated immunity system in terms of dwell time and steps in frequency? Is it dependent on the EUT or the standard?
- Really, it depends on both. The standard will define the frequency step sizes and the minimum dwell times. In many cases, the standard will also include a clause that states that if the functionality of the EUT takes longer than the required minimum dwell time to be fully exercised, then the dwell time must be increased to allow for a full functionality cycle of the EUT.
- Concerning future considerations and expansion - most of the equipment can be sold/donated to other companies & university research groups or somehow tweaked to use in the future. How about Anechoic Chambers? If we need a new (bigger/smaller/better) Anechoic Chamber, how do we 'recycle' the old one?
- While there are certainly options for test equipment like you have mentioned (plus trade-ins), it is significantly more difficult to do the same with anechoic chambers. The market for used anechoic chambers is much smaller than that of used test equipment. Additionally, it can be difficult to move an existing chamber to a new location without severely effecting its performance. That said, it is not unheard of for companies to purchase used chambers from a given company, ship it across the country and reassemble. If you are going to go through this process, be forewarned of the risks. If you’re looking at ‘recycling’ your old chamber though, you may want to contact local sales representatives in the EMC industry who may know of leads.
- What is the definition of spot size?
- Spot size is a quantity determined by the 3dB beamwidth and the antenna to EUT distance. Using these two quantities, one can calculate the area that is equally illuminated by the antenna at a given distance.
- Spot size - It was given as a length ("1.5 meter spot size"). Is that length a radius or a side of a square?
- In reference to the uniform field area defined in IEC 61000-4-3, it is a 2-dimensional square area of field that is located at the EUT site. In terms of antenna spot size, it is determined by the antenna’s 3 dB beamwidth and antenna to EUT distance. The result of this determination gives the diameter of a roughly circular area of illumination provided by the antenna at the defined antenna to EUT distance.
- Have you ever heard of a reputable EMC labs wrapping cables in aluminum foil to prevent auxiliary equipment from emitting EMI into the EUT or the measurement equipment?
- You must be careful when doing this, because in both immunity and emissions testing, the cables must be representative of the cables used in normal operation. This means that if an EUT fails a test under normal conditions, but passes a test with foil wrapped around some cables, those cables must be shielded in its real-world application. If this approach is taken, then the EUT should be retested with the appropriate shielded cables.
- Can a GTEM cell be cost effective for testing Radiated Immunity/Emissions pre-compliance, is so why?
- They certainly can. The primary reason being that GTEMs are significantly less expensive than a chamber. If pre-compliance capabilities are all that is required, this cost reduction can be a compelling driver. This is, of course, if the EUT is small enough to fit into a chamber. Also, EUTs with cabling can pose problems inside a GTEM as repeatability becomes a concern. Remember too that some standards allow for GTEM test methods, but don’t necessarily provide a clear methodology for testing EUTs with cabling.
- Please explain the amplifier power needed to perform the test. You will need more power than you think to perform the test.
- Conservatism is such an important consideration when sizing an amplifier. Often, conservatism is sacrificed for cost. This rarely works to anyone’s advantage. Conservatism is used to accommodate for the unexpected and, unfortunately, in this industry there is a lot of room for unexpected effects. Typically, AR will size amplifiers according to the amp’s P1dB rating and then add an extra 3dB of margin on top of the P1dB rating. Sometimes 6dB depending on chamber and equipment conditions.
- What about conducted transient immunity in designing EMC Test Systems?
- Conducted RF Immunity systems follow many of the same steps that were described in this discussion. Conducted Transient Immunity is a bit different in that there are fewer variables in producing a threat. Typically, conducted transient immunity can be achieved using one-box solutions that can create the transient on-demand and inject this transient into an EUT with a very specific transducer.
- In a radiated Immunity test setup, is there a way to minimize "amp drift", where the power output of the amp changes over time with the same input signal?
- Amplitude drift is much rarer with solid state power amplifiers (SSPA) than with Traveling Wave Tube Amplifiers (TWTA). There are a few steps that can be taken to try to reduce this effect. One is amp warm-up. Sometimes amps (even SSPAs) need some time to warm-up to full operation. The effect is much less pronounced in SSPAs than in TWTAs. If, after warmup, the amp is still not performing to the level expected, the signal generator drive could be adjusted to accommodate for a decrease in gain. Using a directional coupler, one can calibrate the forward power required to produce various fields and adjust drive levels to achieve the required forward power. If frequency drift becomes a concern, the power measured from the directional coupler can also be used to detect these anomalies as well. Again, this effect is seen less often than in TWTAs.
- In a Radiated Immunity test setup, do you recommend having the Field Probe in the test field for measuring the field strength during the test. Or is the effect of the EUT going to make the field probe measurement useless?
- Really, the answer comes down to the test standard. If the method I ‘prefer’ isn’t in agreement with the test standard, I still have no choice and must abide by the standard. That said, my personal preference is to utilize active field leveling when performing a test. In other words, I prefer to leave the field probe in the chamber and near the EUT when performing a test. I like to have the satisfaction that field is being generated while performing the test through active measurement. I certainly agree that the probe will influence the field, but I think that this influence would be minor. The EUT typically would be more of an influence to the field than a field probe. Always remember though, the standard is what drives the method used, not what your preference is.
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