Thanks for all the great questions presented following my recent EMC webinar, sponsored by Rohde & Schwarz and hosted by UBM TechOnline. If you missed the webinar, you may go here to download a copy of the slides and listen to the webinar "on-demand."
As I mentioned in the previous two postings, I've grouped your questions by topic and will be answering them all the best I can. Be advised that for many questions pertaining to EMC, the best answer is, "it depends," so there may not be one answer for all cases. I'll try to include my assumptions in the answers. The questions have been edited for clarity.
This posting will address questions on Pre-Compliance Testing for Radiated Emissions.
Q. Most folks don't have access to a semi-anechoic chamber. Any recommendations for the room used for radiated pre-compliance tests?
A. True; most semi-anechoic chambers are very expensive. I've been very successful using conference rooms or basements to set up a simple 3m measurement range for confirming pass/fail data. Of course, I like to give myself about 6dB of margin, just to lower the risk of failing in a real chamber.
Here is one major issue: Ambient signals (that is, signals from external sources like AM & FM broadcast, television, two-way radios, military transmitters, mobile phone, and wireless communications) can mask the harmonics from the product, unless you can test in a shielded room. Making the measurement in a rural area or basement can help. Knowing where the major emissions lie is important, because you can then narrow down the span to include just that harmonic. If the harmonic in question is still being masked by an ambient signal, often, you can narrow down the resolution bandwidth (assuming the harmonic is a narrow band "CW" signal) and make an approximate amplitude measurement.
Another issue is that very strong ambient signals, say from a nearby FM broadcast or television station, can overload the front end of the EMI receiver (to some degree, depending on whether it has preselection filters) or spectrum analyzer. Because spectrum analyzers have very wideband front end circuitry, a signal that overloads the analyzer can affect (compress) the other measured signal amplitudes.
You'll also need to know the system losses and gains, so you can calculate the voltage at the antenna terminals. This would include coax loss, attenuator losses (if used), preamp gain (if used), and the antenna factor (provided by the antenna manufacturer).
Q. The equipment you show is for pre-compliance or to resolve a problem. For EMC full compliance, which are the advices you give in order to comply with the tests?
A. You really have two choices; assemble the equipment and semi-anechoic chamber required to perform your own radiated emissions testing (can cost $500k to $3M, depending on 3m or 10m chamber), or have a third-party test facility run the compliance testing for you. Unless your company is willing to fund such a project, my advice is to rely on an outside test lab.
Q. How do you set up a pre-compliance test lab, what are minimum possible equipments required with a limited budget?
A. If your budget is limited, the minimum equipment required is a good spectrum analyzer ($2 to $20k), an EMI antenna (or antennas) that can tune from at least 30MHz to 1000MHz (however, the U.S. FCC can require testing up to 6GHz, or more), a turntable to set up the product with its attached cables, and an antenna tower that can be adjusted in height and can turn the antenna from horizontal to vertical polarization.
You may also need a preamplifier to boost the signal from the antenna. It's possible to make a DIY turntable and tower to save a little money. The total cost might run $10k to $50k. Note, also, that the frequency range required also depends on your product family and test standard used. For example, some ITE and medical products require measurement all the way to 30GHz, which could double those costs.
Q. If you use a current loop probe to measure the dBuA on a spectrum analyzer and calculate the current for differential-mode currents, do you still need to input the L & s of the formula you mentioned to calculate E field?
A. Yes, you do. An estimate works OK. The main thing is to capture the length of the circuit trace or cable.
Q. CM and DM calculations -- where did the constant come from? How was it derived?
A. These are derived in Dr. Clayton Paul's book, Introduction to Electromagnetic Compatibility (2nd ed.) starting on page 509 (Differential-Mode Current Emission Model) through page 516 (Common-Mode Current Emission Model). Assuming we start with the Hertzian dipole model, the DM and CM constants are comprised of the intrinsic impedance of free space, phase constant and a factor of 2*π or 4*π.
Q. Why is conducted emission's maximum frequency 152 MHz?
A. I don't recall mentioning 152MHz, however, generally speaking, common-mode emissions start to decrease around 200MHz to 300MHz as a dominant factor in harmonics. That's not saying that CM currents don't exist above that frequency -- just that they aren't the dominant source.
Q. Could you use that scope connected to an antenna for wide band radiated emissions?
A. Interesting question, as most designers have access to oscilloscopes, rather than spectrum analyzers. As well, most digitizing scopes also have FFT functions. The answer is it depends a lot on the sensitivity of the scope in question.
Most scopes can't measure accurately below a few millivolts and the signal levels at the antenna terminals might be in the uV region. So, unless your scope can measure down to the microvolt level, it probably won't work well. Some of the newer scopes from Rhode & Schwarz and LeCroy have sensitivities in the microvolt, however, so they might be worth a try. However, you'll miss out on the convenience of dedicated spectrum analyzer controls, of which there are many. If cost is an issue, you might check out the Rigol DSA815 analyzer that I reviewed recently.
Q. How accurate are FFT readings from a oscilloscope using regular oscilloscope probes compared to near-field probes?
A. Hard to say, as most near-field probes aren't really calibrated. Now, if the question is really, "Can I perform troubleshooting with either near-field probes or scope probes," the answer is "yes."
The other part of your question relates to the accuracy of the FFT data on a scope. I would have to say, most of the newer scopes will be fairly accurate, however input sensitivity and internally generated noise in the front end will come into play and mask very small signals. It's important that the scope have the ability to be adjusted to a resolution bandwidth of 100 to 120 kHz (for the 30 to 1000MHz band) in order to compare accurately with spectrum analyzer measurements. But consider the discussion above, as well.
I have written extensively on how to perform pre-compliance testing for radiated emissions and would invite you all to read some of the postings here on The EMC Blog, as well as the links to some of my articles in other magazines.
Feel free to add additional questions related to PC boards. I'll be posting additional questions asked during this webinar in later blogs. If you missed the webinar, you may go here to download a copy of the slides and listen to the webinar "on-demand."