PCB as reference for EMI measurements - Page 1
Forum section: electronics / rf-microwave

author: Anders Petersson
author_rank: Regular Contributor
timestamp: March 31, 2021, 02:41:31 pm
content: I'm getting some basic equipment to measure and shield EMI (electromagnetic interference). But will my measurements be correct and will my shielding be effective?Is there any PCB with known and documented EMI levels which I can get to compare my readings?A known emitter of E and H radiation would also be useful to test the susceptibility of my own PCBs.I imagine some manufacturer of EMI shields could publish/sell such a board as a demo of their shielding. Probe manufacturers might also use it as marketing or educational kit.

author: jonpaul
author_rank: Super Contributor
timestamp: March 31, 2021, 05:13:35 pm
content: Can you please define the "basic equipment" brands models, totoal cost?What is application/requirement eg learning EMI, design of production device, debug compliance?What is your location and the regions where equipment is to be qualified?Frequency bands of interest?EG 150 kHz...50 MHz will be very different than WiFi/Mobile/3G4G5G frequencies.Usually a calibrated EMI test for radiated needs special calibrated EMI antennas for each band required.For conducted noise, a LISN and protected SA are all thats needed.I have never seen a "PCB reference" for EMI.Suggest to consult the many fine texts on EMI compliance and measurement as well as the app notes from HP, Advantest, R&S, etc.Best Regards,Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 31, 2021, 10:58:58 pm
content: A finished PCB is far too special of a case, to use as a demonstration.You can still test your kit, using basic physics, transformers, transmission lines, coupling networks -- that sort of thing. This will lead to greater understanding of how to use the equipment in the first place, when to apply it, and how to test and solve real problems. Study EM and transmission line theory, and RF circuit analysis. You need to develop an understanding of what a port is (as this is what your equipment provides), how signals get to and from it (coupling networks / antennas), and what the sources or propagation of those signals involves.Once you understand that, you'll understand why a PCB is too complex to use in this way, but you'll also understand the ways in which a PCB can emit or receive signals, and to extend the same thought process to the layout itself (assuming you're doing PCB layout).Tim

author: Anders Petersson
author_rank: Regular Contributor
timestamp: April 01, 2021, 08:34:19 am
content: Thanks for your replies. I'm mostly looking to apply the theory by hands-on measurements. Partly to learn but I also have some noise in a high-resolution ADC that I'd like to track down. And my oscilloscope is picking up noise from some switching power supply, I think. It would be neat to do some pre-compliance tests too. So general use which doesn't warrant paying big bucks. Perhaps frequencies up to MCU digital overtones of 1 GHz?I have a Siglent SSA3021X. I have on order the cheapest near-field probes that Ebay has to offer (this type of $30 deal:https://www.ebay.com/itm/6Pcs-Near-field-Simple-Magnetic-Field-Probe-EMC-EMI-for-Conducted-Radiation/154226885268) so you will understand why I doubt the accuracy. Perhaps I will need better probes and/or a preamp to do any useful measurements, I don't know yet. I don't think theory can bridge the uncertainties of practical test benches, or? How do RF people verify their measurements anyway? Just assume the fields match the theoretical values?A calibrated antenna and full-featured lab would be superior in all respects of course, just as metrology-grade voltage calibrators are superior to $10 Ebay voltage references, but there's room for both ends of the spectrum. I figure many engineers face the task of doing some RF measurements without the time to become an RF expert.If there's no PCB like this, that's the end of that question. But if anyone cares to explain further, I think I have a reasonable grasp of the theory and I don't see why a reference EMI source wouldn't work. Compliance testing is already built on an assumption that PCB behavior and measurements are repeatable, right? The simplest case is a PCB with a single trace and SMA connector, forming a stub or loop antenna when connected to a signal generator. Surely everyone should be able to build that PCB and measure the same radiation levels and patterns? Naturally, at least one person needs to pay a calibrated lab to characterize their copy of the circuit.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: April 01, 2021, 09:32:00 am
content: The closest I've found is documenthttps://www.ti.com/lit/an/swra328/swra328.pdffrom TI:"The main purpose of the CC-Antenna-DK is to ease the decision for which type of low cost antenna can be implemented as well as give an estimation of the performance that can be achieved. The frequency range of the antennas is from 136 MHz to 2480 MHz. The A4 sized PCB panel contains 16 different boards; 13 antenna designs and 3 boards for calibration purposes"There are links to measured radiation patterns and lots of numbers. But it's only for some common radio frequencies and doesn't look that easy to recreate.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: April 01, 2021, 03:39:30 pm
content: You can calibrate H field loops using Helmholtz coils and a signal generator. See:http://wireless.org.uk/pa0se.htmFor E fields, you can produce a field of known Volts per metre using a dipole antenna. This on-line calc gives the field strength:https://www.daycounter.com/Calculators/Maximum-Field-Strength-Calculator.phtmlIt is not easy!

author: jonpaul
author_rank: Super Contributor
timestamp: April 01, 2021, 05:34:58 pm
content: Anders Petersson, Bravo, the TI app note is perfect, mille mercis,Jon