208. Broadband over power lines – interference concerns
Early field trials in UK, Germany and Switzerland showed excessive radiated emissions (up to 40dB) above NB30 RegTP limits, which are about 20dB more relaxed over the 4/2000 RA version of UK MPT 1570 in the short wave spectrum. Broadcast, military, commercial as well as licensed amateur radio services started seriously objecting to a nationwide implementation of PLC.
(PLC = Power Line Communications, basically sending Internet data over existing mains wiring and cables – sometimes called PLT instead – Editor)
Far field effects and underestimated PLC system antenna factors [10] lead to short wave signal mirroring at the ionosphere. That is today not at all taken into account by officials in the ministry of economy in Berlin, which supervises the RegTP agency, the equivalent to the FCC US. These sky wave propagation effects might lead to background noise increase [6][7][10] also outside Europe. Sensitive receiving sites in Germany may experience, based on first simulations, degradations of 10 to 40dB! This is unacceptable for security agencies in the present political scenario.
The introduction of power reduction in broadcasting, due to digital technologies, reducing transmit power and therefore lowering electromagnetic pollution or heath hazards, become useless if at the same time the signal noise ratio will be PLC degraded.
Reports on publicly available, new measurements data from PLC modems/systems (e.g. ASCOM). Some indicating serious legal and technical trouble in wide spread PLC field trials systems.
Suspicion arouses, due to questionable promoter companies, seemingly forcing contracts with non-discloser agreements to be signed by their clients. This could hamper independent measurements.
Everybody is fighting physics. Due to Shannon, signal to noise ratio (typ. 15dB) is EMI relevant. PLC signal level, modulation and existing line noise are important to bridge the distance without costly repeaters. The PLC community is therefore fighting for “better”? less stringent regulations and want new EMC standards.
Little attention was formerly given to commercial System EMC; box testing was rather dominant. Finally, the commercial EMC community is forced into System Thinking!
Cable TV systems started interfering with air traffic control over major German cities.
Typical test problems are identifying PLC Interference in bands <30 MHz, receiver jamming, time variant EMI. It takes wireless experts to be sure it is PLC and not other EMI. Normally at CW, AM, SSB, the whole receive spectrum is experiencing a massive noise floor increase (sounds like an old steam locomotive sometimes), resulting in total blocking. The sensitivity is wiped out.
Generally speaking there is very little willingness of the PLC people to talk technical even today.
On the official side, however, 100 serious, professional NB30 objections, some demanding even lower limits, filed to RegTP, were politically ignored by the ministry of economic affairs last year when NB30 came out. Reliable sources indicate, Federal Cabinet Minister Mueller (Economy) – originating from RWE (a company which is active in the PLC business – Editor) – before entering his political career in the SPD government – wants to return to his old company!
(A number of extracts from: “Update on Power Line Telecommunication (PLT) Activities in Europe” by Diethard Hansen of Euro EMC Services (EES), chairman of ATRT WG PLC, RegTP, Germany, presented at the IEEE’s International EMC Symposium held in Minneapolis, Minnesota, August 19-23 2002, and published in the Symposium Record on pages 17 – 22.)
209. Increasing pollution of electromagnetic environments
Array-pattern nulling effects have become an important field of study recently due to the increased pollution of electromagnetic (EM) environments. These techniques reduce degradation of signal-to-noise ratio (SNR) performance due to undesired interference in radar, sonar, and communications systems.
(Taken from “Reduce SNR Degradation in EM Environments Using a Nulling Technique” on page 56 of Microwaves and RF Journal, September 2002.)
210. Examples of interference from Douglas Brooks
EMI and RFI are not new phenomena. They are problems that have been around for years. When I was a small boy (which is longer ago than I will admit), I grew up across the street from a ham radio enthusiast named Bob Beebe, W71GM.
Bob had a powerful one-kilowatt linear amplifier for his ham rig, and a rotating beam antenna on his roof that covered more area than his roof did. We could hear him on every electrical appliance we owned. His calls are indelibly etched on my memory: “Hello, CQ, CQ, C. Hello CQ, CQ, CQ. This is W71GM, I Got Manilla.” (Manilla was the name of his wife!) Every time we got a new radio, we’d have to call Bob to come over and wrap it in copper or place ground wires all around it in order to shield out his emissions.
In another life I ran a company that made weighing systems for industrial trucks – i.e., scales to make sure trucks were within legal limits. They were portable, could be towed behind a police car on a small trailer, and used portable electronics that plugged into the car’s cigarette lighter. They ran off the same electrical system the police radio did, and the indicators were often placed right next to the radio or on the car roof, right next to the antenna. Immunity to RFI was a significant design requirement.
We had just finished a complete redesign of our indicator family. We had access to a screen room facility and a technician through another company, so we went there to do the EMI/RFI testing. During the very first test, however, the indicator went totally off-scale! No matter what we did we could not quiet down the indicator. After two hours of tweaking we got some improvements and then hit a plateau. No matter what we did we could not quiet down the indicator.
The screen room technician finally spoke up and asked us what the input circuit looked like. We told him it was a high-gain differential amplifier, which then fed an A/D converter. He asked us what part number the amplifier was. We told him. It was a commonly available amplifier made by at least four or five manufacturers. He then asked us who manufactured the part. We told him. He then told us that particular manufacturers often had RFI problems with its parts and why didn’t we buy the same part from a different vendor. We did, and the RFI problems almost totally went away. It took us only a few more hours to achieve the RFI objective and the product then successfully went into production.
There was no clue in any of the published specification from any of the manufacturers of this part number that there would be differences in RFI sensitivity between product offerings. We had no reason whatsoever to suspect that part. We might have struggled with that design for months if that technician had not put us on the right path. There are two morals to this story:
- A good technician with experience can be more valuable than someone else with all the university degrees in the world.
- There can be subtle differences inside IC packages in otherwise identical parts that may only be determined by laboratory testing or trial.
More than one engineer has been ‘burned’ by a part that behaved unexpectedly. Sometimes, as in this case, there are simply differences in design or manufacture or otherwise “identical” parts. Sometimes a supplier changes a manufacturing process without telling anyone. Often this involves the implementation of an improved process, which coincidentally may offer faster rise times.
Perhaps the manufacturer thinks that the change or improvement will have no particular consequence for anyone, and treats it as simply an in-line adjustment. But sometimes the faster rise-time results in timing or EMI problems that didn’t exist in the user’s design before. These can be particularly difficult to trouble-shoot because people rarely equate the problems with a device, particularly a device that used to work just fine.
(Extracts from “Lessons Learned the Wrong Way” by Douglas Brooks, President of UltraCAD Design Inc., http://www.ultracad.com, in Printed Circuit Design mag, Sept 2002, pages 30, 39, http://www.pcdandm.com.)
The regular “Banana Skins” column was published in the EMC Journal, starting in January 1998. Alan E. Hutley, a prominent member of the electronics community, distinguished publisher of the EMC Journal, founder of the EMCIA EMC Industry Association and the EMCUK Exhibition & Conference, has graciously given his permission for In Compliance to republish this reader-favorite column.
The Banana Skin columns were compiled by Keith Armstrong, of Cherry Clough Consultants Ltd, from items he found in various publications, and anecdotes and links sent in by the many fans
of the column. All of the EMC Journal columns are available at: https://www.emcstandards.co.uk/emi-stories, indexed both by application and type of EM disturbance, and new ones have recently begun being added. Keith has also given his permission for these stories to be shared through In Compliance as a service to the worldwide EMC community.
We are proud to carry on the tradition of sharing Banana Skins for the purpose of promoting education for EMI/EMC engineers.
