Diethard Hansen
EURO EMC SERVICE (EES)
Dr.-Ing D. Hansen
Bahnhofstr. 39, CH-8965
Berikon 2, SWITZERLAND
Phone: +41 566337381, Email: euro.emc.service@t-online.de
https://www.euro-emc-service.de
The reason for the removal of the RegTP field trial license, based on numerous protests, is that Digital Power Line Communications (PLC) is trying to increase the speed of data rates into the Megabits per seconds range. Therefore the short wave band of up to 30 MHz will be transmitted over the low voltage distribution network. The recent history of PLC in Germany together with the standardization and measurement procedures used are given. Public opinion in Germany and technological alternatives to PLC like ADSL and wireless communications with low power are discussed and explained. The present approach of the remaining PLC consortia and their attempt to introduce the PLC technology nation wide is questionable.
Digital power line communications, e.g. mains signalling in the frequency range 3 to 148.5 kHz is a well recognized procedure. The EMC details are documented in EN50065-1 1991 as well as the amendments A1 1992, A2 1995 and A3 1996. The German equivalent is DIN EN50065-1 or VDE 0808 part 1 November 1996, respectively. This standard sets among others the limits for the injected signal level into the 230V/50Hz power grid for the various applications, e.g. for operating, controlling and measuring, in particular switching of the mains signalling receivers for low tariff heating systems.
The company NOR.WEB started the most prominent PLC activity in Europe about 3 years ago. The goal was to transmit data over a broadband medium above 148.5 kHz for telecommunications like ISDN and Internet. According to Shannon's information theory several Megabits per seconds data rate requires corresponding transmission bandwidth of several Megahertz and therefore corresponding higher carrier frequencies. After lengthy considerations and pre-tests, regionally limited field trials were conducted in the U.K. The transmission path in this low voltage network started at the power transformer all the way down to the distribution into the individual homes, where the signal was decoupled. The preferred operation frequencies were 3 MHz and 5 MHz. The integral power transmission level was below 1 Watt. Using the standardized EMC receiver bandwidth of 10 kHz, according to CISPR 16 (150 kHz - 30 MHz), this results in approximately 0 dBm or -40 dBm/Hz respectively.
Under the observation of the radio communication agency in London close monitoring of the trials was performed to explore PLC technology in general. In parallel this called for the development of a new measurement procedure MPT1570 with the title: Electromagnetic radiation from telecommunications systems operating over material substances in the frequency range 9 kHz to 3000 MHz. Measurements are executed in peak mode using a magnetic loop and applying the limits of the electrical field strength:
E=20dB(µV/m)-7.7log10(f/MHz) (1)
This formula applies from 1.6 to 30 MHz for radiated fields . Below 1.6 MHz and all the way down to 150 KHz E = 40 dB(µV/m) - 7.7 log10f(MHz) at 1m test distance and 9 KHz measurement bandwidth. These field strength limits try to consider the emission statistics of a wide spread area use of many modems, with hopefully not all modems transmitting the same EMI at a time.
Consequently EMC equipment limits like CISPR 22 do not directly apply and must be reduced. Fig. 1 and 2 give a comparison of various limits.
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Fig. 1: Limits according to Requirements 30 (NB30) of the FreqBZPV, compared with the limit of VDE 0878 part 1/12.1986 class B up to 30 MHz. The diagram also contains the lmits of Draft MPT 1570/Feb 2000 of the British Radio Communications Agency. The field strength is measured with a loop antenna in a distance of 3 m from the line (MPT 1570: 1 m from 9 k to 1,6 MHz). For comparison with the limit, the logarithm of the free-space wave impedance 20log10Z0 is added to the magnetic field strength. This is a reaction of the German Ministry of Economy to the increasing use of frequencies in networks. At the time of VDE0878 Part 1 there was ISDN with a frequency range below 148,5 kHz. (Source: Manfred Stecher, EMC Brugge 2000 "EMC Aspects of PLC" – email: Manfred.Stecher@RSD.rohde-schwarz.com) |
Which disturbance levels are acceptable to radio reception? The problem of compatibility between radiocommunication and high data rate telecommunication services using the power lines or the telephone network is investigated by an ITU-R Study Question. Until now radiocommunication has survived with the help of CISPR emission limits. Now the question is asked whether CISPR conducted emission limits on power ports for (e.g. household or IT equipment) can be used for PLC. Some PLC operators even propose to use the higher emission limits for telecom ports acc. to CISPR22/EN55022. For the definition of EMI limits, the minimum receive field strengths of radio planning are essential. From these values the protection ratio, e.g. 30 dB for AM radio is to be subtracted in order to obtain the ideal emission limit. However radio reception is possible with some compromise.
Which is the nature of radio disturbance emitted by PLC systems? For normal AM receivers, PLC-CDMA or OFDM signals are like Gaussian noise, i.e. the radio listener does not always have the typical impression of interference, like clattering, buzzing, clicking, whistling or similar, in contrast there will be an impression of low sensitivity of the receiver, since the ambient noise is like increased Gaussian noise of the receiver front-end. Therefore the problem of searching and identifying the source of interference will be difficult. This problem would also arise when PLC systems were operated in dwelling areas due to their continuous presence everywhere.
The consequence of using the CISPR-22 emission limits on telecommunication ports as injection levels for PLC, can be illustrated by comparing the field strength, which results from using the permitted common-mode current on telecommunication lines, with the minimum field strengths in the short-wave frequency band (see fig. 2). Please keep in mind, that for the reception of the minimum field strength, a protection ratio of at least 26 dB (better 30 dB) is to be taken into account. It is therefore not sufficient to check whether the received radio spectrum changes significantly by the addition of PLC. It is necessary to check whether the PLC spectrum is 26 dB below the minimum field strength.
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Fig. 2: Values of field strength measured with a loop antenna at 8 m distance from a three-story building. Dotted line: ambient emission; thin continuous line: field strength generated by injection of the common-mode current equivalent to the CISPR22/EN55022 emission limit into the telecommunication network of the building; bold line: minimum useable field strength for radio planning. HFHS-Z2: R&S active loop antenna 9 k to 30 MHz. (Source: Measured data received from the German RegTP) |
The actual test reality revealed excessive emissions above these limits, however detailed individual data are unfortunately commercially classified. It is interesting to note, that in contrast to the U.K. emission limits, the German draft of the RegTP: Measurement procedure 322 MV 05 RegTP, is relaxed by approximately 20 dB. This MV 05 covers telecommunication lines and installations in the frequency range of 9 kHz - 3 GHz, using 3 Meter measurement distance (for short wave frequencies) from the critical installation/system cables or modem equipment under test to the small magnetic loop antenna at 1 m height according to CISPR 16. The convention of free space impedance (377 Ohm) conversion between magnetic and electric field strength (51.5dB) is used in spite of clearly doing the test in the near field of the emitting test object. In this specification the IF-bandwidth of the EMI receiver is set to 200 Hz for the frequency range 9 to 150 kHz and 9kHz bandwidth for bands from 150 KHz up to 30 MHz. Deviating from CISPR, not the 1 sec integrating and audio weighting Quasipeak detector is used in the EMI receiver, but rather the peak function.
Due to the enormous approval difficulties in the UK and the massive protests of civil and military frequency users in the short wave range, the PLC activity was shifted to the continent and Germany in particular.
At the end of 1997 several consortia were formed in the German PLC arena. NOR.WEB tried to implement the British technology in Germany under Nortel DASA. In order to quickly conduct their own field trials with 1 Mbit/s in Herrenberg near Stuttgart, the south German utility company EnBW/Tesion installed the U.K. technology under license from NOR.WEB. BEWAG, the Berlin utility company, got together with HEW Hamburg and GEW Kšln to develop a different approach namely a certain spread spectrum technology, covering the total short wave range. This led to a patent claim 1997 and the granted German patent DE 197 14 386 C1 in 1998. Very limited first field trials of this technology seemed unfortunately not to reveal major savings in transmit power due to non-optimal conditions. Siemens Germany tried to develop their own solution for Internet over the mains, using multi-channel, multi-frequency technology with bit rates below 1 Mbit/s. The West German RWE teamed up with the Swiss Ascom company group to develop and establish a PLC system, which was tested near DŸsseldorf with similar data rates as planned by Siemens. At the CEBIT 99 in Hanover this voice and video transmission system, using ISDN interfaces, was shown in a live presentation.
All the above-mentioned consortia received a test license by the RegTP for locally and temporary restricted field trials. On the other hand the RegTP initiated a round table to deal with the compatibility in telecommunications (ATRT) under the working group 3 for PLC. For this working group the author was elected as chairman by industry, government and trade associations representatives in autumn 1998. Mr. Stecher from Rohde and Schwarz München accepted the secretariat. It was the main goal to evaluate the new PLC technology and use this input to drive national and international standardization.
Discussions at this PLC WG and international CENELEC indicate the proposed PLC injection levels of around 0 dBm at 10 kHz bandwidth would roughly correspond to 60 dB uV/m radio disturbance field strength in 3m distances, which is unacceptably high for short wave bands!
PLC technology can not just simply be categorized by the historically grown EMC standards. One way to look at it, is to put a PLC modem into the framework of the EMC act (EMVG). However, what is going to happen, if this equipment is used in a wide spread network? Moreover, the power distribution grid has not been developed as a symmetrical telecom line. The proper definition, in telecom terms, of the media is almost impossible, because of mismatches, stubs, switches, lighting, outlets etc. Unshielded symmetrical, modern telecom cables have a minimum of 35 dB symmetry attenuation in the used frequency band. Unsymmetrical signals eventually become interfering common mode currents and have to be suppressed as far as economically possible. In the low voltage network inside buildings this value could be anything including 0 dB. Therefore common mode and not differential mode is mainly producing EMI. The equipment is hardly never going to be an impedance match. Additionally, PVC installation cable by their insulation display much more path loss than telecom cables. 60 dB over 100 m at 20 MHz are not unusual. Even on one particular floor there is a lot of difference in attenuation, because outlets are installed in various locations, using different feeder systems of various phases and forming subs. The effective PLC signal transmission in the short wave range (3 MHz to 30 MHz) is perturbed by the legally necessary radio interference protection of the RFI bypass capacitors in the line filters of the associated equipment. These capacitors cause RF short circuits. For these obvious reasons low voltage networks have not been put into the group of telecom networks.
On the other hand, radio transmitters have to go through national approval. This has not changed much with the introduction of TTE and R+TTE directive by the EU, because frequency spectrum use and frequency allocation have historically grown differently in the various countries. A change over night is impossible, harmonization will take time. It is also worth mentioning the duty of the national telecom authorities to ensure a certain transmission quality in the spectrum. Consequently this leads to the right of the German RegTP to control also the situation below 30 MHz along conductors, maintaining a peaceful coexistence of the various services. This may be, among others, public broadcasting, security services, the military, secret service and amateur radio. That is the coordination under the German frequency management plan, which is in its final draft version presently. In former days there was basically no EMC conflict between cable systems and wireless applications. Unfortunately this has changed a lot with the introduction of cable TV systems and high-speed digital signal transmission. In spite of using coaxial cables in the TV distribution systems there is a lot of shield leakage, based on technical imperfections and aging. Catastrophic emissions in the aeronautical security bands are jamming Germany. LANs and WANs are growing increasingly, adding to this critical situation.
The German telecommunication act (TKG ¤45 part 2) regulates the use of frequencies in and along conductors. There is not much of a choice for PLC user. They either use low power spread spectrum systems with limits well below the EMC standards for ITE or apply for a national approval according to TKG, if they intend to use high power systems.
The fight over the last mile to the end customer in modern deregulated telecom and electricity markets requires solutions for wide spread networks on harmonized European level. This is presently happening under ETSI and CEPT/ERC, standardizing functional and EMC parameters more or less under one roof.
An important EMC compromise has been reached in CISPR 22 (EN55022 1998). This led to a relaxation for telecom ports under class B of about 10 dB conducted common mode emission between 6 and 30 MHz. This is, however, a temporary solution, which may have to be revised within the next 3 years, based on interference complaints.
The German standardization subcommittee DKE UK 676.17 has founded a PLC working group in the forth quarter of 1999, closely cooperating with ATRT. The DKE represents the national committee of Germany, dealing with ETSI, CENELEC, CISPR and IEC. Of particular importance is presently CENELEC TC 205A, spanning the frequency range from 3 kHz to 30 MHz. Most PLC interested parties follow the high power approach with subdivisions in various unoccupied short wave windows and creating a product specific standard. This attempt, however, is problematic, due to international frequency allocations at the ITU in Geneva.
On the German side the RegTP under the ministry of economic affairs has issued a decree NB 30 - 1999. This contains the following 3 m limits:
The detailed measurements follow the procedure RegTP 322 MV 05.
Shortly before CEBIT 99, the consortium RWE Ascom announced in a big PR campaign a fundamental breakthrough in PLC. The Ascom stocks rose sharply. However, PLC technical details were never published. It was interesting to watch the reaction of the other consortia in immediately launching their own success stories for projects, which were not even close to finished. Putting the pressure on the consortia in ATRT, clearly demonstrated these experts were not in the position or willing to speak about technical details. Even the presentation of the inventor of the NOR.WEB PLC, Dr. Brown, was only partly covering technical details. How can one standardize a proprietary black box PLC technology? It became finally clear by walking around at the CEBIT 99, that the whole PLC show was marketing driven. Only RWE Ascom demonstrated live. This, however, was not much more than ISDN quality. The show was accompanied by numerous protests of short wave users. The strongest opponents proved to be the RegTP licensed radio amateurs. Their club (DARC) with more than 60,000 members stated correctly PLC to be a lethal threat to the amateur radio service. This was demonstrated in various PLC field trials.
Security services like police, military and intelligence service (BND) contradict the assumption of the PLC activists, regarding the existence of many empty frequency windows. Naturally, these secret windows are not publicly listed, while at the same time radio and TV community was worried about potential increase of the noise floor throughout Germany. It was emphasized, that the introduction of digital broadcasting, leading to less radiation hazards, will only work if the transmitter power is reduced maintaining the same background noise. The more sophisticated the modulation type, the higher the signal noise ratio has to be.
The proof for the premature push of PLC into the standardization becomes evident by analyzing the majority of the symposia, dealing with PLC at that time. Here it was mostly the business case highlighted, but the technical feasibility not to mention EMC was hardly discussed in detail. Services like video on demand, super fast Internet access and Internet telephony kept the project manager's dreaming. This is partly supported by the observation of power network engineers, starting a new telecommunication career. In the mean time German universities started Ph.D. PLC work, which naturally will take 3 to 5 years.
It is important, however, to recognize there are many competing technologies of either conducted or wireless nature outside the field of PLC. There is tough competition in the race to the last mile. The broad band cable TV system can nowadays offer several 10 Mbit/s in about 100 MHz bandwidth at reasonable price. Bi-directional systems with the corresponding modems are commercially available for private homes with charges similar to cellular phones. The provider Netcologne is offering 52 Mbit/s without having to reserve a rented extra line.
The German Telekom is offering ADSL with approximately 1 Mbit/s expanding the old ISDN capabilities. This technology has less EMC problems, due to the symmetrical nature of the telecom lines. Fiber optic links are well known, but normally too costly to connect the end customer.
Wireless LANs today reach 1 to 10 Mbit/s at 20 dBm (100 mW) bridging several 100 m in 2.4 GHz and 5.8 GHz ISM bands. Long distance tests of up to 7.5 km at 2 Mbit/s and 100 mW have already successfully been conducted. Lately 155 Mbit/s have been achieved by wireless loops.
Looking at these advanced alternative technologies, the introduction of PLC within a relatively small window of 1 to 2 years is critical, knowing the PLC community is trying to catch up with the low side of the other high-speed data rates.
After the presentation of the short wave propagation study by Ascom at the end of 1998 it was clear, -40 dBm/Hz PLC transmission level might increase the natural background noise in Germany by as much as 10 dB. The ionosphere would then mirror this effect into overseas regions. Short wave radio communications could therefore be seriously affected.
Amateur radio PLC simulations in the U.K. proved the effectiveness of Milliwatt powers for European wide radio traffic by using typical PLC and even symmetrical, buried cables. These tests were under the supervision of EMC professionals like competent bodies and EMC test labs.
Concern was also expressed by the German armed forces, the BND, the ministry of internal affairs and local federal government state agencies. Telecom experts in the worldwide embassy radio link system also expressed concern. A similar situation was found in the broadcasting over long, medium and short waves. The often-stated argument, this can all be covered by Internet is complete nonsense.
Many serious PLC studies and simulations in academia do not make the author believe PLC could shortly be implemented on a nation wide base.
30 dB to 40 dB in excess of the RegTP NB 30 decree in well-monitored field trials of the consortia, accompanied by accredited test labs and the RegTP, speaks for itself. These results, initially classified, are now publicly available in the minutes of the ATRT PLC meeting from 24 Sept. 1999.
Based on intensive discussions the RegTP has now withdrawn the granted licenses. Another sensational news was: NOR.WEB is pulling out of the PLC business, mainly for economical reasons!
The PLC results, in particular regarding EMC, did by far not meet the expectations. The time pressure in the market place by competing technologies is enormous. The consortia were unable to deliver adequate technical and normative drafts. ATRT, however, was able to input international normative requests for well coordinated PLC standardization. The first working groups are successfully acting. The author assumes that in the near future there is only a chance to introduce PLC technology by using spread spectrum and keeping well below the EMC limits. This, however, will lead to more repeaters along the line. EES Germany has conducted positive pre-investigations. The EMC act with the higher limits, compared to the TKG limits, may have to be reconsidered in view of system and not only box requirements.
To probe further, here are some EU PLC links in English and German language:
The latest news comes from the national association of radio and telecommunications engineers, Inc. (NARTE) USA issue vol 18 No 3 Fall 2000 with a citation of a recent article from the Wall Street Journal, where again German PLC promoters discuss marketing but rather not EMC aspects.
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Dr.-Ing. Diethard Hansen is founder and president of EES Switzerland and Germany, specializing in international consulting, high tech marketing, training, innovative EMC test products, accredited testing and R&D. Further areas: LVD, radio, automotive and medical. He is holding a BS/MS in electrical engineering from Germany and a Ph.D. degree from TU Berlin. More than 20 years of industrial EMC/EMP experience, 35 patents (GTEM, EUROTEM, Poyntor sensor) and 140 professional publications as well as chairmanships are assigned to him. He was the manager of the EES Competent Body and acted as board member of European Competent Bodies ACB - Brussels. Memberships: IEEE/EMC, CENELEC, ETSI and IEC. He is the RegTP ATRT PLC chairman and a lead auditor for EMC labs and Competent Bodies in the German DAR accreditation system. Since 1990, Dr. Hansen is a senior EMC engineer under USA NARTE accreditation. |