National Institute for Subatomic Physics
ETR 2011-01
On‐site EMC measurements of the Nikhef Spark Chamber at the Nemo Museum.
T.Sluijk ([email protected]),
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Abstract
Nikhef, Department of Electronic Technology
Science park 1051098 XG Amsterdam, NL
Nikhef, tested its own Spark Chamber - Particle Detector De-monstrator setup, to see if we can comply to European rules for CE marking especially with respect to good EMC behavior.
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Contents 1 Introduction ......................................................................................................................... 2
1.1 General ......................................................................................................................... 2 1.2 A word about CE marking ........................................................................................... 2 1.3 Measurements we want to do ....................................................................................... 2 1.4 Immunity , measurements we will not do .................................................................... 3
2 E-field measurements .......................................................................................................... 3 2.1 Method ......................................................................................................................... 3 2.2 Trigger pickup .............................................................................................................. 4 2.3 Resulting Radio disturbance of the short chamber pulses ............................................ 7 2.4 Conclusion of E-field measurements: .......................................................................... 8
3 Pacemakers of the public: .................................................................................................... 8 3.1 Pacemaker measurement Conclusion ........................................................................... 9
4 Current injection measurement in the Mains Cable: ......................................................... 10 4.1 Cable disturbance Conclusion .................................................................................... 10
5 Peak Searching measurement method with Efield antenna: ............................................ 12 5.1 Conclusion for peak searching: .................................................................................. 12
6 Overall Conclusion: ........................................................................................................... 14 7 Bibliography ...................................................................................................................... 15
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1 Introduction 1.1 General
In October 2010 Nikhef, tested its own Spark Chamber - Particle Detector Demonstrator setup,
to see if we can comply to European rules for CE marking especially with respect to good EMC be-havior.(Photo1.1)
Photo 1.1, 2010-10-11 Test Situation Overview. The grey Nikhef Spark Chamber with the
blackened windows in the hall at Nemo. There are 3 measurement testers, Tektronix battery scope TDS3034 a Hameg Analyzer HM5011 with E-field-antenna HM500E(10cm) and FCC23 current probe (next to spark chamber on the floor)
1.2 A word about CE marking For free movement of goods European environmental norms are being adopted, also for EMC.
Europe devised CE (Compatibilite Europeain) Marking rules for this. EMC (Electro Magnetic Compatibility) means a device can work, fully functionally, together with other devices in the Electro Magnetic environment without disturbing the other devices (interfe-rence), or being disturbed by them (immunity). The device is compatible with its environment. This report describes EMC pre-compliance measurements of the Nikhef Spark Chamber for the Ne-mo museum.
1.3 Measurements we want to do For fixed installation-devices we measure the interference produced on site. We don’t require
an official submittal of the verification measurements. The manufacturer (Nikhef) must keep data on hand.
The emitted Electro Magnetic field should be little, not to disturb radio reception and communication especially communication of services like fire brigade and medical assistance (ambulance)
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We also looked at the noise that could possibly de-regulate pacemakers of the public, when they are looking at the display. The Nikhef Spark Chamber carries high currents during sparking (~100A) like a small lightning. This could cause pulsed magnetic fields that could de-regulate the pacemaker, or produce a current in the wires of the implant, causing a fake cardiac pulse. Pulses of the hart are in a rhythm of 1 per second. The rhythm of the Spark chamber is 1 spark per few seconds. [ (1) pace-makers] To prevent interference Nikhef build a closed faraday cage around the unit, and covered the windows with special electrically screening glass (blackened copper foil with 80% optical transparency). The mains cable is especially filtered, to reduce outgoing noise. This report describes the measurement of radio frequent interference measured at different distances of the spark chamber. Second we test the noise level that is injected into the mains cable. (Cable conducted noise) This is the only cable the device has. This injection can cause the cable to act as antenna. (2)
1.4 Immunity , measurements we will not do We did not work on immunity measurements for this device. It is a scientific demo setup, and
the importance to stay functional under all conditions of external interference is less important. Of course we did not notice any malfunctioning under normal working circumstances.
2 E-field measurements 2.1 Method
We can use standard methods described in CISPR 16 (3), using a 6m long ground plane on the floor. We used copper foil on a Kapton base. We can make a roll of it and then the transport is easy compared to metal plates. Because of lack of room we used 6m instead of 10m distance of the anten-na and converted that to 10m as prescribed in the norm. We are using a battery supplied Tektronix TDS-3034 oscilloscope with a battery supplied Hameg HM500E E-field active probe giving 50 mVeff for 1Veff/m with a 10cm long antenna. We placed them on a wooden chair of 60cm high. This makes it possible to measure free from contact with the outside world, often a cause of measurement errors. We have to watch the EMC-field of the scope screen itself, when the antenna comes close to the scope. We stay away more than 1m. We measured without the standard 30*30cm ground plane around the probe, so we can expect somewhat higher field values. We started with measuring the E-field already present, with no power on the Nikhef-Spark Chamber (in Volt per meter) The interference of switching supplies is already prominent, with bursts of 400ns with large pulsed 25MHz components at a distance of 3m from the Chamber. Interference from the environment (the room where the apparatus is) is about 60mVtop-top on the scope, meaning a E-field of 1.2Vtt/m= 0.4Veff/m during 0.2us (Photo2.1 and 2.2 )
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Photo2.1. Surrounding noise, medium size HF noise burst(25Mc)
,(~60mVtoptop=1.2Vtt/m=0.4Veff/m)
Photo 2.2 : Same signal measured with longer time base.
2.2 Trigger pickup Now, in this environment, to be sure we are looking at the interference caused by the Nikhef Spark Chamber, we made a scope-trigger with a magnetic pickup close to the device, only starting a scope acquisition when a spark occurs. The trigger is not disturbed by the all around electrical field because it is so close to the chamber and gets a strong chamber signal. (photo 2.3) We used this to snap the chamber interference signal during a spark.
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Photo 2.3 Magnetic pickup to trigger on a spark. Using this trigger, the E field antenna signal is measured at different distances.
Photo 2.4 : A Spark Pulse after triggering with the close-by pickup (trigger on ch2 not shown
on scope), channel1 is the E-field antenna 1m away from window, 200mVpp from the antenna means E-field= 4Vpp/m
We noticed the disturbance signal was not especially coming out of the window area , or the
side door, even though it was not completely closing on the special contact strips.
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Photo 2.5: Same as p5 , not much difference from pulse to pulse even at 3m away
the transmission in air reaches further with a ground plane, giving a non- linear reduction of field with distance.
Photo 2.6: At 6m distance 60cm above ground plane 100mVpp on scope =
2Vpp/m=0.7Veff/m@6m=0.5Veffmax/m@10m 3Mc component. during 0.4us
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Photo 2.7: Left from chamber: at 1m distance, no ground foil there, 200mVpp so
Efield=4Vpp/m
Fig. 2.8 Left from the chamber at 5m distance away from ground foil (side) 100mVpp so E=
2Vpp/m
2.3 Resulting Radio disturbance of the short chamber pulses To know the interference on radio we have to convert the oscilloscope pulses from the time
domain to the frequency domain (FFT). According to cispr16 the receiver works with a quasi peak detector with an intermediate frequency of 9KHz bandwidth upto 30MHz. Above that 120KHz (like for FM band) A standard EMC detector follows the peaks of the rf bursts, and has a slow diminishing time of 0.16 s. The EMC detector measures levels selectively for individually measured frequencies. The oscillos-cope adds all levels of all frequencies, but the levels can also be displayed for individual frequen-cies.(FFT) For 9KHz bandwidth it means the the pulse has to stay “on” for 100usec. This means for a 1usec pulse the signal can only rise to 1% of its full level . It would take a duration equal or longer than 100usec for full amplitude. (run-in effect)
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Also we look at one frequency component at a time, this means from photo 5 and 6 (1m and 3m away) the 1MHz component (1usec period time) is most pronounced but comes up and dies quickly. The level in a standard receiver (9khz BW) would be 1% of 4Vpp/m= 40mVpp/m=15mVeff/m =5mVeff/m @10m distance (lineair reduction with distance)
fig 2.9 a basic quasi peak detector (2)
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table 2.1 quasi peak detector time constants (2) , t-charge, t-discharge, t-meter
The demanded level for continuous disturbance is very low : 30uVeff/m, the mentioned 5mVeff/m @ 10m, of the pulse is too high. And indeed on a radio one hears a click at the time of the spark. In the norms there is also mention of a factor related to the occurrence frequency of the click. The factor of 10-20db however does not help enough inside the building reducing the level only from 5mVeff/m to 0.5mVeff/m. This is inside the building however. Outside the building the ticks cannot be noticed anymore, so for the outside world there will be no disturbance. Inside the building civil services can still work be-cause of the low occurrence rate of the ticks.
2.4 Conclusion of E-field measurements:
Strictly speaking we do not fulfill the disturbance requirements of the CE norm. For class A fixed installations the levels disturbance for radio reception and communication (telephone , ambulance) are acceptable, but too high for the norm
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3 Pacemakers of the public: We also have consider danger for people in the public carrying a pacemaker. (1) For a pace-
maker we have to convert the energy of the very short pulse to the amount of energy it would take if the pulse was able to use the full time a normal hart pulse takes to get exited (~0.1sec), see also fig-ure 3.1. In this case we could do with an amount 100 000 times smaller than the 1 us sec pulse we have now.
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fig 3.1: the form of a hart pulse
This means 5Vtt/m reduces to 50uVtt/m for a pacemaker wire of 0.5m this would give too lit-tle signal to excite the hart through the wires. For stimulation (4) a current of a few milliamp at 1kohm is needed. We can also compare this to the advise to keep a cell phone 15cm or more away from a pacemaker (not in the blouse pocket). A phone however produces large fields at this distance, 90V/m with long HF bursts repeating with 150 Hz (3 ms on, 3ms off) In the second generation pacemakers from around the 80’s the sense wire of the electronics was sensitive for interference, giving the effect of even halting the pulsing with 200 of 1000 tested devices. The present generation has build in filters on this sense wire so that the hardware hardly be disturbed with hf signals (highly immune). The Apple Ipod safety instructions warn not to place the Ipod within 6 inches of the pacemaker, in view of the used WIFI communication [ref 5] . Strong magnets may produce fields of about 20 gauss [ref 6] disturbing the electronics (shielding at very low frequencies is not working well) This all is not the case at the Nikhef Spark Chamber, the levels are much smaller and the distance bigger.
3.1 Pacemaker measurement Conclusion The measured fields of a few volts per meter at high frequencies of very short duration will not
affect the functioning of heart pacemakers.
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4 Current injection measurement in the Mains Cable:
Fig 4.4 The special absorbing clamp to measure the mains line disturbance. The cable is put in the
centre and then the box is closed.
We also checked the currents injected in external cabling to see if we introduce noise on the power line. This so called “conducted” interference can be measured with a special Ferrite clamp( fig 4.4) , and should be as low as 5uA or less (3) otherwise the cabling can start to radiate frequencies where the cable length is around half a wavelength or longer. The disturbance is measured per frequency component, with a bandwidth of 20kHz upto frequencies of 30MHz, and 400KHz above that. It can then be converted to standard bandwidths. With a clamp transformer factor of 1:1 and a 50 ohm load it means 50*5uAeff=250uVeff on the scope/analyzer With a bottom line of the analyzer at 0 uV , the components should stay below the 5th line from the bottom= 300uVeff (50 dBuV) Photo 4.1 comes over this level, but the used bandwidth is still 400 kHz. In Photo 4.2 the bandwidth is selected to be 20kHz,(9KHz not available) then all components up to 100Mc stay below 100uV=2uAeff. Photo 4.3 shows the nature of the interference at 23Mc is wideband, because the signal is present around 23 Mc over more than 5MHz over the whole screen. (Photo4.3, BW 400KHZ)
4.1 Cable disturbance Conclusion The interference level for 0-100Mc is acceptable, and stays just below the norms in cispr 16 (3) The new build in filters are a big improvement with respect to the previous same type of unshielded and badly filtered Spark chamber. Photo’s of the clamp measurements are shown below:
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Photo 4.1 2010-10-11-11:17 frequency spectrum of clamp current from 0-100Mc measured
without spark chamber, measured with 400kHz bandwidth
Photo 4.2 2010-10-11-11:20 dito with 20kHz bandwidth instead of 400 kHz =15dblower level
of the 23MHz bump, only there when the spark chamber is on.
Photo 4.3 2010-10-11- 11:21 The 23 Mc peak spectrum looked at with 0.5Mc /div scan bw
400khz, to check nature of signal, it appears to be wide band spread spectrum noise.
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5 Peak Searching measurement method with Efield antenna: A measurement is also done, scanning the frequency range from 0-100MHz for a long time
searching for peak levels. This is done with the aid of a laptop with a accompanying program to store all highest values at each frequency, that occurred during a run of about 10 minutes. This way we could subtract the background signal.
This technique also reduces the chance that a disturbance peak is missed because at the mo-ment of the disturbance the analyzer was scanning at another frequency area. Scanning for a long time enhances the chance to catch the disturbance at the cost of long measurement time (10 min). see Figure 5-3.
It appeared that switching on the spark chamber only added the already known 6 MHz wide bump at 23 MHz that stayed below the levels of the broadcast transmitters of for instance the FM band. It was too difficult to subtract the already present level from the actual with the chamber on. The le-vels of the background have to stay the same within a few percent,to be able to look at the low levels added by the Spark Chamber. A reasonable plot cannot be presented. Below the some photo’s of this test (Figure 5-1Figure 5-2Figure 5-3)
5.1 Conclusion for peak searching:
It is not possible to see the difference between only the background disturbance and the disturbance with the spark chamber switched on, the background noise is too high. See figures below:
Figure 5-1 Ffield measurement closely above ground plane (10mVeff=200mVeff/m)
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Figure 5-2 Spectrum with antenna close to, and well connected to the ground plane Apart from
a additional small bump at 23 Mc the frequency components of the environment stay the same
Figure 5-3 the E-field Spectrum from 0-100 MHz ( . Peaks between 90 and 100MC are FM
transmitters. Environment levels are below 60dBuV= 1mVeff = 20mVeff/m on the analyzer input (IF=400KHz). The added 23MHz bump has a level of 30uV=300uV/m , too high for a normal device but acceptable for a fixed installation in one’s own building. Civil services communication can still work in the building
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6 Overall Conclusion: We can draw the conclusions of our EMC measurements on the Nemo site. 1. The 220V mains Cable Interference level: is sufficiently below levels demanded. The dis-
turbance was measured with a clamp device up to 100MHz. 2. The Electro Magnetic Radiation: is too high for CE certification. If the device stands in a
big building, the disturbance for others, outside the building, can still be negligible . But the device cannot be used in an arbitrary environment. For this we have to certify for the dis-turbance produced by the equipment under test (EUT) in a free space area at a distance of 10m.
3. Pacemakers: the radiated field is reasonably low, and will not disturb pacemakers, this is also thanks to the very high immunity of present pacemakers. Pacemakers carriers are more sensitive for low frequency, strong magnetic fields. The spark chamber does not produce these kind of fields, so there is no danger.
Special thanks is for Henk Peek for his valuable advice and assistance with the measurements.
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7 Bibliography
1. EN 50527‐1 Assessment of human exposure at the workplace for persons bearing Active Implantable Medical Devices (AIMD) in electric magnetic and electromagnetic fields with frequencies from 0‐300GHz‐part 1: General. 2008. EN 50527‐1‐2008.
2. Goedbloed, Dr. J.J. EMC Analyse en Onderdrukking van Stoorproblemen. sl : Kluwer, 1997.
3. Cispr16 Specification for radio disturbance and imunity measuring apparatus and methods. sl : Cispr, 2004‐2011. CISPR 16.
4. R. M. Heethaar, R. van Poelgeest, F, L. Meijler , C. van der Zouw . Computer‐controlled stimulator for clinical cardiac study, dep of cardiol, hosp Utrecht. Medical & Biological Engineering & Computing. 1977, march 1977.
5. Mehul B. Patel, MD, Jay P. Thaker, Sujeeth Punnam, MD, Krit Jongnarangsin. Pacemaker interference with an iPod. 2007, From the Thoracic and Cardiovascular Institute, Michigan State University, Lansing, Michigan.
6. Morphy, Erika. Study: Headphone Magnets Mess With Pacemakers. [Online] 10 nov 2008.
7. T.Sluijk. EMC test report of the Nikhef Spark Chamber 13‐2‐2009. sl : Nikhef, 2009.
8. RICHTLIJN 2004/108/EG VAN HET EUROPEES PARLEMENT EN DE RAAD van 15 december 2004 betreffende de onderlinge aanpassing van de wetgevingen van de lidstaten inzake elektromagnetische compatibiliteit en tot intrekking van Richtlijn 89/336/EEG, Artikel 13 , Va. sl : European commision, 2004. 89/336/EEG.
9. Cispr 11: Industrial Scientific and Medical (ISM) radio frequency equipment‐ Electro magnetic disturbance characteristics limits and methods of measurement. sl : Cispr, 2002. CISPR 11.
10. CISPR22: Information Technology Equipment, Radio disturbance characteristics Limits and methods of measurement. sl : Cispr, 2003. CISPR 22.
11. T.Sluijk. EMC Improvement tests of the small Nikhef Spark chambers. sl : Nikhef, 2009.
