Re: Shield Room Grounding
Bob: Barry is trying to say that a dummy power load is only resistive at the power frequency, and may not truly represent the EUT at all of the other frequencies within the test range. He's right. Bob suggests a powerline filter be added to my El Cheapo lightbulb load bank to more closely approximate an EUT. He's right, too. My bank of lightbulbs is versatile, but it IS spread out over a few square feet. There are stray lead inductances and capacitances, and the lamp filaments are also inductors. At a low frequency starting point of 10KHz (or 30Hz for me), this is negligible. But yes, as we go up to 10MHz or 30MHz, these undefined and uncontrolled variables will start causing some weird effects. A better load bank could be constructed of non-inductive bulk silicon carbide resistors, but these are expensive ($150 each) and not as versatile. (OTOH, I have NEVER had one burn out during a test!) Practically, I assume that the load is mostly resistive, having had the experience of using ordinary light bulbs on the end of a coax cable for loading amateur transmitters in the HF and VHF regions. Lamp filaments do a good job of converting RF to heat and light, so I trust that they retain a considerable resistive component of the impedance at any of these frequencies! But, I don't think we need to know what the impedance curve of my loadbank is, because there is no standard to compare it against. What we are groping toward here is that there seems to be a need to define a standard dummy power load. This load would have an impedance similar to the typical EUT. Placing this dummy load into the test setup would then allow test conductors to verify the quality of the supplied power and the sensitivity of the measurement equipment. So, if there is a need to have a defined dummy load impedance, then we'll have to start with a survey of typical EUT's to find the value and variation of the impedance. Maybe we'll need several categories of dummy loads (appliance, computer, entertainment)? But, as far as I'm concerned, I still have a few cases of light bulbs to use up first! ;-) Ed From: Robert Macy m...@california.com Subject: Re: Shield Room Grounding Date: Fri, 9 Oct 1998 16:27:11 -0700 (PDT) To: b...@namg.us.anritsu.com, Bailin Ma@unspecified-domain Cc: emc-p...@ieee.org Lightbulbs make great AC loads, but use a line filter in series with them and you'll probably duplicate most EUT's since they'll be using filters. - Robert - On Fri, 9 Oct 1998 b...@namg.us.anritsu.com wrote: Hi Ed, I appreciate your kindness to share your experience with us about constructing equivalent load: I constructed a load bank consisting of 16 surface mount light bulb sockets, all wired in parallel. I just screw in an array of 25/60/75/100 Watt rated lamps until I get the necessary current. Sure, there's some unknown slight lead inductance and capacitance. But all I want to do is draw a few amps DC I have two questions: (1)What is the impedance of your bulb array at 30 MHz? i.e., Zb=? @30 MHz. (2)What is the impedance of EUT at 30 MHz? i.e., Ze=? @30 MHz (DETAILS SNIPPED) Suggestion: We might need to check the equivalence of Zb and Ze @30 MHz by using an Impedance Analyzer, e. g., HP4191A(?). Thank you. Please correct me. Best Regards, Barry Ma -- Ed Price ed.pr...@cubic.com Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: emc-pstc@listserv.ieee.org Date: 10/12/1998 Time: 10:47:22 -- - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
Re: Shield Room Grounding
From: Scott Roleson sc...@hpsdde.sdd.hp.com Subject: Re: Shield Room Grounding Date: Thu, 8 Oct 1998 14:17:56 -0700 To: emc-p...@ieee.org Cc: sc...@hpsdde.sdd.hp.com Barry Ma forwarded to me a message from Peter Hays that said: Can someone tell me what is the best method to find out and ensure that a screen room is adequately grounded? (SNIP) There is some debate on this single-point ground approach. I know some people who think it doesn't matter, so long as the room doesn't have any gaps so that ALL potentially interfering currents stay on the outside surface of the room. This may be true, but in practice it's not always possible to have a room without any holes or gaps. -- Scott Roleson Scott: Grounding for safety is a must, since a shielded enclosure will almost always have a set of low-pass powerline filters which bring the 60Hz power into the equipment within the room. These filters usually have a few large (15uF or so) capacitors from line to filter case to bypass RF currents. Since the capacitors also have a modest Xc at 60 Hz, there will also be a 60 Hz current component to the filter case. If the room isn't grounded, a hazardous voltage potential can exist on the room wall. A person standing on the concrete floor of the parent structure can get a very serious shock just by reaching out to open the door handle or to connect a coax cable to a port. So, for almost every situation, we have to ground the room to protect the people around it. But grounding for RF shielding effectiveness isn't needed. A copper spherical Faraday cage floating in mid air (what a sight!) would make an fine RF shield. The shielding would be limited primarily by the gaps, intentional seams and accidental cracks and gaps. But... it's not very useful. Let's land that baby and now think of it as a shielded room. To be of any use, this room has to be big, say 10 feet tall. And maybe 30 feet wide. Now, let's get very simplistic. To an RF wave, propagating along happily in air, your room looks like a little old antenna. I mean, it's conductive, and it has a height above ground. An effective height. Right, it looks like a stubby, broadband vertical monopole above a ground plane. And that RF wave gives that antenna a present; it induces some RF current into the conductive structure. Now, that RF current would like to flow, along the outer surface of the conductor (skin depth effect), somewhere. By providing a single, well defined ground path, you prevent that current from flowing along paths which would create problems. What problems? Well, imagine the current flowing to ground through the outer jacket of a coax cable connected to a grounded spectrum analyzer. The noise currents would sum with the valid RF currents on the analyzer coax. Now, with all that said, let me tell you a story. At both General Dynamics and Cubic in San Diego, I needed a large shielded room (for the EUT) and a smaller shielded room or antechamber (for the program support equipment. (Support equipment is notorious for being built just barely able to work, with a rat's nest construction and no thought to EMC.) At GD, I had a solid, welded main room and a modular, 8' cube antechamber. At Cubic, I have a modular main room and a modular, screen antechamber. Both locations used the same technique to join the large and small rooms; a penetration port was located in each facing wall, and a steel tunnel was fabricated to bolt onto each room's penetration port bolt pattern. In effect, the shielded volume turned into a dumbbell shape, with the tunnel at the waist of the dumbbell. Then, a penetration port cover plate, equipped with multiple signal line filters, is bolted across the tunnel throat at one end. (This isolates the two test chambers from each other.) Each shielded room has it's own set of powerline filters (400Hz three phase, 60Hz three phase and two DC lines). The modular panel (on each room) that carries the filter sets has it's own ground well. The two powerline filter sets were over fifty feet apart in both examples. I can't remember when anybody would have recommended a design like this. I know I wouldn't. But the GD example grew from merging existing facilities, and it worked. It worked good enough that we NEVER had a trace of any ambient signals in the many emission tests that were performed there over about 15 years (using Eaton receivers and HP spectrum analyzers, often with active antennas). It was good enough to do TEMPEST testing at the facility. It worked good enough that I decided to deliberately emulate the design here at Cubic. And it's working again, good enough that my fancy HP-8571A Receiver (a re-worked 8566B), even operating with external pre-amps in some bands, doesn't see any ambient distractions. So what's the lesson in all of this? Well, I was scrupulous about seam quality and using very good powerline
Re: Shield Room Grounding
Hi Ed, I appreciate your kindness to share your experience with us about constructing equivalent load: I constructed a load bank consisting of 16 surface mount light bulb sockets, all wired in parallel. I just screw in an array of 25/60/75/100 Watt rated lamps until I get the necessary current. Sure, there's some unknown slight lead inductance and capacitance. But all I want to do is draw a few amps DC I have two questions: (1)What is the impedance of your bulb array at 30 MHz? i.e., Zb=? @30 MHz. (2)What is the impedance of EUT at 30 MHz? i.e., Ze=? @30 MHz If we are not sure Zb=Ze @30 MHz, I am afraid, it's hard to say the spectrum analyzer would receive the same RF emission at 30 MHz from noise sources other than EUT, although the bulb array draws the same current at 60 Hz as EUT does. In other words, Zb=Ze @60 Hz is one thing, and Zb=Ze @30 MHz would be another. Let's see an example, assuming Ze=Re+jXe, where Xe=Omega*Le, and Omega=2*Pi*F. Re=20 Ohm, Xe=0.1 Ohm @60 Hz, Ze=20+j*0.1=20 Ohm Be=20 Ohm, Xe=5 Ohm @30 MHz,Ze=j*5 Ohm Conclusion: As far as the equivalent load is concerned, we can only pay attention to the equivalence of resistance part of Zb and Ze @60 Hz. At 30 MHz, however, we should pay more attention to the equivalence of reactance part of Ze and Zb instead. Suggestion: We might need to check the equivalence of Zb and Ze @30 MHz by using an Impedance Analyzer, e. g., HP4191A(?). Thank you. Please correct me. Best Regards, Barry Ma - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
RE: Shield Room Grounding
Hi Barry et al, I'm I missing something here? Is this exercise worth the trouble? If one doesn't have a clear margin to any commercial limit line with any kind of resistive dummy load attached to your LISN shouldn't you examine the your system setup? Accurate determination of your measurement system noise floor with a known impedance through the frequency range is really only of academic use in this case, isn't it? This should have no bearing on pass/fail of an EUT as the measurement system noise floor and the limit line should be well separated for conducted emissions in a shielded room. Regards, Kevin Harris -Original Message- From: b...@namg.us.anritsu.com [SMTP:b...@namg.us.anritsu.com] Sent: Friday, October 09, 1998 1:33 PM To: emc-p...@ieee.org Subject: Re: Shield Room Grounding Hi Ed, I appreciate your kindness to share your experience with us about constructing equivalent load: I constructed a load bank consisting of 16 surface mount light bulb sockets, all wired in parallel. I just screw in an array of 25/60/75/100 Watt rated lamps until I get the necessary current. Sure, there's some unknown slight lead inductance and capacitance. But all I want to do is draw a few amps DC I have two questions: (1)What is the impedance of your bulb array at 30 MHz? i.e., Zb=? @30 MHz. (2)What is the impedance of EUT at 30 MHz? i.e., Ze=? @30 MHz If we are not sure Zb=Ze @30 MHz, I am afraid, it's hard to say the spectrum analyzer would receive the same RF emission at 30 MHz from noise sources other than EUT, although the bulb array draws the same current at 60 Hz as EUT does. In other words, Zb=Ze @60 Hz is one thing, and Zb=Ze @30 MHz would be another. Let's see an example, assuming Ze=Re+jXe, where Xe=Omega*Le, and Omega=2*Pi*F. Re=20 Ohm, Xe=0.1 Ohm @60 Hz, Ze=20+j*0.1=20 Ohm Be=20 Ohm, Xe=5 Ohm @30 MHz,Ze=j*5 Ohm Conclusion: As far as the equivalent load is concerned, we can only pay attention to the equivalence of resistance part of Zb and Ze @60 Hz. At 30 MHz, however, we should pay more attention to the equivalence of reactance part of Ze and Zb instead. Suggestion: We might need to check the equivalence of Zb and Ze @30 MHz by using an Impedance Analyzer, e. g., HP4191A(?). Thank you. Please correct me. Best Regards, Barry Ma - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
