Re: pulse modulation in reverb chambers
Ken Javor wrote: My way of thinking about this problem and others like it is to reduce it to the simplest possible problem, and then look at the general case as a linear extrapolation. In this case I look at the interaction of the direct and single bounced ray during a site attenuation measurement on a perfect OATS, and then extrapolate from that to the reverberant chamber via linear superposition. I say this by way of an attempt to establish a common point of reference to enhance mutual understanding. I too have thought about an OATS as a simplification. I think here I prefer an unterminated transmission line. Supporting multiple reflections, it is (unlike an OATS and similarly to a chamber) a resonator as well as a reflector. If the time delay between arrival of direct and bounced ray is much shorter than the pulse duration, then there is interference but insignificant spread or smearing. If the delay is a sizable fraction of the pulse duration, the received pulse will appear longer than the transmitted pulse. This is multi-path interference. If the time delay is well in excess of the pulse duration (but shorter than the pulse repetition period), then there is no constructive/destructive interference effect, but a second pulse appears at the receive antenna. That is essentially my understanding; a really short pulse is received once undistorted. A longer pulse, however, could be lengthened, how much depending on pulse length, chamber size and Q. In the general case of a reverberant chamber pulse spreading would be contained if the difference between the shortest and longest paths of commensurate power density were equal to or less than 30 meters. One needs to insure there are few enough reflections to keep this true, no? My one-meter experiment suggested that around 50 MHz, absorption was quite low; 50 dB cancellation implies that less than that was absorbed in the chamber walls. What can one expect at 1 GHz? Skin depth being less, surface resistivity would increase losses and reflections would be attenuated more quickly. How long do reflections actually remain a problem? Wish I could repeat the experiment, but I don't have access to a chamber just now. Cheers, Cortland This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
My way of thinking about this problem and others like it is to reduce it to the simplest possible problem, and then look at the general case as a linear extrapolation. In this case I look at the interaction of the direct and single bounced ray during a site attenuation measurement on a perfect OATS, and then extrapolate from that to the reverberant chamber via linear superposition. I say this by way of an attempt to establish a common point of reference to enhance mutual understanding. If the time delay between arrival of direct and bounced ray is much shorter than the pulse duration, then there is interference but insignificant spread or smearing. If the delay is a sizable fraction of the pulse duration, the received pulse will appear longer than the transmitted pulse. This is multi-path interference. If the time delay is well in excess of the pulse duration (but shorter than the pulse repetition period), then there is no constructive/destructive interference effect, but a second pulse appears at the receive antenna. Given the speed of light at roughly 300 meters per microsecond, if the bounce path is say 30 meters or less longer than the direct ray, there should be insignificant spreading (30 meters would spread the pulse from 1 to 1.1 us). In the general case of a reverberant chamber pulse spreading would be contained if the difference between the shortest and longest paths of commensurate power density were equal to or less than 30 meters. My initial supposition was that the limitation on pulse length would be room size, since as room dimensions increase delays increase. It was pointed out by members of the list that there is a strong dependence on chamber Q since a high Q means less power loss and longer delays (higher order modes) can still have the same intensity as more direct paths. That is where my understanding lies at this point, and the test I described earlier is my only way to gauge the extent of the problem. From: Cortland Richmond 72146@compuserve.com Date: Tue, 29 Jul 2003 14:05:16 -0400 To: Ken Javor ken.ja...@emccompliance.com, ieee pstc list emc-p...@ieee.org Subject: Re: pulse modulation in reverb chambers Ken Javor wrote: My thinking is just the opposite. The duration of the pulse should be long relative to the time it takes to travel from transmit to receive antennas. Then there is no smearing It seems to me that you may be overlooking the effect of the reflected wave on a received pulse's shape. Consider the step function, which is certainly longer than the time to propagate across a chamber. Won't its leading edge be reflected, and subtract from the pulse? Won't subsequent reflections either (momentarily) increase or decrease it? And, given an imperfectly symmetrical chamber, won't there be a confusing multitude of such reflections? Of course, half the reflected energy incident on the antenna would be re-radiated, and so on for each reflection, diminishing their effect with time, but still, there's distortion not compensated for by a long pulse. An analogy might what happens with a two dimensional cavity -- an unterminated transmission line -- when we send a fast rise-time pulse down it. We are not, I think, precisely in disagreement; I do agree a short pulse will be smeared (after some time). It's just that I think reliance on a longer one won't insure that it remains undistorted. If rise and fall times were longer than the chamber transit time, then I'd expect no problem. I've not studied the chamber/pulse problem, mind. I did -- almost 20 years ago -- set up two bicons a meter apart in a 4 meter long reverberant chamber, and look at site attenuation. Nulls over 40 dB; high Q, indeed! See the May issue of the Transactions on Antennas and Propagation for a similar look (with regard to antenna impedance) at Statistical Properties of Linear Antenna Impedance in an Electrically Large Cavity.* That may have some relevance to your query. Ad astra per aspirin. Cortland *Statistical Properties of Linear Antenna Impedance in an Electrically Large Cavity, L.K. Warne, K.S. H. Lee, H.G. Hudson, W.A. Johnson, R.E. Jorgenson, and S.L Stronach. Volume 51 Number 5, page 978. This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
Ken Javor wrote: My thinking is just the opposite. The duration of the pulse should be long relative to the time it takes to travel from transmit to receive antennas. Then there is no smearing It seems to me that you may be overlooking the effect of the reflected wave on a received pulse's shape. Consider the step function, which is certainly longer than the time to propagate across a chamber. Won't its leading edge be reflected, and subtract from the pulse? Won't subsequent reflections either (momentarily) increase or decrease it? And, given an imperfectly symmetrical chamber, won't there be a confusing multitude of such reflections? Of course, half the reflected energy incident on the antenna would be re-radiated, and so on for each reflection, diminishing their effect with time, but still, there's distortion not compensated for by a long pulse. An analogy might what happens with a two dimensional cavity -- an unterminated transmission line -- when we send a fast rise-time pulse down it. We are not, I think, precisely in disagreement; I do agree a short pulse will be smeared (after some time). It's just that I think reliance on a longer one won't insure that it remains undistorted. If rise and fall times were longer than the chamber transit time, then I'd expect no problem. I've not studied the chamber/pulse problem, mind. I did -- almost 20 years ago -- set up two bicons a meter apart in a 4 meter long reverberant chamber, and look at site attenuation. Nulls over 40 dB; high Q, indeed! See the May issue of the Transactions on Antennas and Propagation for a similar look (with regard to antenna impedance) at Statistical Properties of Linear Antenna Impedance in an Electrically Large Cavity.* That may have some relevance to your query. Ad astra per aspirin. Cortland *Statistical Properties of Linear Antenna Impedance in an Electrically Large Cavity, L.K. Warne, K.S. H. Lee, H.G. Hudson, W.A. Johnson, R.E. Jorgenson, and S.L Stronach. Volume 51 Number 5, page 978. This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
The pulse is a modulation waveform. At say 1 GHz the signal source is gated on and off for a duration of 1 us at a 1 kHz rate. The pulse rise/fall-time is limited by the rate of change associated with the microwave frequency. I think understand what you are saying about the transmit antenna, that if you could switch it off the signal source then it wouldn't load the chamber Q. I don't think that is an issue in this case, but it is an interesting idea for a low duty cycle modulation like this. I really don't have a feel for what kind of load an unterminated or shorted horn would present to the field. Do you? From: drcuthb...@micron.com Reply-To: drcuthb...@micron.com Date: Tue, 29 Jul 2003 09:49:12 -0600 To: j...@jmwa.demon.co.uk, emc-p...@majordomo.ieee.org Subject: RE: pulse modulation in reverb chambers Ken, do you mean you pulse an antenna with a square wave at some specific repetition rate? If so, The pulse rise time and duration can be selected to contain the frequency components you want. I think that if the pulse remains on two long, the generator will act as a 50 ohm load and absorb energy from the chamber. Switching the generator from a 50 ohm state to a high-Z or low-Z state could be beneficial. Dave Cuthbert Micron Technology -Original Message- From: John Woodgate [mailto:j...@jmwa.demon.co.uk] Sent: Sunday, July 27, 2003 11:53 AM To: emc-p...@majordomo.ieee.org Subject: Re: pulse modulation in reverb chambers I read in !emc-pstc that Ken Javor ken.ja...@emccompliance.com wrote (in bb49768d.3717%ken.ja...@emccompliance.com) about 'pulse modulation in reverb chambers' on Sun, 27 Jul 2003: So if I am worried whether the 1 us pulse width can be sustained, and I don't know how to determine it analytically, my plan is as follows: I put a wire probe in the room, run it to a spectrum analyzer tuned to the transmit frequency, put the analyzer in zero span mode with a 1 or 3 MHz bandwidth and look at the modulation waveform. If it is significantly longer than 1 us, I know my constructive interference path delays are smearing the modulation away. I originally thought you wanted to establish a reverberant field with a 1 us pulse. Now I'm not so sure. By 'modulation', you mean the envelope of the 1 us pulse? Isn't the idea of a reverberation chamber that once it is excited the energy takes a long time to die away? That means that the original pulse will be s t r e t c h e d, dying away exponentially if the room is a good one. A too-short pule would almost certainly produce a delay that was not exponential. You can only 'sustain' the 1 us pulse if your receiver is in the 'direct field' of the antenna, i.e. the field strength due to the direct propagation path from sending antenna to receiving antenna is at least 3 dB above the combined field strength due to all indirect paths. You can easily see this if you consider just a few simple specular reflections, which appear at the receiver as a series of overlapping (since their path lengths are less than 300 m) 1 us pulses of varying level. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc
RE: pulse modulation in reverb chambers
Ken, do you mean you pulse an antenna with a square wave at some specific repetition rate? If so, The pulse rise time and duration can be selected to contain the frequency components you want. I think that if the pulse remains on two long, the generator will act as a 50 ohm load and absorb energy from the chamber. Switching the generator from a 50 ohm state to a high-Z or low-Z state could be beneficial. Dave Cuthbert Micron Technology From: John Woodgate [mailto:j...@jmwa.demon.co.uk] Sent: Sunday, July 27, 2003 11:53 AM To: emc-p...@majordomo.ieee.org Subject: Re: pulse modulation in reverb chambers I read in !emc-pstc that Ken Javor ken.ja...@emccompliance.com wrote (in bb49768d.3717%ken.ja...@emccompliance.com) about 'pulse modulation in reverb chambers' on Sun, 27 Jul 2003: So if I am worried whether the 1 us pulse width can be sustained, and I don't know how to determine it analytically, my plan is as follows: I put a wire probe in the room, run it to a spectrum analyzer tuned to the transmit frequency, put the analyzer in zero span mode with a 1 or 3 MHz bandwidth and look at the modulation waveform. If it is significantly longer than 1 us, I know my constructive interference path delays are smearing the modulation away. I originally thought you wanted to establish a reverberant field with a 1 us pulse. Now I'm not so sure. By 'modulation', you mean the envelope of the 1 us pulse? Isn't the idea of a reverberation chamber that once it is excited the energy takes a long time to die away? That means that the original pulse will be s t r e t c h e d, dying away exponentially if the room is a good one. A too-short pule would almost certainly produce a delay that was not exponential. You can only 'sustain' the 1 us pulse if your receiver is in the 'direct field' of the antenna, i.e. the field strength due to the direct propagation path from sending antenna to receiving antenna is at least 3 dB above the combined field strength due to all indirect paths. You can easily see this if you consider just a few simple specular reflections, which appear at the receiver as a series of overlapping (since their path lengths are less than 300 m) 1 us pulses of varying level. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
RE: pulse modulation in reverb chambers
Hi Ken, A lot of that work is I belive being done at NIST right here in Boulder under the direction of Dr R. Johnk. Some presentations he has made can be downloaded from our website at http://www.ieee.org/rmcemc under Archives. Search for Johnk. Best Regards Charles Grasso Senior Compliance Engineer Echostar Communications Corp. Tel: 303-706-5467 Fax: 303-799-6222 Cell: 303-204-2974 Email: charles.gra...@echostar.com; Email Alternate: chasgra...@ieee.org From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Sunday, July 27, 2003 3:35 PM To: Cortland Richmond; ieee pstc list Subject: Re: pulse modulation in reverb chambers That mostly makes sense, except I'm not sure about this part: I'd expect a pulse to excite many modes within a chamber as long as either its length or its transition times are shorter than the time it takes a wave to travel across the chamber and back. That'll smear the pulse. My thinking is just the opposite. The duration of the pulse should be long relative to the time it takes to travel from transmit to receive antennas. Then there is no smearing. I also recall work using network analyzers and time windowing to electronically create an anechoic chamber out of an unlined screen room. From: Cortland Richmond 72146@compuserve.com Date: Sun, 27 Jul 2003 16:44:20 -0400 To: Ken Javor ken.ja...@emccompliance.com, ieee pstc list emc-p...@ieee.org Subject: Re: pulse modulation in reverb chambers Ken, A recent article on reverberant chambers mentions a Q of 83,000 or so. I'd expect a pulse to excite many modes within a chamber as long as either its length or its transition times are shorter than the time it takes a wave to travel across the chamber and back. That'll smear the pulse. On the other hand the FIRST pulse received shouldn't be smeared unless the pulse is longer than the time it takes a reflected wave to arrive at the antenna. If I recall correctly, there've been some papers on time gating to take advantage of this. Cortland This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
I read in !emc-pstc that Ken Javor ken.ja...@emccompliance.com wrote (in bb49ae98.374a%ken.ja...@emccompliance.com) about 'pulse modulation in reverb chambers' on Sun, 27 Jul 2003: Sorry, but I don't understand the physics here. Could you please explain how a 10 us delay could add 0.01 us to a 1 us pulse? A typical pulse rep rate is 1 kHz. To me it seems that a 10 us delay would cause no interference effect at all, since the first pulse is over and another isn't due to arrive for another millisecond. What you said makes sense to me if you meant 10 ns, but that is not what your message said. Was that a typo? Yes, it was a typo, sorry. Speed of light 1 ns a foot. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
So if I am worried whether the 1 us pulse width can be sustained, and I don't know how to determine it analytically, my plan is as follows: I put a wire probe in the room, run it to a spectrum analyzer tuned to the transmit frequency, put the analyzer in zero span mode with a 1 or 3 MHz bandwidth and look at the modulation waveform. If it is significantly longer than 1 us, I know my constructive interference path delays are smearing the modulation away. Comments yea or nay? And thanks for all comments so far! From: John Woodgate j...@jmwa.demon.co.uk Reply-To: John Woodgate j...@jmwa.demon.co.uk Date: Sun, 27 Jul 2003 07:36:15 +0100 To: emc-p...@majordomo.ieee.org Subject: Re: pulse modulation in reverb chambers I read in !emc-pstc that Ken Javor ken.ja...@emccompliance.com wrote (in bb48596e.36a1%ken.ja...@emccompliance.com) about 'pulse modulation in reverb chambers' on Sat, 26 Jul 2003: What is the limitation on minimum pulse width in reverberation chambers? I expect it relates to room size, but does anyone have either a functional relation or a rough order of magnitude? Light travels 300 meters per microsecond, so I would think a 1 microsecond pulse width would work just fine, but nanosecond rise-times would be lost. Judging by what happens a million times slower in acoustics, I think 1 microsecond could be quite a bit short. Obviously it depends on the size of the chamber. If there is a paddle, it might be necessary to allow several turns of it to establish a cyclically stable field pattern. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
I read in !emc-pstc that Ken Javor ken.ja...@emccompliance.com wrote (in bb48596e.36a1%ken.ja...@emccompliance.com) about 'pulse modulation in reverb chambers' on Sat, 26 Jul 2003: What is the limitation on minimum pulse width in reverberation chambers? I expect it relates to room size, but does anyone have either a functional relation or a rough order of magnitude? Light travels 300 meters per microsecond, so I would think a 1 microsecond pulse width would work just fine, but nanosecond rise-times would be lost. Judging by what happens a million times slower in acoustics, I think 1 microsecond could be quite a bit short. Obviously it depends on the size of the chamber. If there is a paddle, it might be necessary to allow several turns of it to establish a cyclically stable field pattern. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
Sorry, but I don't understand the physics here. Could you please explain how a 10 us delay could add 0.01 us to a 1 us pulse? A typical pulse rep rate is 1 kHz. To me it seems that a 10 us delay would cause no interference effect at all, since the first pulse is over and another isn't due to arrive for another millisecond. What you said makes sense to me if you meant 10 ns, but that is not what your message said. Was that a typo? From: John Woodgate j...@jmwa.demon.co.uk Reply-To: John Woodgate j...@jmwa.demon.co.uk Date: Sun, 27 Jul 2003 21:20:07 +0100 To: emc-p...@majordomo.ieee.org Subject: Re: pulse modulation in reverb chambers I read in !emc-pstc that Ken Javor ken.ja...@emccompliance.com wrote (in bb4987fa.372a%ken.ja...@emccompliance.com) about 'pulse modulation in reverb chambers' on Sun, 27 Jul 2003: As long as those delays are much shorter than 1 us (path difference much less than 300 meters), the original modulation is received. Not really. If a reflection arrives with a delay of 10 us, the received pulse is 1.01 us long. Some 'rays' suffer multiple reflections, which increases the delay considerably, and the reflections in a reverberation chamber must be low-loss. How much stretching can you accept? This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
That mostly makes sense, except I'm not sure about this part: I'd expect a pulse to excite many modes within a chamber as long as either its length or its transition times are shorter than the time it takes a wave to travel across the chamber and back. That'll smear the pulse. My thinking is just the opposite. The duration of the pulse should be long relative to the time it takes to travel from transmit to receive antennas. Then there is no smearing. I also recall work using network analyzers and time windowing to electronically create an anechoic chamber out of an unlined screen room. From: Cortland Richmond 72146@compuserve.com Date: Sun, 27 Jul 2003 16:44:20 -0400 To: Ken Javor ken.ja...@emccompliance.com, ieee pstc list emc-p...@ieee.org Subject: Re: pulse modulation in reverb chambers Ken, A recent article on reverberant chambers mentions a Q of 83,000 or so. I'd expect a pulse to excite many modes within a chamber as long as either its length or its transition times are shorter than the time it takes a wave to travel across the chamber and back. That'll smear the pulse. On the other hand the FIRST pulse received shouldn't be smeared unless the pulse is longer than the time it takes a reflected wave to arrive at the antenna. If I recall correctly, there've been some papers on time gating to take advantage of this. Cortland This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
Ken, A recent article on reverberant chambers mentions a Q of 83,000 or so. I'd expect a pulse to excite many modes within a chamber as long as either its length or its transition times are shorter than the time it takes a wave to travel across the chamber and back. That'll smear the pulse. On the other hand the FIRST pulse received shouldn't be smeared unless the pulse is longer than the time it takes a reflected wave to arrive at the antenna. If I recall correctly, there've been some papers on time gating to take advantage of this. Cortland This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
I read in !emc-pstc that Ken Javor ken.ja...@emccompliance.com wrote (in bb4987fa.372a%ken.ja...@emccompliance.com) about 'pulse modulation in reverb chambers' on Sun, 27 Jul 2003: As long as those delays are much shorter than 1 us (path difference much less than 300 meters), the original modulation is received. Not really. If a reflection arrives with a delay of 10 us, the received pulse is 1.01 us long. Some 'rays' suffer multiple reflections, which increases the delay considerably, and the reflections in a reverberation chamber must be low-loss. How much stretching can you accept? But if delays are too long, then the pulse smears. Yes. How long is 'too long' for you? If you can find someone who has ray-tracing software for either electromagnetic **or acoustic** applications, you can run some simulations. Just remember if you use acoustic to treat seconds as microseconds. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
You were correct in your initial interpretation. I am wondering if a 1 us pulse can be established and not smeared over a longer period of time. The source antenna emits a coherent wave with this pulse modulation envelope, but many different rays taking different paths converge at the receive antenna at different arrival times. As long as those delays are much shorter than 1 us (path difference much less than 300 meters), the original modulation is received. But if delays are too long, then the pulse smears. What I was looking for was how to relate the smearing to room size. As Mr. DeWitt pointed out, the dependence is not on room size alone, but also on the Q of the room. Since I don't know how to analytically predict any of this, and I don't know the Q of the room, I was asking if the previously submitted check seemed reasonable. From: John Woodgate j...@jmwa.demon.co.uk Reply-To: John Woodgate j...@jmwa.demon.co.uk Date: Sun, 27 Jul 2003 18:52:33 +0100 To: emc-p...@majordomo.ieee.org Subject: Re: pulse modulation in reverb chambers I read in !emc-pstc that Ken Javor ken.ja...@emccompliance.com wrote (in bb49768d.3717%ken.ja...@emccompliance.com) about 'pulse modulation in reverb chambers' on Sun, 27 Jul 2003: So if I am worried whether the 1 us pulse width can be sustained, and I don't know how to determine it analytically, my plan is as follows: I put a wire probe in the room, run it to a spectrum analyzer tuned to the transmit frequency, put the analyzer in zero span mode with a 1 or 3 MHz bandwidth and look at the modulation waveform. If it is significantly longer than 1 us, I know my constructive interference path delays are smearing the modulation away. I originally thought you wanted to establish a reverberant field with a 1 us pulse. Now I'm not so sure. By 'modulation', you mean the envelope of the 1 us pulse? Isn't the idea of a reverberation chamber that once it is excited the energy takes a long time to die away? That means that the original pulse will be s t r e t c h e d, dying away exponentially if the room is a good one. A too-short pule would almost certainly produce a delay that was not exponential. You can only 'sustain' the 1 us pulse if your receiver is in the 'direct field' of the antenna, i.e. the field strength due to the direct propagation path from sending antenna to receiving antenna is at least 3 dB above the combined field strength due to all indirect paths. You can easily see this if you consider just a few simple specular reflections, which appear at the receiver as a series of overlapping (since their path lengths are less than 300 m) 1 us pulses of varying level. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: pulse modulation in reverb chambers
I read in !emc-pstc that Ken Javor ken.ja...@emccompliance.com wrote (in bb49768d.3717%ken.ja...@emccompliance.com) about 'pulse modulation in reverb chambers' on Sun, 27 Jul 2003: So if I am worried whether the 1 us pulse width can be sustained, and I don't know how to determine it analytically, my plan is as follows: I put a wire probe in the room, run it to a spectrum analyzer tuned to the transmit frequency, put the analyzer in zero span mode with a 1 or 3 MHz bandwidth and look at the modulation waveform. If it is significantly longer than 1 us, I know my constructive interference path delays are smearing the modulation away. I originally thought you wanted to establish a reverberant field with a 1 us pulse. Now I'm not so sure. By 'modulation', you mean the envelope of the 1 us pulse? Isn't the idea of a reverberation chamber that once it is excited the energy takes a long time to die away? That means that the original pulse will be s t r e t c h e d, dying away exponentially if the room is a good one. A too-short pule would almost certainly produce a delay that was not exponential. You can only 'sustain' the 1 us pulse if your receiver is in the 'direct field' of the antenna, i.e. the field strength due to the direct propagation path from sending antenna to receiving antenna is at least 3 dB above the combined field strength due to all indirect paths. You can easily see this if you consider just a few simple specular reflections, which appear at the receiver as a series of overlapping (since their path lengths are less than 300 m) 1 us pulses of varying level. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
pulse modulation in reverb chambers
List members, What is the limitation on minimum pulse width in reverberation chambers? I expect it relates to room size, but does anyone have either a functional relation or a rough order of magnitude? Light travels 300 meters per microsecond, so I would think a 1 microsecond pulse width would work just fine, but nanosecond rise-times would be lost. Thanks in advance for any inputs. Ken Javor This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
