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: [email protected] > Reply-To: [email protected] > Date: Tue, 29 Jul 2003 09:49:12 -0600 > To: <[email protected]>, <[email protected]> > 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:[email protected]] > Sent: Sunday, July 27, 2003 11:53 AM > To: [email protected] > Subject: Re: pulse modulation in reverb chambers > > > > I read in !emc-pstc that Ken Javor <[email protected]> wrote > (in <bb49768d.3717%[email protected]>) 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: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > 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: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > 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: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] 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
