Filtering options for reflected signal with noise
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From: John121 on 15 Apr 2008 10:39 Hi, My groups project involves detecting the presence of a train. Our idea is: - send a square wave (one pulse) with a highly skewed duty cycle down the railway track - if a train is present it will create a short circuit which our pulse would reflect off - we would then measure the 'echo' time difference between our sent pulse and the received reflection A sub-system which needs to be looked at is filtering the reflection signal we receive. There will be a lot of noise which is imparted onto the signal from several areas such as the traction return current (probably going to be 50Hz noise in its nicest form) and noise from the trains motors (rectification process). I was wondering what the best options would be, in terms of filtering. As a square wave is made up of a fundamental frequency and then the odd harmonics thereafter, how can I ensure the filter outputs a signal which still represents a square wave? I have to give several suggestions which could be used. I was thinking I would look into passive filters as probably the worst solution (due to component drift etc....), with active filters using op-amps etc being another. But the main solution would be to use dsp, however I have very little knowledge on this subject and therefore I do not know what sort of filter I would be looking for (eg: FIR, IIR) Any input into this would be hugely helpful. Thank you for any help
From: Jerry Avins on 15 Apr 2008 11:44 John121 wrote: > Hi, > > My groups project involves detecting the presence of a train. Our idea > is: > - send a square wave (one pulse) with a highly skewed duty cycle down the > railway track You need to be clearer. "One pulse" doesn't fit with "duty cycle".\ > - if a train is present it will create a short circuit which our pulse > would reflect off > - we would then measure the 'echo' time difference between our sent pulse > and the received reflection Will it? Are the rails insulated one from the other? What is the characteristic impedance and propagation velocity of the rail pair considered as a two-wire transmission line? Given the spacing, what is the lower usable frequency in that mode? > A sub-system which needs to be looked at is filtering the reflection > signal we receive. There will be a lot of noise which is imparted onto the > signal from several areas such > as the traction return current (probably going to be 50Hz noise in its > nicest form) and noise from the trains motors (rectification process). Why does the reflection need to be filtered? What part of it needs to be removed? > I was wondering what the best options would be, in terms of filtering. > > As a square wave is made up of a fundamental frequency and then the odd > harmonics thereafter, how can I ensure the filter outputs a signal which > still represents a square wave? So, which is it: a square wave, or a single pulse? > I have to give several suggestions which could be used. I was thinking I > would look into passive filters as probably the worst solution (due to > component drift etc....), > with active filters using op-amps etc being another. You haven't said what the filter is for, so suggestions must be vague. Component drift is only a problem if the cut-off frequency must be precise. I don't understand why that should be. > But the main solution would be to use dsp, however I have very little > knowledge on this subject and therefore I do not know what sort of filter I > would be looking for (eg: FIR, IIR) > > Any input into this would be hugely helpful. > > Thank you for any help Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Noway2 on 15 Apr 2008 16:39 On Apr 15, 11:44 am, Jerry Avins <j...(a)ieee.org> wrote: > John121 wrote: > > Hi, > > > My groups project involves detecting the presence of a train. Our idea > > is: > > - send a square wave (one pulse) with a highly skewed duty cycle down the > > railway track > > You need to be clearer. "One pulse" doesn't fit with "duty cycle".\ > > > - if a train is present it will create a short circuit which our pulse > > would reflect off > > - we would then measure the 'echo' time difference between our sent pulse > > and the received reflection > > Will it? Are the rails insulated one from the other? What is the > characteristic impedance and propagation velocity of the rail pair > considered as a two-wire transmission line? Given the spacing, what is > the lower usable frequency in that mode? > > > A sub-system which needs to be looked at is filtering the reflection > > signal we receive. There will be a lot of noise which is imparted onto the > > signal from several areas such > > as the traction return current (probably going to be 50Hz noise in its > > nicest form) and noise from the trains motors (rectification process). > > Why does the reflection need to be filtered? What part of it needs to be > removed? > > > I was wondering what the best options would be, in terms of filtering. > > > As a square wave is made up of a fundamental frequency and then the odd > > harmonics thereafter, how can I ensure the filter outputs a signal which > > still represents a square wave? > > So, which is it: a square wave, or a single pulse? > > > I have to give several suggestions which could be used. I was thinking I > > would look into passive filters as probably the worst solution (due to > > component drift etc....), > > with active filters using op-amps etc being another. > > You haven't said what the filter is for, so suggestions must be vague. > Component drift is only a problem if the cut-off frequency must be > precise. I don't understand why that should be. > > > But the main solution would be to use dsp, however I have very little > > knowledge on this subject and therefore I do not know what sort of filter I > > would be looking for (eg: FIR, IIR) > > > Any input into this would be hugely helpful. > > > Thank you for any help > > Jerry > -- > Engineering is the art of making what you want from things you can get. > ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Jerry, FYI - this guy already admitted on eng-tips.com that this was a student post. I am guessing it will get red flagged and removed PDQ.
From: Jerry Avins on 16 Apr 2008 11:13 Noway2 wrote: ... > Jerry, > > FYI - this guy already admitted on eng-tips.com that this was a > student post. I am guessing it will get red flagged and removed PDQ. Hi. I assume you mean red flagged and removed from eng-tips.com. As far as I know, that doesn't happen at comp.dsp. I don't mind talking to students. Some of the regulars here were students once. Not doing someone's homework isn't the same as encouraging him to think more clearly. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Philip Martel on 4 May 2008 10:01
"Steve Underwood" <steveu(a)dis.org> wrote in message news:fvkcjl$djv$1(a)nnews.pacific.net.hk... > Jerry Avins wrote: >>>> Eric Jacobsen >>> >>> We had another "toy" in the playground, that we called a >>> "merry-go-round". You know, that huge circular platform on a central > > I thought Americans called those carousels. We called them merry go rounds > in England. > Both term are used in the US, at least in New England. I think merry go round is more common. >>> pillar of rotation. A half dozen kids would sit on the platform and >>> then other kids would start spinning the platform around just as damned >>> fast as they could. depending on the speed, there was certainy >>> screaming, and sometimes crying from the littler kids. >>> If a passenger lost their grip, they flew off the spinning paltform into >>> the dirt. >> >> I see those occasionally. The last one I saw had a friction governor to >> limit the speed. > > That's not to limit the speed. Its to limit the fun. That way the kids > don't want to play on the thing. Then the thing can be shown to be hardly > used. Then it can be removed. Then all safety issues are solved. Such are > the twisted ways of the modern world. > > Regards, > Steve |