On Apr 27, 5:29�pm, "John121" <adriane...@hotmail.com> wrote:> >I am not aware of any reason to consider the frequency rather than the > >pulse width and edge rate. �There are both radars and sonars which > >send single single pulses and watch for the return. �The rep rate is > >very low and therefore the frequency. �But the pulses are short with a > >fast rise time. �This allows the round trip time to be measured very > >accurately. > > Ok. This is what the thinking behind not being able to send a pulse down > the line is: > > We want to send out a single pulse and wait for the reflection. The time > taken between sent pulse and returned relection is recorded and with the > known velocity of the pulse the distance to the S/C or O/C can be worked > out. > > Now we want to have sections roughly 1/2 mile long, therefore the signal > would have to at most travel 1 mile. > > Here are some figures of velocity of travel and attenuation with > frequency: > > 1kHz: attenuation constant: 0.0011 velocity: 3% c > 10kHz: attenuation constant: 0.0037 velocity: 7% c > 100kHz: attenuation constant: 0.0104 velocity: 18% c > > I'm slightly confused because when I say pulse I mean just 1 period of a > sqaure wave. And a square wave is made up of a fundamental freq and > infinite odd harmonics. If we were to send a 1kHz sqaure wave (1 period) at > 20V then it could only get 1 mile before being less than 0.5V. The > wavelength of this would also be around 9000m therefore a full period would > not even occur along a 1 mile track. > > A 100kHz, 20V sqaure wave would attenaute below 0.5V after only 200m which > makes it useless in terms of this idea. > > Now am I completely wrong in my thinking of a pulse? > > All we really need is a spike in voltage for a very short time which would > propagate down the line and then return some reflection. > > Is a pulse with a very small width and fast rise time not made up of very > high frequencies? This is what is confusing me?I'm going to jump in here a bit.. there is some confusion. there are 2 methods being discussed.. 1) measure the impedance (not the characteristic impedance) looking into the shorted line at some frequency. This can tell you the length of the line and hence the position of the train. You need to measure the STEADY STATE impedance looking into the line in this case... You can use 1 kHz if you wish to make this measurement 2) measure the round trip time delay of a pulse. this will also tell you rhe distance and location but is different from method 1. I think your first step is to decide which of these 2 methods you want to use.. Mark
Filtering options for reflected signal with noise
Started by ●April 15, 2008
Reply by ●April 27, 20082008-04-27
Reply by ●April 27, 20082008-04-27
>I'm going to jump in here a bit.. >there is some confusion. >there are 2 methods being discussed.. > >1) measure the impedance (not the characteristic impedance) looking >into the shorted line at some frequency. This can tell you the length >of the line and hence the position of the train. You need to measure >the STEADY STATE impedance looking into the line in this case... You >can use 1 kHz if you wish to make this measurement > > >2) measure the round trip time delay of a pulse. this will also tell >you rhe distance and location but is different from method 1. > >I think your first step is to decide which of these 2 methods you want >to use.. > >MarkSorry about the confusion. Yes there are 2 techniques being discussed here. I would prefer to do the time delay method, however as described above I didn't think it would be possible due to the need for a very small pulse (and to my undestanding this would be made up of very high frequencies, which are attenuated a huge amount and therefore cant be used). If I'm wrong please explain it to me. If this is correct then the other method would be the way I am going to proceed.
Reply by ●April 27, 20082008-04-27
John121 wrote:>> I am not aware of any reason to consider the frequency rather than the >> pulse width and edge rate. There are both radars and sonars which >> send single single pulses and watch for the return. The rep rate is >> very low and therefore the frequency. But the pulses are short with a >> fast rise time. This allows the round trip time to be measured very >> accurately. > > Ok. This is what the thinking behind not being able to send a pulse down > the line is: > > We want to send out a single pulse and wait for the reflection. The time > taken between sent pulse and returned relection is recorded and with the > known velocity of the pulse the distance to the S/C or O/C can be worked > out. > > Now we want to have sections roughly 1/2 mile long, therefore the signal > would have to at most travel 1 mile. > > Here are some figures of velocity of travel and attenuation with > frequency: > > 1kHz: attenuation constant: 0.0011 velocity: 3% c > 10kHz: attenuation constant: 0.0037 velocity: 7% c > 100kHz: attenuation constant: 0.0104 velocity: 18% cThese are much lower velocities than I had guessed.> I'm slightly confused because when I say pulse I mean just 1 period of a > sqaure wave. And a square wave is made up of a fundamental freq and > infinite odd harmonics.The fundamental frequency is the repetition rate of the pulse train. The spectrum of a pulse depends on the steepness of the edges. The width of the pulse determines whether the particular components of the leading and trailing edges reinforce or cancel. The velocities you posted above imply large dispersion, so you can't expect a broadband pulse to maintain its shape over much distance.> If we were to send a 1kHz sqaure wave (1 period) at > 20V then it could only get 1 mile before being less than 0.5V. The > wavelength of this would also be around 9000m therefore a full period would > not even occur along a 1 mile track. > > A 100kHz, 20V sqaure wave would attenauate below 0.5V after only 200m which > makes it useless in terms of this idea. > > Now am I completely wrong in my thinking of a pulse?When characterizing its "frequency", yes.> All we really need is a spike in voltage for a very short time which would > propagate down the line and then return some reflection.That spike is a very narrow pulse which therefore has a very broad spectrum.> Is a pulse with a very small width and fast rise time not made up of very > high frequencies? This is what is confusing me?Every waveform can be decomposed to a superposition harmonics. As the edges become steeper, higher harmonics need to be included in the summation to turn the corners faster. As the pulse becomes narrower, lower frequencies diminish in importance. Frequencies whose period are much less than a single pulse's width become irrelevant. Look at the principles of Fourier analysis. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 29, 20082008-04-29
Hi, So we are going to measure the steady state input impedance seen. Here is a sketch of how we intend to do this: http://img241.imageshack.us/my.php?image=conceptuo6.jpg Now I have a few questions regarding exactly what would be the best way to connect to the rails and the implications on the filtering. If we were to use the bottom rail as our 'gnd' would this not be unsuitable as there is going to be noise on this rail. Should we therefore use our own ground and insert the filtering between the measuring circuitry. Here is another picture of what I mean: http://img236.imageshack.us/my.php?image=concept2ye4.jpg Is this correct? This set up should remove the noise from the line and allow us to measure the input impedance seen (which in turn can be used to calculate distance of train)
Reply by ●April 29, 20082008-04-29
John121 wrote:> Hi, > So we are going to measure the steady state input impedance seen. Here is > a sketch of how we intend to do this: > http://img241.imageshack.us/my.php?image=conceptuo6.jpg > > Now I have a few questions regarding exactly what would be the best way to > connect to the rails and the implications on the filtering. > > If we were to use the bottom rail as our 'gnd' would this not be > unsuitable as there is going to be noise on this rail.There will be noise on all the rails. To measure impedance, you need to connect to both.> Should we therefore use our own ground and insert the filtering between > the measuring circuitry. Here is another picture of what I mean: > http://img236.imageshack.us/my.php?image=concept2ye4.jpg"Circuit" is a synonym for "loop". Where is the circuit in concept2ye4.jpg? If you try to show the path (closed path; circuit; loop) of the current, you will find none.> Is this correct? This set up should remove the noise from the line and > allow us to measure the input impedance seen (which in turn can be used to > calculate distance of train)You don't need DC for your measurement. Connect to the rails with a transformer. A step-down transformer -- one with few turns on the rail winding and many turns on the instrument winding -- will match what I believe is very low impedance of the rails to more reasonable impedance for instrumentation. Each rail probably has fairly large "noise" to ground. It is reasonable to hope that is is nearly the same for each rail, so that the noise from rail to rail will be lower. (This is called a differential measurement.) Is there a rail yard where you can take some preliminary measurements? An initial idea of the rail characteristics will help guide your plans to fruitful approaches. For instance, if you were to use a car battery, jumper cables, and a headlamp, I think you will find the lamp lit to full brightness even with the rails in the loop. If my guess is correct, connecting a 12-volt battery directly from rail to rail would result in a current of a few hundred amps and a terminal voltage at the battery of about a volt or two. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 29, 20082008-04-29
>You don't need DC for your measurement. Connect to the rails with a >transformer. A step-down transformer -- one with few turns on the rail >winding and many turns on the instrument winding -- will match what I >believe is very low impedance of the rails to more reasonable impedance >for instrumentation. Each rail probably has fairly large "noise" to >ground. It is reasonable to hope that is is nearly the same for each >rail, so that the noise from rail to rail will be lower. (This is called>a differential measurement.)If we assume the noise on both the rails is going to be equal then doesn't that mean there is no need to filter the noise. If we assume the noise is going to be different then the noise will have to be filtered from both the rails, correct?
Reply by ●April 29, 20082008-04-29
John121 wrote:>> You don't need DC for your measurement. Connect to the rails with a >> transformer. A step-down transformer -- one with few turns on the rail >> winding and many turns on the instrument winding -- will match what I >> believe is very low impedance of the rails to more reasonable impedance >> for instrumentation. Each rail probably has fairly large "noise" to >> ground. It is reasonable to hope that is is nearly the same for each >> rail, so that the noise from rail to rail will be lower. (This is called > >> a differential measurement.) > > If we assume the noise on both the rails is going to be equal then doesn't > that mean there is no need to filter the noise. > > If we assume the noise is going to be different then the noise will have > to be filtered from both the rails, correct?If the noise -- or any other signal -- is identical on both rails, then it will not excite the transformer or interfere with the differential signal. Search for "common mode" and "differential mode". If you have access to a wire-line telephone, check the signal from either wire to ground. It is likely that you will hear a great deal of power-line hum. The signal from wire to wire, though, is usually quite clean. (If not, notify the phone company.) Use a transformer to read the differential-mode signal and reject the common mode. A filter will help with the residual noise. Get ahold of a pair of headphones with good low-frequency response and preferably the kind with soft cups to block outside noise. Bring them, a hammer, some long clip leads, and an iron rod to a railroad siding. Drive the rod into the ground (not just the stone ballast) and connect the headphones between the rod and a rail. You will probably hear noise. Then connect the headphones from rail to rail. You will probably hear much less. But don't accept my guesses. Try it and know. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 29, 20082008-04-29
On Apr 27, 6:00 pm, "John121" <adriane...@hotmail.com> wrote:> >I'm going to jump in here a bit.. > >there is some confusion. > >there are 2 methods being discussed.. > > >1) measure the impedance (not the characteristic impedance) looking > >into the shorted line at some frequency. This can tell you the length > >of the line and hence the position of the train. You need to measure > >the STEADY STATE impedance looking into the line in this case... You > >can use 1 kHz if you wish to make this measurement > > >2) measure the round trip time delay of a pulse. this will also tell > >you rhe distance and location but is different from method 1. > > >I think your first step is to decide which of these 2 methods you want > >to use.. > > >Mark > > Sorry about the confusion. Yes there are 2 techniques being discussed > here. > > I would prefer to do the time delay method, however as described above I > didn't think it would be possible due to the need for a very small pulse > (and to my undestanding this would be made up of very high frequencies, > which are attenuated a huge amount and therefore cant be used). If I'm > wrong please explain it to me. > > If this is correct then the other method would be the way I am going to > proceed.Yes, you are wrong. It does not give you a correct result when you try to treat a pulse as a repeating sine wave. I can send a step function down a wire and measure the length of the wire by measuring the time between the transmission and reception of the leading edge (only edge actually). According to your analysis, since the setp function has frequencies down to DC, it can not measure *any* length no matter how long. The resolution of a time measurement is limited by the accuracy of detecting the pulse edge. As the pulse degrades, it will become harder to decide when the edge has occurred. But there are ways of restoring the pulse shape by filtering. Do you know how much your pulse edge will degrade in the 1 mile round trip? BTW, where did you get the numbers for attenuation and velocity?
Reply by ●April 29, 20082008-04-29
On Apr 29, 12:09 pm, Jerry Avins <j...@ieee.org> wrote:> For instance, if you were to use a car battery, > jumper cables, and a headlamp, I think you will find the lamp lit to > full brightness even with the rails in the loop. If my guess is correct, > connecting a 12-volt battery directly from rail to rail would result in > a current of a few hundred amps and a terminal voltage at the battery of > about a volt or two.Why would you think that two parallel, insulated rails spaced some 6 foot apart would constitute a near short circuit at *any* frequency, much less DC? The old style train detector uses the rails as a pair of wires. A battery is connected to one end of a section of track and the voltage is conducted down the track to the other end to a signal. The relay in the signal keeps the signal off as along as there is a voltage on the coil. When a train is on that section of the track, the rails are shorted (a resistance protects the battery) and the current to the signal relay stops which trips the signal to activate. If a rail breaks (a very dangerous condition) the current to the signal relay also stops and the signal activates. If the rails had any sort of a low impedance between them this circuit would not work. Rick (a former member of a CSX railroad signal gang)
Reply by ●April 29, 20082008-04-29
rickman wrote:> On Apr 29, 12:09 pm, Jerry Avins <j...@ieee.org> wrote: >> For instance, if you were to use a car battery, >> jumper cables, and a headlamp, I think you will find the lamp lit to >> full brightness even with the rails in the loop. If my guess is correct, >> connecting a 12-volt battery directly from rail to rail would result in >> a current of a few hundred amps and a terminal voltage at the battery of >> about a volt or two. > > Why would you think that two parallel, insulated rails spaced some 6 > foot apart would constitute a near short circuit at *any* frequency, > much less DC?They are not actually insulated. The rails I played around with were periodically bonded together.> The old style train detector uses the rails as a pair of wires. A > battery is connected to one end of a section of track and the voltage > is conducted down the track to the other end to a signal. The relay > in the signal keeps the signal off as along as there is a voltage on > the coil. When a train is on that section of the track, the rails are > shorted (a resistance protects the battery) and the current to the > signal relay stops which trips the signal to activate. If a rail > breaks (a very dangerous condition) the current to the signal relay > also stops and the signal activates. > > If the rails had any sort of a low impedance between them this circuit > would not work. > > Rick (a former member of a CSX railroad signal gang)I guess railroads in different places adhere to different standards. In the New York area, the concern seemed to be preventing potentially dangerous rail-to-rail voltages from building up. Was the CSX track electrified? The New Haven RR used (or at least experimented with carrier current on the catenary for voice communication with train crews. The main rails were the return (as they are for the traction current). Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
