So I've done a quick read through of common-mode and differential-mode and I'll just put out there what I think I need to do. So here is a quick drawing I did of what a possible 'filtering' of noise could look like: http://img507.imageshack.us/my.php?image=concept4om3.jpg So this setup should remove most of the common-mode noise leaving only the differential-mode, which is the signal being sent down the line. There is going to be some common-mode noise which gets through the common-mode choke so could I use an active bandpass filter centered around my signal frequency. As there should be a lot less noise the roll-off of this filter shouldn't have to be too severe. Once again I am unsure of this so if you could again rip my confidence apart that would be nice.... :( (I really am grateful or all the input anyones gives!) I'm sorry if I seem to know little, but this is the trouble with University today, very theoretical based and very little practical and hands on.
Filtering options for reflected signal with noise
Started by ●April 15, 2008
Reply by ●April 29, 20082008-04-29
Reply by ●April 29, 20082008-04-29
So I've done a quick read through of common-mode and differential-mode and I'll just put out there what I think I need to do. So here is a quick drawing I did of what a possible 'filtering' of noise could look like: http://img507.imageshack.us/my.php?image=concept4om3.jpg So this setup should remove most of the common-mode noise leaving only the differential-mode, which is the signal being sent down the line. There is going to be some common-mode noise which gets through the common-mode choke so could I use an active bandpass filter centered around my signal frequency. As there should be a lot less noise the roll-off of this filter shouldn't have to be too severe. Once again I am unsure of this so if you could again rip my confidence apart that would be nice.... :( (I really am grateful or all the input anyones gives!) I'm sorry if I seem to know little, but this is the trouble with University today, very theoretical based and very little practical and hands on.
Reply by ●April 29, 20082008-04-29
John121 wrote:> So I've done a quick read through of common-mode and differential-mode and > I'll just put out there what I think I need to do. > > So here is a quick drawing I did of what a possible 'filtering' of noise > could look like: > http://img507.imageshack.us/my.php?image=concept4om3.jpgThe common-mode choke is useful, but an impedance-matching transformer would make better use of the iron core while providing the same common-mode rejection.> So this setup should remove most of the common-mode noise leaving only the > differential-mode, which is the signal being sent down the line. > > There is going to be some common-mode noise which gets through the > common-mode choke so could I use an active bandpass filter centered around > my signal frequency. As there should be a lot less noise the roll-off of > this filter shouldn't have to be too severe.It would make sense to me to see what signal you actually get before trying to figure out how to improve it.> Once again I am unsure of this so if you could again rip my confidence > apart that would be nice.... :( (I really am grateful or all the input > anyones gives!)You're thinking about your proposals. That's good.> I'm sorry if I seem to know little, but this is the trouble with > University today, very theoretical based and very little practical and > hands on.Theory is important, but one needs facts to theorize about. Armchair theorizing can confine itself to "what if". Practical engineering necessarily deals with "what is". Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 30, 20082008-04-30
On Apr 29, 4:15 pm, Jerry Avins <j...@ieee.org> wrote:> 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).Yes, of course, if the rails are being used for carrying current, especially such high currents as used to power a locomotive, then you will find a lot of things different about the structure of the rails. No, the CSX railroad, like most, use non-electric engines. Electrically powered railroads are a whole different ball of wax. If the rails are shorted together, I don't expect you would be able to measure the location of a train anywhere past the first short.
Reply by ●April 30, 20082008-04-30
John121 wrote:> So I've done a quick read through of common-mode and differential-mode and > I'll just put out there what I think I need to do. > > So here is a quick drawing I did of what a possible 'filtering' of noise > could look like: > http://img507.imageshack.us/my.php?image=concept4om3.jpg > > So this setup should remove most of the common-mode noise leaving only the > differential-mode, which is the signal being sent down the line. > > There is going to be some common-mode noise which gets through the > common-mode choke so could I use an active bandpass filter centered around > my signal frequency. As there should be a lot less noise the roll-off of > this filter shouldn't have to be too severe.What Jerry said! Another reason to use a transformer is for safety. I had forgotten that you are working with an electric railway. If so, you need to be aware that a rail that is part of the return path for the engine current can also carry high voltages in a ground fault situation. It doesn't even have to be an open. The subway here has 700 amps flowing through the rails and if you got just 100 mohm resistance, that becomes 70 volts which is enough to give a severe shock if not fatal. Using a transformer to couple the signals to the rail will preclude the circuit from ever experiencing dangerous voltages. As a bit of practical knowledge, common mode chokes are much more commonly used for DC than for AC. The problem with using them for AC signals is that they will attenuate the signal as well as the noise. So the only noise that can be significantly attenuated is significantly higher frequency noise. The noise in the same general band as the signal will not be affected. But in DC circuits, a common mode choke has virtually no effect on the DC while attenuating the noise as much as you wish depending on the inductance.
Reply by ●April 30, 20082008-04-30
>What Jerry said! Another reason to use a transformer is for safety. >I had forgotten that you are working with an electric railway. If so, >you need to be aware that a rail that is part of the return path for >the engine current can also carry high voltages in a ground fault >situation. It doesn't even have to be an open. The subway here has >700 amps flowing through the rails and if you got just 100 mohm >resistance, that becomes 70 volts which is enough to give a severe >shock if not fatal. Using a transformer to couple the signals to the >rail will preclude the circuit from ever experiencing dangerous >voltages.>As a bit of practical knowledge, common mode chokes are much more >commonly used for DC than for AC. The problem with using them for AC >signals is that they will attenuate the signal as well as the noise. >So the only noise that can be significantly attenuated is >significantly higher frequency noise. The noise in the same general >band as the signal will not be affected. But in DC circuits, a common >mode choke has virtually no effect on the DC while attenuating the >noise as much as you wish depending on the inductance.The signal we are going to be using will be somewhere around 500-1kHz, 20V. The main component of noise is going to be centered around 50Hz (England) and then several harmonics above this. If I were to connect to the rails with an isolation transformer this would remove the common-mode noise (most of the 50Hz and harmonics). Would the isolation transformer have any negative impact upon the differential-mode signal when it comes to measuring them for impedance calculations. My section cannot effect the next section. All I ultimately have to do is get the signal off the lines and remove any noise which is present, before the readings can occur. Thanks....
Reply by ●April 30, 20082008-04-30
John121 wrote:>> What Jerry said! Another reason to use a transformer is for safety. >> I had forgotten that you are working with an electric railway. If so, >> you need to be aware that a rail that is part of the return path for >> the engine current can also carry high voltages in a ground fault >> situation. It doesn't even have to be an open. The subway here has >> 700 amps flowing through the rails and if you got just 100 mohm >> resistance, that becomes 70 volts which is enough to give a severe >> shock if not fatal. Using a transformer to couple the signals to the >> rail will preclude the circuit from ever experiencing dangerous >> voltages. > >> As a bit of practical knowledge, common mode chokes are much more >> commonly used for DC than for AC. The problem with using them for AC >> signals is that they will attenuate the signal as well as the noise. >> So the only noise that can be significantly attenuated is >> significantly higher frequency noise. The noise in the same general >> band as the signal will not be affected. But in DC circuits, a common >> mode choke has virtually no effect on the DC while attenuating the >> noise as much as you wish depending on the inductance. > > > The signal we are going to be using will be somewhere around 500-1kHz, > 20V. The main component of noise is going to be centered around 50Hz > (England) and then several harmonics above this.Depending on the impedance, several hundred amperes might be needed to attain that voltage. You really need some preliminary measurements or a confirmed model in order to settle on such decisions.> If I were to connect to the rails with an isolation transformer this would > remove the common-mode noise (most of the 50Hz and harmonics).An isolation transformer is, in theory, sensitive to differential-mode voltages only. In practice, parasitic capacitance may give it weak sensitivity to common-mode voltage, but "weak" is the operative word.> Would the isolation transformer have any negative impact upon the > differential-mode signal when it comes to measuring them for impedance > calculations.No.> My section cannot effect the next section. All I ultimately > have to do is get the signal off the lines and remove any noise which is > present, before the readings can occur.If sections are isolated, what completes the traction-current circuit? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 30, 20082008-04-30
>> The signal we are going to be using will be somewhere around 500-1kHz, >> 20V. The main component of noise is going to be centered around 50Hz >> (England) and then several harmonics above this. > >Depending on the impedance, several hundred amperes might be needed to >attain that voltage. You really need some preliminary measurements or a >confirmed model in order to settle on such decisions. > >> If I were to connect to the rails with an isolation transformer thiswould>> remove the common-mode noise (most of the 50Hz and harmonics). > >An isolation transformer is, in theory, sensitive to differential-mode >voltages only. In practice, parasitic capacitance may give it weak >sensitivity to common-mode voltage, but "weak" is the operative word. > >> Would the isolation transformer have any negative impact upon the >> differential-mode signal when it comes to measuring them for impedance >> calculations. > >No. > >> My section cannot effect the next section. All I ultimately >> have to do is get the signal off the lines and remove any noise whichis>> present, before the readings can occur. > >If sections are isolated, what completes the traction-current circuit?I meant my filtering section/subsystem cannot effect the readings which are going to be taken. There is no break in the lines so the traction current can continue along. Track circuits do use impedance bonds which only allows the traction current through and filters the signal. I was thinking that to ensure successive devices do not effect each other, each would operate on a slightly different frequency. I could then place a bandpass active filter after the transformer which filters the frequency for that particular section. There would only need to be 3-4 different frequencies before they can be repeated.
Reply by ●April 30, 20082008-04-30
>> The signal we are going to be using will be somewhere around 500-1kHz, >> 20V. The main component of noise is going to be centered around 50Hz >> (England) and then several harmonics above this. > >Depending on the impedance, several hundred amperes might be needed to >attain that voltage. You really need some preliminary measurements or a >confirmed model in order to settle on such decisions. > >> If I were to connect to the rails with an isolation transformer thiswould>> remove the common-mode noise (most of the 50Hz and harmonics). > >An isolation transformer is, in theory, sensitive to differential-mode >voltages only. In practice, parasitic capacitance may give it weak >sensitivity to common-mode voltage, but "weak" is the operative word. > >> Would the isolation transformer have any negative impact upon the >> differential-mode signal when it comes to measuring them for impedance >> calculations. > >No. > >> My section cannot effect the next section. All I ultimately >> have to do is get the signal off the lines and remove any noise whichis>> present, before the readings can occur. > >If sections are isolated, what completes the traction-current circuit?I meant my filtering section/subsystem cannot effect the readings which are going to be taken. There is no break in the lines so the traction current can continue along. Track circuits do use impedance bonds which only allows the traction current through and filters the signal. I was thinking that to ensure successive devices do not effect each other, each would operate on a slightly different frequency. I could then place a bandpass active filter after the transformer which filters the frequency for that particular section. There would only need to be 3-4 different frequencies before they can be repeated.
Reply by ●April 30, 20082008-04-30
>> The signal we are going to be using will be somewhere around 500-1kHz, >> 20V. The main component of noise is going to be centered around 50Hz >> (England) and then several harmonics above this. > >Depending on the impedance, several hundred amperes might be needed to >attain that voltage. You really need some preliminary measurements or a >confirmed model in order to settle on such decisions. > >> If I were to connect to the rails with an isolation transformer thiswould>> remove the common-mode noise (most of the 50Hz and harmonics). > >An isolation transformer is, in theory, sensitive to differential-mode >voltages only. In practice, parasitic capacitance may give it weak >sensitivity to common-mode voltage, but "weak" is the operative word. > >> Would the isolation transformer have any negative impact upon the >> differential-mode signal when it comes to measuring them for impedance >> calculations. > >No. > >> My section cannot effect the next section. All I ultimately >> have to do is get the signal off the lines and remove any noise whichis>> present, before the readings can occur. > >If sections are isolated, what completes the traction-current circuit?I meant my filtering section/subsystem cannot effect the readings which are going to be taken. There is no break in the lines so the traction current can continue along. Track circuits do use impedance bonds which only allows the traction current through and filters the signal. I was thinking that to ensure successive devices do not effect each other, each would operate on a slightly different frequency. I could then place a bandpass active filter after the transformer which filters the frequency for that particular section. There would only need to be 3-4 different frequencies before they can be repeated.
