I would rule out mobile phones, but there is an awful lot of other intermitent airbourne RF noise from other transmitters. Emergency services radio transmitters are very powerful for example.

What about broad spectrum RF noise from sparking? either from OHLE, or from good old 750V third rail.



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Possbly a bit off topic for a undergrad project, but the "lateral thinking" apprach would say "why fight the noise?" Why not [electronically] look for a distinctive traction motor noise as the signal you are trying to detect, rather than eliminate

Hello, and "bon courage" for your project!

The current track-circuit systems vary according to the status of the line: electrified or not, 750 V DC 3rd rail (ex Network SouthEast and various metros); 1500 V DC usually overhead (France, Netherlands); 3kV DC (Belgium...);15 kV 16,66 Hz (Germany, Austria and Switzerland) and 25 kV 50 Hz (England, France...).

Usually, over non electrified lines, or DC electrified lines, 50Hz track circuit was used.

With AC electrification, systems have evolved to higher frequencies, of course off the harmonics of the traction current. More recent systems use periodic impulsions of high frequency (something like three impulsions at ~150Hz every second), process which proved to be efficient short sections of rail in dirty environment (leaves, oil...) especially in station areas.

Coming back to your solution, you mentionned a number of external sources of noise. DC usually does not create noise, provided that it is properly filtered in the substation, but DC trains are now equiped with thyristors which "cut" periodically the current in order to deliver the adequate power to the motors. The frequency they cut is around 400 Hz and is perfectly audible on recent trains (jubilee line for instance). Your system will have to avoid these frequencies.

Last question: if you process the signal on the reception side, how long will the process take, and how long is the free distance that it will represent ?

Good luck! (In case your project is not yet done, which I -now- doubt, according to the date of the former posts!!)

Hi SDX, thanks for some intersting info there.

In the UK, the last couple of generations of 750 V DC mainline EMUs have used AC motors. This started with 465/466 Networkers in the 1980s , and has been continued with the more recent 375/376/377/378 series. The main benefit of this is that the motors can be used for dynamic (regenerative) braking. This not only saves on brake disc usage and is a more 'powerful' brake, but also keeps the Green lobby happy as it enables some of the train's kinetic energy to put back into the Traction Current supply, or even the National Grid theoretically.

Because the motor frequency varies as a function of train speed, some units like 465/2 and 466 (ie, the MetCam units) have a device called an Interference Monitor, which will actually cut off traction power if the train's return current contains a frequency near to 50Hz into the running rails - which I'm told is to prevent damage to the ac track circuit.

And it's not just EMUs; Diesel locos are undergoing similar development. The ubiquitous Class 66 loco was conceived with DC traction motors, but the new Genesis Project hybrid loco (powered byboth Diesel & AC) which GE is currently developing for Freightliner is to have AC traction motors.

Hello, and thank you for your comments. As far as I know, there are in France some areas where using "récupération" (i.e. producing and sending it back to the over-head line) is forbidden due to incompatibility with track-circuit. So, either they use the rehostatic braking (dissipating energy through resistors) or use engines without this feature!
The idea is, on the long term, to modify track-circuit in order to protect them from these effects.