MTH DCS TIU Termination
(This 2008 information has been
updated on July 21, 2016 to correct the PN code frequency.)
The information below describes how the filter works and some of the design criteria used to choose the filter topology and component values.
Track/Controller System Description
Based on available literature, it appears as though the MTH DCS controller utilizes a 3.75 3.27 MHz PN code between the TIU and the engine. The transmission path between the TIU and the engine consists of the lead wire to the track (from the TIU) as well as the track itself. This PN signal appears to be without further modulation (baseband) and has frequency components extending to 10 MHz and above.
The fidelity of this PN signal is impacted by the integrity of the ‘transmission line’ between the TIU and the location of the engine at any given point along the track. Typically, transmission line integrity is only significant when the length of the transmission line approaches 1/8th of a wavelength, or longer. At the higher frequencies of the baseband PN signal on the track (10 MHz), the wavelength is approximately 98 feet. Train layouts where distance between the TIU and the farthest point on the track from the TIU exceeds approximately 98/8 feet, or about 12 feet may begin to experience data transmission issues between the TIU and the engine.
Enhancing the Transmission Line
Transmission lines work best when both ends are terminated with an impedance that matches the characteristic impedance of the transmission line itself. Unfortunately, transmission line characteristic impedance is not well controlled in a typical train layout. Bundles of wires, distributed accessories, and varying wire routing techniques all contribute to this unknown.
Also unknown is the characteristic impedance of the PN signal transmitter located with the TIU. Based on reported “magic light bulb” accounts, it appears as though light bulb impedances below 50 Ohms begin to take a toll on the signal strength available to the engine (due to series loss across the PN signal source resistance). This would suggest that the PN signal source resistance (Rs in Figure 1) is 50 Ohms or smaller. This sets the floor value for a shunt impedance matching element that might be located at the opposing end of the transmission line.
Figure 1. Simplified TIU/Track Model with Terminating Elements
The goal of these matching terminations is to reduce the impact of signals reflecting between the end of the transmission line and the TIU. These reflections can cause swings in amplitude as well as distortions in the PN waveform that make edge detection difficult.
Based on reported success with the “magic light bulb”, it appears that these terminations can be successfully used to improve communication performance to and from the engine.
The drawbacks to the light bulb, however, include:- There is a significant amount of wasted power from the TIU
- The light bulbs can get very hot
- The impedance of the light bulb may not be well behaved at the upper-end of the PN signal spectrum.
By placing a capacitor in series with a resistive element, the transmission line termination function of the light bulb can be realized without the drawbacks.
Figure 2 illustrates the shunt impedance characteristics of a typical light bulb and of an 0.1 µF capacitor in series with a 221 Ohm resistor – both of which are selected for their impedance characteristics up to and beyond 10 MHz.
Figure 2. Shunt Impedance Characteristics of the Termination Assembly.
The capacitor significantly reduces the power consumption of the termination assembly at the 60 Hz track frequency (as well as DC), but presents a controlled 221 Ohm impedance throughout the vast majority of the PN signal spectrum.
Based on experience with very large layouts, these terminating elements significantly improve the reliability of engine/TIU communication without the drawbacks associated with previously documented techniques.