KB283: High Frequency Filters?

This article was last updated on Jan. 17, 2012, 12:17 p.m. | Print Article | Leave Feedback

Q: I read in the February 2008 "Scale Rails" on page 11 that I need to add some "High Frequency Filter" gizmos on my layout to make my trains run properly.

I have been using Digitrax equipment for 12 years, do I need to make these changes?

We do not feel that this is necessary.

With modern, well designed decoders, almost any combination of transmission-line ringing and other mismatch effects described in this article are completely filtered and ignored by the decoder.

This is a superior solution rather than trying to design filters for unpredictable limiting cases of wiring arrangements that this article supposes.

If you read the NMRA article in depth on the need for HF filters on a DCC layout, you could be excused for thinking that "the sky is falling"!

We take your trust seriously, and Digitrax has gone to great lengths to insure ease of use and a satisfactory experience with our products.

Many of the issues brought up in this article are issues that Electrical Engineers discuss in serious arcane presentation papers which are given at conventions in Las Vegas.

For the vast majority of Digitrax users, the issues discussed in this article do not materially affect the operation of their trains and the performance of their Digitrax systems.

We have had no significant issues in 15 years of production, sales or support that have involved transmission line effects on rails and feeders.

Most practical real-life layouts simply do not attain the complex configurations where the issues raised by this article would occur.

Or to put into Electrical Engineer terms, we have not heard from operators of Digitrax systems with problems that could be described as transmission line propagation at the low edge rates which became unmanageable.
A more important issue is the Radio Frequency Interference (RFI) that is caused by any High Frequency (HF) energy radiated by electronics systems.

In this case, this RFI is handled at the decoder and command stations.

Or, to put it in EE terms at a Las Vegas convention, it is most important to slew-limit the DCC transitions, since the bit times are fixed by the DCC definitions.

Basic physics.


The DCS100 Super Chief was not presented for "conformance" evaluation with the NMRA because so few of the unit's features fall within any of the limits of the NMRA Standards or RP's.

Yet, the Super Chief shown as an example in the article actually has the same Booster as the DB150, which received an "NMRA Conformance Warrant" when it was introduced in 1998.

In fact, the Super Chief has several advanced features that are not in the "NMRA Standards"; such as (again, back to Las Vegas) interleaved multi-format track signals that the Standards do not address, much less explain how to design or test.

The majority of the DCS100's functionality is related to system operation such as; throttles, detectors, computer operations, configuration, wireless connections, signal systems etc.

It is a sophisticated command station / booster in a single package.

We feel that the Super Chief is a very worthy unit, with features, available customization and expandability, one that has been extensively tested and proven.

Given the wide variability of model railroad layouts, our customers will invariably need some support or questions resolved from the manufacturer and their support team.

There are a very large number of DCS100's in the model railroad world that are working reliably; Super Chief has been the choice of many very large model railroads, systems which use all features of this unit and LocoNet to great success.

As we get ready for our next Electrical Engineer's convention at Vegas, we are planning a symposium on "Evaluation of the addition of HF loads or terminations which may adversely affect any track feedback systems."

We believe that this will fill a crucial gap in the body of knowledge about the tested system, addressing the big issues of track overvoltage and ringing, even absent any external loads.

Dust off your slide-rule and get out your Smith Charts and wail away at the impedance matching issues.... Meanwhile, the rest of us are going back to having fun and running our trains with Digitrax.

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