Model Forum / General / Railroads / January 2008

Erm, on re-reading the article, I realise I may have erred. I inferred
that the Conformance Warrant (CW) was granted after the tests reported
in the article. Although the article refers to passing CW tests, it is
not clear whether the specific problems discussed were the reason for
granting or refusing CWs. I don't know why the two systems without CWs
don't have them. IOW, "passed" just means that the systems have a CW.

So with that caveat in mind, read on. The following represents what I
understood the article to say.

The article actually reports on the effect of the layout itself on the
DCC signal, and specifically the occurrence of high-frequency "bounce"
or "ringing" around the transition from -15V to +15V. This transition
should be as sharp and smooth as possible, since the quality of the
signals received by the decoder chip depends on it. If the transition is
bad, the the information packet may not be read correctly. The HF bounce
or ringing is an effect of the impedance of the power bus/track.

So, first of all, all systems showed this ringing or bounce at the far
end of the layout when tested without a "terminator" or "snubber." IOW,
high frequency filters are essential for trouble free DCC. The two
systems without CWs showed bad transitions at the command station, too.

NB that all five systems listed below showed acceptable waveforms when a
terminator (150 Ohm resistor plus 0.1mFd capacitor in series) was used.
IOW, if a high-frequency filter type of terminator is used, all systems
are acceptable. However, none of the DCC systems supply a terminator,
which means that a plug'n'play test simulates what people actually do:
plug in the system and go. The implication is that the two failed
systems have unacceptable range and duration of the high frequency
bounce during the transition.

BTW, all five systems showed a degradation of the square wave towards a
sine wave form when a 1.A load was placed across the track. This implies
that boosters must be used.

The terminator was placed at the end of the 66ft "single track line",
which was simulated electronically by using two "white boxes", each of
which had the same impedance characteristics as 33ft of track.

I take the results to mean that three systems passed a "plug'n'play"
test, while the failed systems would require a terminator, ie, an
additional component supplied by the purchaser. NB that several
manufacturers indicate that such terminators or snubbers should be used.
NCE recommends one every 20 ft or so.

The article also gives circuit diagrams for the white box and the
terminator.

I infer from the technical discussion included with the test results
that a terminator should be placed at the most distant point from the
booster or command station. The degradation of the signal at the end of
the 66ft track indicates to me that boosters should be used when the
total signal path reaches roughly this figure. Ie, keep power districts
at 20-30ft or thereabouts. I am about to convert a 16ft shelf layout to
DCC, so this consideration won't apply.

With CW: Hornby Elite, Lenz LZV100, NCE Power Pro.
Without CW: MRC Prodigy Wireless, Digitrax Chief.