Re: DCC occupancy detections false positives

David Nebenzahl wrote:
On 5/17/2008 8:52 PM Charles Davis spake thus:

David Nebenzahl wrote:

On 5/17/2008 4:46 PM Charles Davis spake thus:

David Nebenzahl wrote:


>

On 5/17/2008 2:16 PM Charles Davis spake thus:

Charles Davis wrote:

The available DCC signal, can be loaded (some current drawn) by either capacitance or resistance.
Capacitance can be as simple as the two wires paralleling each other over a long distance.



This is so unlikely (capacitive loading from just the 2 rails) as to be outside the realm of possibility.

If you had a pair of insulated wires tightly twisted together for a long run, you *might* start to see some effects due to capacitive loading, but even this is unlikely.

Not that UNLIKELY at the frequencies (& harmonics) generated by a DCC signal. Remember, they TRY for a 'Square Wave' with DCC. You can't look at things as if you were working with Analog Sine-Wave RF, and draw conclusions based on that universe.

When I typed the above ----
">> Capacitance can be as simple as the two wires paralleling each other
>> over a long distance."
I wasn't thinking of a 10' run to a close block. more on the order of exceeding 75'.



Sorry, TYPING IN ALL CAPS still doesn't make it any more correct. Even taking harmonics into account, a DCC signal is still well down in the kilohertz region, nowhere near radio frequencies. Capacitance effects, if any, are negligible.

Right, but the poster was talking about a 'bleeding edge' type of sensitivity.
And no, the fact of the 'Square Wave' signal wave form generates all sorts of higher frequency harmonics. In face, those harmonics are a not insignificant portion of the power being pumped out by the DCC Power Station/ Booster.


So show me where I disagreed with that; I said above, if you bothered to read, "even taking harmonics into account".

So let's look at those harmonics, shall we? First of all we need to know the fundamental frequency of a DCC signal. Referring to the DCC standard itself (I'm reading from one of the PDFs on the NMRA site), the shortest interval defined is for a "1" bit, which they define as being about 100 microseconds wide. (They actually allow more time than that, but let's use the lower figure for the sake of discussion.)

Since freqquency is the reciprocal of bandwidth, that means the the highest frequency in a DCC signal *should* be about 10 kHz. For the sake of discussion, let's double that, to 20 kHz.

As we all know, square wave harmonics are the odd multiples of the fundamental frequency. These harmonics diminish the higher they go; in other words, the strongest harmonics are the lowest-order ones.

So we now have harmonics at the following frequencies:

- 60 kHz (3f)
- 100 kHz (5f)
- 140 kHz (7f)
- 180 kHz (9f)
- 220 kHz (11f)

and so on. So we can see that we have a diminishing set of harmonics that is *well* below a megahertz until we get well up into the series. Which says that any higher-frequency harmonics are going to be so weak as to have negliglbie effect. Which means that the capacitive effect of two conductors in the form of HO track can be disregarded.


Hi David;
Sorry you are so hung up on "theory'.
The 'Old Professor', was asking about a 'Real World' problem.
In the 'Real World', observed problems, 'Trump' theory every time.

Capacitive loading cannot be ignored. It may be only a minute part of the detected load, but that 'minute' part may be the final 'smidgen' that trips the detector.

Go argue with Greg for a while.

Chuck D.

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Relevant Pages

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    ... can be loaded by either capacitance or resistance. ... Capacitance can be as simple as the two wires paralleling each other over a long distance. ... Even taking harmonics into account, a DCC signal is still well down in the kilohertz region, nowhere near radio frequencies. ... Since freqquency is the reciprocal of bandwidth, that means the the highest frequency in a DCC signal *should* be about 10 kHz. ...
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  • Re: DCC occupancy detections false positives
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