RC Filter Name: Why?

This section talks about DCC and the application of a Resistor Capaciotors circuit that goes by many names and consequently comes with confusing rules.


1) WHY RC FILTER NAME?
2) TERMINOLOGY TRAP CONFUSION. (Why Electrical Engineers do not appear to agree).
3) TERMINOLOGY COMPARISON (RC Filter is a neutral name)
4) WHY WE CAN'T CALL IT A TERMINATOR?
5) WHY WE CAN'T CALL IT A SNUBBER?


1) WHY RC FILTER NAME?

The RC Filter consist of a Resistor and a Capacitor of specific values that when put together provide a general purpose DCC filter that addresses both electrical noise and voltage spike.

I carefully choose the name "RC Filter" because it:

1) EASY TO UNDERSTAND.  Make it easy for the not technical layman to understand what it is.

2) AVOIDS CONFUSION.   Intentionally disconnect it from other technical Nicknames that come with "technical baggage" that does not apply to our DCC situation.   See terminology trap.


2) TERMINOLOGY TRAP CONFUSION. (Why Electrical Engineers do not appear to agree).

Two Electrical Engineers (EE) can look a the SAME DCC bus and come away with two totally independent electrical recommendations at the same time because of their different area of expertise within electrical engineering.

An EE who’s expertise is on high speed data communication bus problems will LIMIT his comments to the data bus side of DCC.     
An EE who’s expertise is on power problems will LIMIT his comments to the high current short circuit voltage spike side of DCC.

The solutions to BOTH PROBLEM involve a Resistor and Capacitor circuit AKA “RC” circuit.  The RC circuit is visually recognized in many different fields of electrical engineering.  But depending on the person technical background, there are corresponding DIFFERENT Nicknames for this same exact RC circuit.

The communication EE will call it a terminator and recommend the terminator use rules.
The power EE will call it a snubber and recommend the snubber use rules.

Each of these Nicknames come with VERY DIFFERENT "design and installation rules" which at face value appear to be in direct conflict with each other.    Hence the source of confusion about how to install the RC filters.

ROOT CAUSE OF  CONFUSION:   DCC is BOTH a digital communication bus and a high power bus at the same time.   The Communication EE and Power EE fail to see/understand the WHOLE PROBLEM because they do not completely understanding the required "dual function" of the DCC signal.  They ignore the other half of the problem in making their recommendations since they fail to understand it.


3) TERMINOLOGY COMPARISON (RC Filter is a neutral name)

Here is a comparison table of the various EE name for the same RC circuit.

Circuit
Name
Engineering
Field
Designed to attack
voltage
spikes?
Designed
to attack
general noise?
Designed
to limit
maximum
frequency?
Designed
to work
with
DCC?
Designed to preserve
waveform
shape?
Simple
part
requirements.
Simple
usage
rules for
installation.
Snubber
Power Supply or
Electromechanical
Yes
No
No
No
No
No
No
Terminator
Networking
No
No
No
No
Yes
No
No
RFI Filter
Radio Engineering
FCC compliance
No
Yes
Yes
No
No
No
No
RC FILTER
DCC
Yes
Yes
Yes
Yes
Yes
Yes
Yes


Conclusion:  The Prefix term "RC" in "RC filter" does NOT limit the application rules because "RC" is referring to physical parts that make up the filter.  Think Generic.

Table Defintions:

Designed to attack voltage spikes:   The circuit is designed to absorb the undesired and damaging voltage spikes and convert them to harmless heat.  Voltage spikes, with enough energy in them, represent a real destructive threat to decoders.

Designed to attack general noise:  The circuit filters out random high frequency electrical noise that is not part of the DCC signal.

Designed to limit maximum frequency:  The circuit filters out repetitive high frequencies signal unrelated to the DCC signal

Designed to work with DCC:  The RC part values used in the filter design take into account the frequency needs of the DCC signal.

Designed to preserve waveform shape:  Maintain the original signal waveform shape over the length of the cable.

Simple part requrements:  The circuit uses "off the shelf", low cost common parts to implement.  There are no precision parts required.

Simple usage rules for installation: You can place the circuit anywhere where it is needed most.   It does not have to be installed at very specific location to be effective.   You can also use as many as you want.


4) WHY WE CAN'T CALL IT A TERMINATOR?

Single Sentence why?  A DCC bus is not just a data bus.

DATA BUS SOLUTION GOALS:  Terminators are all about signal waveform preservation with minimum distortion as the rectangular AC waveform travels down the length of the wire.  The data cable that is used to carry the data is specifically designed and manufactured to have controlled electrical characteristic over the lenght of the cable.   Example: Ethernet cable.   The Terminator resistors value is very precise and chosen to match the specifics electrical characteristics (impedance) of the data cable wires being used to carry the high speed digital data.  The terminator is ONLY placed at the far end of the cable to absorb the electrical energy behind the AC signal when it reaches it.  Why?  The absorption of the energy prevents a reflection of the AC data signal back upon itself the way it came.  Analogy: You want to stop the bounce of a ball back at you when it hits a solid wall.   A series capacitor is not required but can be used to save power since it blocks DC current flow without effecting the AC function.

The GOOD:  Terminator eliminate signal reflections which maximizing data signal wave shape fidelity which in turn eliminate data errors caused by waveform distortion.  No false detection of a high or low signal states.   In DCC terms, decoders will never misunderstand a DCC command and have to throw the DCC command away.  A  given decoder's speed in response to a given DCC command is maximized.  Do not have to wait for another transmission of a redundant DCC command to come by later.

The BAD:   Terminators installation rules INCORRECTLY ASSUMES the following

1) compared to DCC, the communication bus power is a very very low. 1/10 or far less of an amp depending on the bus
2) there are never any short circuits on the communication bus.
3) the communication bus wires (cable) used to carry the data signal have a controlled impedance over its entire length.
4) all electrical problems can all be solved from a single location at the end of the communication bus.

What is being missed when applied to DCC:

1) Booster put out AMPS of current. 
2) Short circuit are a common event on the track.
3) Real World layout wiring is often done in loose and random format tracing back to pure DC days.  There is no controlled impedance.
4) High power voltage spikes due to short circuit occur at the location of the short circuit.  The terminator's location at the end of the bus is to far away from the location of the short to be fully effective.

CONCLUSION:  TERMINATORS DO NOT FIX THE HARMFUL VOLTAGE SPIKE PROBLEM

Terminator design and installation rules prevent placement at the source of the voltage spike problem.  Not flexible enough to address all the problems on the DCC bus.


5) WHY WE CAN'T CALL IT A SNUBBER?

SNUBBER SOLUTION GOALS:  A Snubber are all about reducing the voltage amplitude of the high energy voltage spike to a non destructive level.   A Snubber's resistor and capacitor values are calculated to absorb enough of the energy behind the voltage spike such that the voltage spike amplitude is reduced or clamped to a safe value that can do no harm while at the same time, they do not present an unmanageable load to the power source.  A Snubber is ONLY placed at the root cause location of the voltage spike.  Why?  The voltage spike has combination of both high voltage and high frequency content in the form or ringing.   The addition of any wires between the Snubber's circuit location and the root cause voltage spike circuit location interferes with and consequently reduces with the Snubber's effectiveness in attacking the highest frequency content.   Snubbers work because the following is true

a) The energy behind the voltage spike is known because the circuit that causes the voltage spike never changes. 
b) The location of the voltage spike is known because the circuit that causes the voltage spike never moves.


The GOOD:  By minimizing the amplitude of high energy voltage spikes, you protect all of your decoders from destruction.

The BAD:  Snubber installation rules INCORRECTLY ASSUME the following

1) The location of the short is constant. 
2) The energy behind the voltage spike is constant. 
3) The power does not contain any Data.
4) There are no special waveform requirements.


What is being missed when snubber rules are applied to DCC:

1) The location of short on the track can happen anywhere on the layout.  Furthermore every layout is unique in it design.
2) Voltage spike energy is not constant.    To many variables.  The amount of energy behind the voltage spike is function of:
2a) the booster current rating (Amps).  Different booster have different current rating.  The higher the booster current rating, the worse the problem gets.
2b) the length of the track bus wiring (Wire inductance value) involved relative to the location of the short.   Further away from the booster is worse than near the booster.  If the short location moves, the voltage spike energy changes with it.

3) DCC is the combination of both power and DCC data on the same signal.  Improper design of a Snubber on DCC will draw excessive current and consequently generate to much wasted heat in the snubber.   Worse case it can overload the booster.
4) DCC is the combination of both power and DCC data on the same signal.   Snubbers are NOT concerned about DCC waveform fidelity like terminators can.

CONCLUSION:  A SNUBBER DOES NOT TAKE INTO ACCOUNT ALL OF DCC SIGNAL REQUIREMENTS

Snubber design and installation rules prevent placement at the end of the track bus to serve the same function as a terminator.   Improperly design they can harm the DCC signal.  Not flexible enough to address all the problems on the DCC bus.


10/27/15