DCC Waveforms

Many thanks to Don Vollrath for providing these 'scope traces.

Purpose:
These "scope traces" are intended for the technically savy and support the recommendations on the Wiring For DCC "Track Wiring - Part II" webpage regarding long bus runs. Click here to get back to the page on track wiring.

It is beyond the scope of this website to train you to read 'scope traces. If you don't understand what you see here, don't worry about it. The advice in this website does not require that you understand this particular webpage. If you are so inclined, a brief tutorial in reading 'scope traces is provided below.

Test Set-up:
NCE PowerHouse Pro DCC system, circa January, 1999.
NCE later furnished an add-on EMI filter (basic ferrite beads) for user install.
DCC bus = twisted pair w/ 3-5 turns/foot

1. Track signal voltage right at booster terminals. Proper amplitude, 28.8Vpp = 14.4 DCC track volts. No ringing. Very abrupt transitions, 240 nano-Seconds rise time, 177 n-Sec fall time. This is a "good, clean" DCC signal.

 

2. Booster signal right at exit of installed EMI filter, located less than 8 inches from booster terminals. Note rise and fall time slowdown (644/536 nano-Sec rise/fall times) but now with visible ringing that increases peak amplitude to +/-20V but not the primary DCC flat-top signal.

 

3. Track voltage positive transition 5ft from booster. Ringing now peaks up to 30V, but same basic DCC track voltage.

 

4. DCC unloaded track voltage at end of twisted pair bus 25 feet from booster/EMI filter on open unloaded track. Note severe ringing, now to 50 Vpeak.

 

5. Close-up of ringing from #4. 25 feet from booster. Unloaded track.
Vpeak is almost 60V. Ringing is at 2.5MHz. Ringing actually causes multiple voltage polarity reversals. Note that not only are the high voltages displayed here potentially damaging to a locomotive decoder, but the multiple zero crossings by the severe ringing might confuse some decoders.

6. Adjacent track voltage. 25 ft from booster, also unloaded. Very similar to #5.

 

7. Track voltage of #4 with R/C (0.1 uF, 100 ohms). Note ringing now reduced to 24 Vpeak and damped ringing cycles. Single voltage transition.

 

8. Same track as 4, 5, & 7, 25ft from booster, but with a decoder equipped loco on the track. Motor ON or OFF or headlight ON or OFF made little difference in signal amplitude or ringing. A constant load of resistor or incandescent lamp did not have the same damping quality as did the loco decoder.

How to Read a 'Scope - A very brief introduction (aka 'Scope 101):

A 'scope trace shows how voltage changes over time. Voltage is displayed vertically as time passes from left to right. Zero volts is represented by the horizontal dashed line. Positive voltage is above the line. Negative voltage is below the line. Each square is called a division. The bottom display, left of center, says 10V 500nS. That means each vertical division represents 10V and each horizontal division represents the passage of 500nS (500E-9 or 500 billionths of a second).

Free counters provided by Andale.