Zener Diodes – Tradeoffs and Downsides of Clamping Signals

I have a signal source that sends fast pulsing square waves in the MHz range (encoder pulses from a motor) down a long transmission line to a receiver. I have tried many solutions and different types of terminations to mitigate the transmission line effects that showed up as overshoot/ringing on rising/falling edges (that exceed the receiver's absolute maximum input voltages), as well as crosstalk coupling between channels.

The solution that worked the best for this problem turned out to be diode clamps using 4.7V zeners arranged in the schematic below:

^{simulate this circuit – Schematic created using CircuitLab}

The effect of these diode clamps can be seen in the below oscilloscope screenshot, where the clamped purple trace is juxtaposed with the unclamped yellow trace. As you can see, the rising/falling edges get clamped to a clean corner for the purple trace, which is desirable.

However, I am worried about the tradeoffs and downsides to doing this. Few things I have in mind:

• No series current limiting resistor.
  • The encoder (with a line driver) is specced to source a maximum of 20
    mA. Using a 4.7/0.02=235≈220 ohm resistor negated the clamping
    effects completely. Even using a lower 100 ohm resistor erased any
    clamping effects. I think it is hard to choose a good resistance to
    use because I cannot identify the other impedances accurately
    (encoder output impedance, transmission line impedance, input
    impedance, zener impedance)
  • However, I am thinking this might be fine because of the origin of these voltage spikes are from impedance mismatch reflections, so its not like its drawing current directly from the encoder driver.
• Ambient current draw. Say one of the channels ends up at logic high 5V, the zener will conduct and sink away current continuously.

Are there any things I'm missing? What do you guys think of this solution?

EDIT1 (additional information requested by @AnalogKid):

1. The "long transmission line" is a 6ft long unshielded PVC encapsulated cable. There are no twisted pairs and all signals are single ended. However I have estimated the impedance of the transmission line to be around 50-100 Ohms (from measuring inductance/capacitance with LCR meter, then corroborated by trying different parallel termination resistances). I have tried parallel (and AC) termination but ditched it because it didn't get rid of falling edge oscillations, and I didn't like that extra power was dissipated in the parallel path. Series termination cannot be implemented at this time.

2. The driving device is a Maxon ENX 10 EASY encoder with a built in line driver.

3. The driver is not source terminated. It is very difficult to source terminate because the mechanical design of that portion is frozen, although could possibly be a last resort.

4. I am mainly worried about the ringing damaging the receiver (LS7366R) as the oscillations exceed the absolute maximum input voltage rating of the IC. If the energy of the ringing is so small, do you think it is OK to just let it happen? FWIW the system has been working perfectly fine over hours of testing.

5. I am using the 4" long alligator clips. Although I did use the short ground springs and compared it to the 4" long alligator clips and found the difference to be negligible, so I stuck with the alligator clips due to ease of use. Perhaps I can revisit this.