The Encoder Cable That Picked Up Noise: Differential Signals, Shielding, and Twisted Pairs
A rotary encoder on a servo motor. The position feedback was noisy — the motor jerked at low speed. The encoder was new. The cable was 5 meters long, running alongside the motor power cable. The encoder’s differential signal was picking up noise from the motor’s PWM drive. This is how encoder cables should be routed and shielded.
Why encoder signals are sensitive
An encoder outputs A/B/Z pulses at 5V differential (RS-422). The signal is small (about 1V peak-to-peak over the differential pair). Any noise coupled onto the cable appears as false edges. The servo drive counts the noise as position pulses. The motor moves the wrong amount. At low speed, the noise is visible as jitter. At high speed, it causes following errors.
The cable requirements
| Requirement | Why |
|---|---|
| Twisted pair (A+, A- and B+, B-) | Noise induces equally in both wires, differential cancels it |
| Shielded (foil + braid) | Blocks electric field coupling |
| Shield grounded at one end (drive end) | Prevents ground loop |
| Separate from motor power cable | Motor PWM is the main noise source |
| Max length per spec (usually 15 m) | Signal degrades over distance |
What we did
1. Separated the cables. The encoder cable ran in the same tray as the motor power cable. We moved the encoder cable to a separate tray, 200 mm away. The noise dropped by 50%.
2. Used the correct cable. The original cable was a generic shielded cable, not an encoder-specific one. We replaced it with a servo-rated encoder cable (twisted pairs, foil + braid shield). The noise dropped further.
3. Grounded the shield at the drive end. The shield was grounded at both ends (motor and drive). That created a ground loop. We grounded it only at the drive end. The remaining noise disappeared.
The differential advantage
An RS-422 differential signal sends A+ and A- on a twisted pair. The receiver measures the difference between A+ and A-. Noise couples equally onto both wires (they’re twisted together). The difference cancels the noise. This is why encoder signals use differential pairs — they’re inherently noise-immune. But if the cable isn’t twisted or shielded, the advantage is lost.
The motor cable
The motor power cable should also be shielded. The shield grounded at both ends (motor and drive). The motor cable’s PWM (20 kHz, high dv/dt) radiates noise. A shielded motor cable contains it. An unshielded motor cable radiates everywhere, including onto the encoder cable.
The encoder cable rule: Use twisted pair, shielded encoder cable. Ground the shield at the drive end only. Separate it from motor power cables by 200 mm. If the motor jerks at low speed, check the encoder cable routing and shielding. The encoder isn’t noisy — the cable is picking up the motor’s PWM noise.
