Measuring Common-Mode Current: Why Coax Is Easy and Open-Wire Is Hard

Measuring common-mode and stray return current (CM) on an open wire or twisted pair is far more difficult than on coax. Here’s a compact guide explaining why, what your clamp-on RF current probe really measures, and how to make your readings repeatable.

Why Coax Is Easy

In differential-mode (DM) operation, equal and opposite currents flow on the coax center conductor and the inside of the shield, so external fields cancel. Any CM current rides on the outside of the shield and returns through the environment. That clear separation—inside vs. outside—makes it simple to clamp around the coax and read those stray currents directly.

Why Open Wire or Twisted Pair Is Hard

There’s no shield, and the CM return path is undefined—it closes via stray capacitance to the bench, chassis, cable trays, or your hand. Move the cable and the reading changes. To get repeatable numbers, you must define the geometry and the common mode impedance seen by the pair.

What the Clamp Actually Measures

Clamp-on RF current probes are broadband transformers. They report a voltage proportional to the enclosed current through the probe’s transfer impedance Z_t(f):

I_CM(f) = V_out(f) / Z_t(f)

In dB form (for a 50 Ω receiver):
I_CM(dB µA) = V_out(dB µV) − Z_t(dB Ω)

Example — using a Fischer F-33-1 (Z_t ≈ 5 Ω ≈ +14 dBΩ from 1–250 MHz): if your analyzer reads −40 dBm (≈ 67 dBµV), then I_CM = 67 − 14 = 53 dBµA ≈ 0.45 mA.

Measuring on Coax — Quick and Reliable

1. Clamp placement: Close the RF current probe around the outside of the coax at any point along the run.
2. Instrument: Terminate the probe into a 50 Ω spectrum analyzer or EMI receiver.
3. Convert: Use the probe’s Z_t curve to convert V→I.

Because DM currents cancel outside the shield, this directly measures the CM current.

Twisted Pair or Open Wire — Making It Repeatable

You have two options depending on whether you need relative troubleshooting data or absolute, standards-based data.

A) Troubleshooting (Fast, Relative)

• Reference environment: Use a metal sheet as a ground plane and route the pair consistently above it. Keep height and path identical for each test.
• Clamp orientation: Pass both conductors through the jaw in the same direction—DM cancels, CM adds. Passing opposite directions measures DM instead.
• Terminations: Run the pair normally (e.g., 100 Ω load for Ethernet). Avoid moving hands or nearby cables during measurement.
• When to use: Ideal for bench debug. Add a ferrite, adjust a shield, or move routing and watch CM current change. Works well ~30–500 MHz depending on probe.

B) Standards-Based (Absolute, Comparable)

For conducted emissions (150 kHz–30 MHz), use an ISN/AAN (Impedance Stabilization Network) per CISPR 32. It defines a 150 Ω common-mode impedance and outputs CM voltage to the EMI receiver. Convert via I_CM = V_CM/150 Ω. ISNs are available for 1–4 pairs and shielded/unshielded variants.

(Above 30 MHz, radiated tests per CISPR 32 apply; current clamps remain excellent diagnostic tools but not compliance substitutes.)

Probe Selection & Handling Tips

• Pick Z_t for your job: Higher Z_t = more sensitivity; lower Z_t handles higher primary current. Mind DC & 50/60 Hz limits to avoid core saturation.
• Know your Z_t curve: The Fischer F-33-1 is roughly flat ~ 5 Ω (+14 dBΩ) across 1–250 MHz — great for general cable CM checks.
• Do the math right: Use I = V/Z_t or dB form above; remember dBµV = dBm + 107 (50 Ω).
• Troubleshooting strategy: Cable CM often predicts radiated failures. Measuring it early is one of the best pre-compliance checks you can do.

Minimal, Repeatable Bench Recipe (Twisted Pair)

1. Set up a reference plane and fix the cable path.
2. Run the link normally with proper termination.
3. Clamp both conductors in the same direction about 5–10 cm from the device.
4. Measure with a 50 Ω analyzer, apply the Z_t(f) curve.
5. Change one variable at a time and log Δ CM current vs frequency.

Quick Sanity Checks & Pitfalls

• Clamp closure: Fully close the jaw; even small gaps hurt accuracy.
• Probe saturation: High LF currents can flatten readings — use lower-sensitivity probe or keep LF current path outside.
• Environment: Twisted-pair readings vary with cable or hand movement — keep geometry fixed.
• Know what you’re measuring: Same direction = CM; opposite = DM. Use intentionally.

Rule-of-Thumb Targets

Henry Ott suggests keeping cable CM currents below a few µA for Class B radiated emission goals — a helpful benchmark for pre-compliance troubleshooting.

References

• Henry Ott — Measuring Common-Mode Currents on Cables
• Tektronix / Kenneth Wyatt — The HF Current Probe: Theory and Application
• Fischer Custom Communications & ETS-Lindgren Manuals — Transfer Impedance and Conversion Data
• CISPR 32 / ITU-T K.123 — Use of ISNs/AANs for Balanced Pairs
• EMC FastPass — Practical Ranges and Usage of Current Clamps

Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.

Questions or experiences to share? Contact RF.Guru.