Evenly Spaced RFI: Find the Clock, Then Find the Current Path
Evenly Spaced RFI: Find the Clock, Then Find the Current Path
A row of spectral lines across several HF bands often points to a switching clock, pulse repetition rate or nonlinear mixing process. It may arrive by common mode, but the line spacing alone cannot tell you whether the coupling is common mode, differential mode, direct radiation or receiver overload.
When a waterfall from 20 to 10 metres looks like a piano keyboard, I take the spacing seriously. It is often the fastest route back to a clocked device. But I do not call it common mode until a current-path test says so. The spectrum tells me what rhythm to hunt; the probes tell me how that rhythm reaches the receiver.
Joeri’s short version: equal spacing is a source-mechanism clue. Common mode is a coupling-mode conclusion. Write down the spacing, check for receiver overload, correlate the pattern with equipment state, and measure the conductors before choosing a choke or filter.
What an RFI Comb Actually Tells You
A periodic waveform has spectral components related to its repetition rate. A switching converter, digital clock, pulse train or data-dependent process can therefore produce a family of lines at integer multiples of a fundamental spacing, or sidebands around another carrier. The visible spacing is often close to the switching or repetition frequency, but it may also be a subharmonic, harmonic or aliased result.
fn = foffset + n · ΔfIn that useful description, Δf is the observed line spacing and foffset is any offset from zero or a carrier. It is a pattern model—not proof that the physical source clock equals exactly Δf. Frequency conversion, modulation and receiver processing can move or fold the apparent components.
The line amplitudes need not be equal. Edge rise/fall time, pulse width, source impedance, filters, cable transfer functions, installation resonances, antenna response and receiver settings shape the envelope. Similar-looking levels across several bands can be real, but they are not evidence that no resonant structure is involved.
Harmonics and Intermodulation Can Build Similar Patterns
A nonsinusoidal periodic source produces harmonics at integer multiples of its repetition frequency. Multiple clocks or switching states can produce sidebands. A nonlinear junction—inside a supply, corroded connector, overloaded amplifier, semiconductor input or even the receiver—can mix two or more signals into products such as:
fIM = |m f1 ± n f2|If the original frequencies share a common grid, the resulting intermodulation products can also look evenly spaced. That is why “find the clock” includes looking for more than one oscillator, load-dependent spacing, drifting offsets and changes in the comb envelope as equipment operating state changes.
Some switch-mode controllers change frequency with load, temperature or regulation mode. Others skip pulses or use spread-spectrum modulation. The comb may move, broaden or break into clusters. Those changes are valuable fingerprints, but none of them identifies the coupling mode on its own.
Common Mode Is One Possible Route
Common-mode current flows in the same direction on a group of conductors relative to another return path in the surrounding environment. On coax, current on the outside of the shield is the familiar example. On a DC pair, clamping both conductors together can reveal net current that did not cancel inside the probe aperture.
Differential-mode current flows around the intended circuit: out on one conductor and back on the other. A loop formed by separated conductors can radiate even when the net common-mode current is small. Direct electric or magnetic coupling can also inject the comb into the receiving antenna or receiver wiring.
Common-mode paths are not non-resonant by definition. Cable exteriors, protective conductors, speaker wires, metal rails and building structures all have frequency-dependent impedance, loss and coupling. Their resonances can emphasize some teeth and suppress others. The absence of a 50-ohm matched line does not mean the path has no impedance transition or selectivity.
Spectral signature and coupling mode are orthogonal questions. The line spacing points toward the source timing. Differential/common-mode current, near-field and substitution tests identify the route.
Check Whether the Receiver Is Creating the Comb
A strong broadcast, amateur, local oscillator or switching signal can overload a receiver input, preamplifier, active antenna or external amplifier. Nonlinearity then creates harmonics and intermodulation products that were not present at the antenna at the displayed levels. A wide waterfall can make those internally generated products look like external RFI.
Insert a known attenuator ahead of the suspected nonlinear stage, switch off the preamplifier or use a more selective preselector. A genuine external signal within the receiver’s linear range should fall predictably with added attenuation until it approaches the noise floor. Products created by overload often fall much faster, rearrange or disappear. Repeat with a second receiver or a passive antenna when available.
Terminate the receiver input with the correct load before concluding that a pattern enters through the antenna. If the comb remains, investigate local power, USB, network, display and audio connections. Keep the receiver and instrument within their input limits during every test.
Measure Common and Differential Current Deliberately
A current probe measures the net current passing through its aperture. Put both conductors of a balanced or DC pair through the aperture in their normal orientation: equal and opposite differential currents largely cancel, leaving a response to net common-mode current. Put one conductor through the aperture to observe the current in that conductor, which contains both intended and unwanted components.
The measurement is not automatically quantitative. Probe transfer impedance, frequency response, aperture position, conductor placement, analyzer input impedance and calibration all matter. A clamp-on ferrite with a pickup winding can be excellent for relative A/B/A work, but it is not a calibrated current probe unless the completed fixture has been characterized.
Use a near-field magnetic loop to follow current-bearing conductors and a small electric-field probe to find high-voltage coupling regions. These are localization tools. Keep probe distance and orientation fixed, and do not interpret a larger near-field response as a standards-compliance result.
Use A/B/A to Connect the Comb to a Source
- Capture the baseline. Save the frequency span, line spacing, receiver bandwidth, detector, gain/attenuation state, antenna and time.
- Change one state safely. Switch one suspected device or subsystem between operating and non-operating states using its normal controls. Do not defeat protective earth or open hazardous equipment.
- Return to the baseline. Restore the original state and confirm that the comb returns. A single disappearance may be coincidence.
- Repeat at another point. Compare the station antenna, a portable loop and current/near-field probes while preserving settings.
- Vary the source load. When safe and normal for the equipment, note whether spacing, offset or envelope follows load, temperature or operating mode.
- Test receiver linearity. Add known attenuation and use a second receiving chain to reject overload-generated products.
Correlation is not complete causation proof, but a repeated A/B/A result plus a localized current or field path is far stronger than recognizing a familiar-looking waterfall.
Apply the Remedy to the Demonstrated Mode
If common-mode current is demonstrated, place suitable common-mode impedance at the boundary where that unwanted current should stop. Treat all intended conductors of the circuit together when their wanted currents must cancel through the core. Select the completed choke by measured complex impedance, frequency, current, temperature and safety requirements.
If differential-mode noise dominates, the remedy belongs across or in series with the intended circuit and depends on source and load impedances. A common-mode choke may have little effect. Cable-pair routing, loop area and a manufacturer-approved differential filter may matter more.
If direct radiation dominates, enclosure seams, cable entry, shielding termination and physical separation become relevant. If receiver overload creates the teeth, input attenuation, preselection or removal of excessive upstream gain can solve the apparent interference without changing the source.
Do not scatter ferrites until one happens to move the display. A changed comb can mean reduced current, shifted resonance, added differential impedance or a different receiver-overload condition. Measure the mode before and after the change.
Sources and Engineering Context
- Keysight — Spectrum Analysis Basics: harmonics, intermodulation and dynamic range
- Rohde & Schwarz — Interference hunting, characterization and documentation
- Keysight — Differential, common-mode and mixed-mode measurements
- CISPR 16-1-1:2019 — Radio-disturbance measuring apparatus
- CISPR 16-2-1:2014+A1:2017 — Conducted-disturbance measurement methods
- CISPR 16-2-3:2016+A1:2019+A2:2023 — Radiated-disturbance measurement methods
- ITU-R — Spectrum Monitoring Handbook supplement: harmonics, intermodulation, overload and conducted interference
Joeri’s Bottom Line
If the spectrum looks like a piano keyboard across several bands, suspect a periodic or nonlinear source first. Measure the spacing, watch how it moves with operating state and use that fingerprint to shorten the hunt.
Then earn the common-mode conclusion. Check receiver linearity, measure net current around conductor bundles, probe the near field and repeat the source change A/B/A. The comb tells you what rhythm is present. The current path tells you where the fix belongs.
Have a repeatable comb signature or a well-documented cure? Share it with RF.Guru with frequency spacing, settings and A/B/A evidence.
Mini-FAQ
- Do evenly spaced RFI lines prove common-mode coupling? No. They indicate a periodic or nonlinear mechanism. Current, near-field and substitution measurements are needed to identify common mode, differential mode, direct radiation or receiver overload.
- Does the line spacing always equal the source switching frequency? No. It may be the clock, repetition rate, a harmonic, subharmonic, sideband spacing, intermodulation grid or an aliased result.
- Why are some teeth stronger than others? Source waveform, edge speed, filters, cable and structure resonances, antenna response, propagation and receiver settings shape the comb envelope.
- How can receiver overload imitate external RFI? A nonlinear receiver or upstream amplifier can generate harmonics and intermodulation internally. Added input attenuation, preselection and a second receiver help expose that behaviour.
- How do I test common-mode current on a cable pair? Pass all intended conductors through a characterized current probe together. Equal and opposite differential currents largely cancel, leaving the net current, subject to probe calibration and placement.
- Should I add a common-mode choke as the first fix? Only after common mode and its boundary are demonstrated. Differential noise, direct radiation or receiver overload require different remedies.