Technical Articles

Troubleshooting Hum, Buzz, Crosstalk, and RF Pickup

Hams are typically less precise in their use of technical terms than are professional engineers and technicians. Nowhere is this more true than in descriptions of signals contaminated with unwanted signals. Since the terms refer to very different problems, using the wrong term can make troubleshooting very frustrating! This overview is intended to help customers and customer service reps describe problems accurately so they can solve problems more quickly.

Hum

Hum is caused by an AC (alternating current) magnetic field inducing a voltage in a continuous loop of cable or wire. The amount of voltage induced by the field is proportional to the area enclosed by the loop. It typically sounds like a fairly pure tone (sine wave) at the frequency of the local AC power grid (60 Hz in the US and Canada) or of a rotating ac motor. The field is most often generated by power transformers, high- current AC wiring, and AC motors. Because of the low frequency, the field penetrates conventional cable shielding and enclosures very easily.

To reduce hum, break the loop (using a hum eliminator transformer in an audio circuit) or minimize loop area by twisting or tying cables together. If the hum is caused by a transformer, power cable, or motor, move the affected conductors away or reorient them to minimize the induced voltage. Bonding equipment together with heavy wire or strap “short out” some of the induced voltage if the loop is formed by inter-equipment cabling.

Buzz

Buzz is a higher-frequency signal than hum, usually at harmonics of the AC power frequency. A common source of buzz is half- or full-wave rectifiers in electronic power supplies and from leakage currents through stray and bypass capacitance. Rectifier circuits create current pulses at 60 or 120 Hz with a lot of harmonics and so sound “sharper” (“buzzier”) than hum. Another source of buzz is voltage differences from currents in AC power neutral and ground conductors, particularly in large buildings.

To minimize buzz, keep ac power and audio cables separated to minimize coupling. Then, try to power your radio, accessories, and computer from power outlets that use the same ground conductor (“green wire”) to the AC service panel. This minimizes any noise caused by voltages between different ground conductors. Finally, bond the affected equipment together using heavy wire or strap. If the buzz is from currents carried by the relatively light shields on most data and audio cables, bonding will give the undesired current a low-resistance path away from the desired signal path

Crosstalk

If audio from one source is added to audio in a second circuit, the combination of signals is called crosstalk or bleed-through. This often happens when the two sources share a common signal return path through a typical light cable shield or ground wire. This allows the return current to develop a small voltage across the shared ground resistance which then becomes an input signal just like the desired signal.

Crosstalk sounds like low-level audio and generally goes away only when the common return path is eliminated by removing a cable or disconnecting a return circuit. Consumer audio products often use a cable shield as a signal return which creates problems when connected to more than one piece of equipment or multiple audio sources.

Bonding equipment together may help by providing a low-resistance shared ground. A better solution is to use separate signal return paths for each source, but this is often infeasible in the typical Ham station. Ground isolation transformers (see the section on hum) may also help.

RF Pickup

The symptom of RF pickup is usually distortion of speech or data audio when transmitting. In data circuits, RF pickup results in higher error rates or data connection failures when transmitting. The problem is almost always caused by common-mode RF current flowing on cable shields.

Common-mode RF current can be blocked by winding cables on ferrite cores (see “Ferrite – What Is It Used For Anyway? (Part 2)”) to keep it out of equipment. Ethernet cables can use an RF isolator. As with hum, wire or cable loops also pick up RF so minimize the loop area. A very effective technique to reduce RF pickup is good bonding between the equipment to minimize any RF voltage differences between enclosures that cause RF current to flow.

Consumer electronics and some amateur gear often connect cable shields directly to the internal circuit ground or common. This is a common problem for equipment in plastic enclosures. RF picked up by the shield is then conducted directly to the internal electronics. If the enclosure is metal, be sure cable shields are connected only to the enclosure which should be bonded to other enclosures. If the enclosure Is plastic or the signal return and shield are combined, ferrite cores will be needed.

For Further Reading

These three publications go into considerable detail about audio and RF pickup problems. The first two are online publications by Jim Brown, K9YC, a retired professional audio engineer.

http://audiosystemsgroup.com/GroundingAndAudio.pdf

http://audiosystemsgroup.com/RFI-Ham.pdf

The third is a book by Ward Silver, NØAX, that discusses grounding and bonding specifically from the perspective of the Ham operator and his or her station.

https://www.arrl.org/shop/Grounding-and-Bonding-for-the-Radio-Amateur

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