Buzzing interference on a digital DMR signal

 

On the 4th Tuesday of every month, we hold a local digital net here in Cork and this weeks net was on the DMR Talk Group 2723.

During the net, it was noted that Robby EI3GGB had quite a loud buzzing noise on his audio. His voice blocked out the buzzing/hum but the sound was very noticeable in between his words.

Rob was using a TYT MD-380 handheld with a magmount antenna on 70cms.

The buzzing sound sounded very much like mains hum, as if he was using a battery charger at the same time as when he was transmitting. He tried switching from the mains power supply to just a battery pack on its own but to no avail.

Through a process of elimination, we eventually traced the problem back to an external speaker mike.

It would seem that the problem is the RF signal on transmit is getting into the speaker mike and causing interference. We don't know yet whether the RF is getting into the electronics in the microphone or via the lead and into the radio.

Robby bought the 'Kenwood' speaker from China but it's not clear whether it's the real deal or just a clone. Hopefully the RF interference can be cured by using a simple clip on split ferrite core which Robby is going to source.

If you are using DMR and people start saying that your audio is distorted, noisy or buzzing then try removing the speaker mike if you have one.

Videos about VDSL Broadband interference

VDSL (Very-high-bit-rate Digital Subscriber Line) is currently the main way to deliver broadband to  residential customers in the UK. A similar ADSL system is used in Ireland.

VDSL uses the existing twisted-pair traditional telephone cables to carry high speed broadband data signals between the network provider’s “cabinet” (typically an above ground street-furniture box) and the customers premises.

The frequencies used in the UK are shown below...

The problem however is that the existing telephone cables were only supposed to carry a simple telephone signal in the audio spectrum up to several kHz and not signals of several MHz. As a result, VDSL broadband signals can cause widespread interference as seen in the plot below...

VDSL interference to the 7 MHz (40m) & 10 MHz (30m) bands

Videos... Two videos featuring presentations by John Rogers, M0JAV have been uploaded to YouTube recently.

The first one on the Mid-Ulster Amateur Radio Club account outlines what VDSL is and ways to deal with interference...



The second one on the Radio Society of Great Britain (RSGB) account describes VDSL and how people in the UK can complain to OFCOM, the UK regulator...



For further reading, RSGB have provided some additional information...

RSGB Frequently Asked Questions about interference

VDSL Interference Reporting

VDSL Interference Advice Leaflet