Mobile phone affects vintage FM receivers

[stevehertz]

I've noticed that when my mobile phone is near to my Roberts RM33 (switched to FM), the latter sporadically emits a variety of loud 'dot, dot dot' and other strange noises picked up from the former. I wasn't sure if it was the phone causing the problem, but I have since experienced the same interference pattern on late 70s hifi receivers that I happen to be working on and have the phone of the work surface nearby.

Do mobile phones affect modern FM sets so badly - or at all? What is the technical nature of the interference given that FM receivers should not be unduly affected by anything other than an FM signal? Maybe the interference is getting in through the power supply side of things being as the phone is so close to the set, I reckon that's the issue

[Nuvistor]

It's a technical deficiency in the HI Fi or radio, you will probably find it's the audio stages that are picking up the signal and detecting it, something an audio state shouldn't do.

It's a well known problem, just keep the mobile phone away from the radio.

Frank

[G8HQP Dave]

These older items were not designed to ignore nearby UHF/microwave transmitters because few people had such things in their pockets in the 1970s. They would have been designed to reject MW and HF broadcasting at much lower field strengths.

[G6Tanuki]

Yes, the issue is that the audio-stage circuitry of your radio has some lengths of PCB-track/component-leads that are inconveniently resonant at the frequencies used by cellphones - and so work really well to pick up the signal when your phone periodically 'calls-home'.

A few centimetres of PCB-track and the lead-in to a transistor is a resonant circuit at cellphone frequencies. It doesn't help that pretty much all the general purpose silicon ~~audio frequency~~ transistors from the 1970s onwards would work rather well as detector diodes up into the UHF region.

You *could* re-engineer your radio [with ferrite-beads, quarter-wave stubs and stopper-resistors] to better reject cellphone intrusion. But simply putting your phone further away from the radio is a lot easier.

[SiriusHardware]

I've always had this on my car audio systems as well, if there is a mobile phone in 'waiting' mode somewhere near the centre console I occasionally hear kerklunk-kerklunk-kerklunk whenever the phone negotiates with new cells as I'm driving along.

[stevehertz]

Thanks guys, that'll do me. Like I proposed and confirmed by everyone, it's the proximity of the phone causing the interference.

[paulsherwin]

This has been a huge problem ever since GSM launched. Everyone in a broadcast environment is always told to turn their phones off, as even professional equipment can't keep the interference out. Sometimes somebody will ignore the request or forget to do it with obvious results, sometimes at very unfortunate moments such as at a police briefing announcing multiple deaths. Generally people know what's required nowadays though.

The regulators were aware of the interference produced by GSM devices when they were introduced, but chose to permit it as the benefits were felt to outweigh the drawbacks.

The interference does normally originate in the audio preamp stages.

[stevehertz]

In fairness - at least in my experience - the phone has to be within a metre or so of the FM receiver. Three metres away and I get no interference.

[G4YVM David]

Im surprised youve only recently noticed this. I thinknyou are lucky if youve not experienced it before...but grateful that you rfaised it as i hadnt seen an explanation before.

D

[cashaw1972]

The old TACS analogue phones, before GSM, used to cause the stereo indicator on my Hi-Fi to flash. I maybe remembering wrongly but I am sure this happened when the hi-fi was switched off !

[G3VKM_Roger]

I've heard this effect on TV news broadcasts, when a PA system is being used for a speech or press briefing. Someone near the mic or amplifiers has a cellphone on and it breaks through on the audio, the sound as quite distinctive as has already been mentioned.

Roger

[mhennessy]

Even DECT phones can break through sometimes. They're much lower power than a mobile...

In fairness, newer professional audio gear is much better at rejecting it. You're much more likely to hear the "beep beep" of an accidentally un-silenced phone than RF breakthrough in a modern studio. But, audio gear has a much longer service life than other stuff, so it'll take a while before all the older gear has passed into retirement.

[kalee20]

I've had mobile phone breakthrough on all sorts of equipment, but usually if it's less than a couple of feet away. Even a 4-valve battery LW/MW radio isn't immune!

[Skywave]

Originally Posted by paulsherwin

The regulators were aware of the interference produced by GSM devices when they were introduced, but chose to permit it as the benefits were felt to outweigh the drawbacks.

"The regulators": would that have been OfCom?

Al.

[paulsherwin]

It would have formed part of the technical negotiations over the GSM standard: https://en.wikipedia.org/wiki/GSM#History

[Skywave]

Thanks Paul.

Al.