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View Full Version : SOLVED Any electrical/electronics boffins able to advise?


Ian
16th November 2016, 10:05 PM
Simple question; use of ferrite rings or beads with USB and audio cables to suppress RF interference; I see some cables with a single loop around the ring, so the cable passes twice through its centre and with some cables the ring simply surrounds the unlooped cable. Do you need to loop the cable for the ring to be effective?

Thanks!

Ian

raichea
16th November 2016, 10:47 PM
The quick answer is "not necessarily".

The ferrite core can be to reduce interference generated by the cable that might affect other devices. It can also be to reduce susceptibility to interference from other devices. It could be for both reasons.

The cable passing straight through the ring creates a single turn (loop) of cable around the core. Any cable that forms a loop creates an inductor, even if it is looped around air or a vacuum - looping with a ferrite core increases the value of its inductance, making it more effective at rejecting high frequencies. If the cable passes through the ring twice, you now have two turns of cable linked to the core. Increasing the number of turns also increases the inductance.

High inductance values also reject lower frequencies, so the cable designer will choose a configuration that will give the rejection that the design requires. This is usually driven by regulations that specify what levels of interference a device can generate. In the case of susceptibility, the product designer will usually have a specification to define what levels of external RF interference are allowed while correct operation is maintained.

If you're attempting an ad-hoc solution to an interference problem, it's probably a case of trying with and without multiple turns.

Naughty Nigel
16th November 2016, 11:55 PM
Spot on! *yes

Most cables are supplied without ferrite cores. Cables fitted with ferrite cores are usually supplied with specific devices, such as power supplies, cameras, computers or computer peripherals. As Raichea says their purpose is to contain interference generated by the device so that it doesn't affect surrounding equipment; or sometimes to protect the device itself from outside interference.

It might also help to know that inductors used to be known as 'chokes' because they restrict (or attenuate) the passage of high frequency signals whilst allowing lower frequencies to pass freely.

Simply passing a cable through the centre of a ferrite ring will only attenuate very high (radio) frequencies of maybe 1 GHz or more. (Mobile phones operate from about 900 MHz - the old UHF television frequency, to 1,900 MHz.)

Winding the cable around a ferrite core several times progressively reduces the frequency at which signals are 'choked'.

However, I suspect the problem that Ian has is caused not by the mobile phone transmitter as such, but by the switch-mode power supply or power adaptor that powers it in the car. These things usually operate at around20,000 to 25,000 Hz - just above audio frequency, but they create nasty harmonics over a much wider frequency range which can be difficult to filter out.

Otto
17th November 2016, 10:04 AM
Switch mode power supplies (i.e. most of them nowadays) are notorious for causing interference. In the darkroom (remember those?) I replaced the failed transformer on my enlarger with a 12v lighting transformer. It works a treat but renders the radio unusable during an exposure!

Naughty Nigel
17th November 2016, 10:30 AM
Switch mode power supplies (i.e. most of them nowadays) are notorious for causing interference. In the darkroom (remember those?) I replaced the failed transformer on my enlarger with a 12v lighting transformer. It works a treat but renders the radio unusable during an exposure!

Dimmer switches often cause similar problems.

Dimmers for LED lamps have to switch the LED's on and off several thousand times a second to create a 'dimming effect'. (Unlike incandescent bulbs the colour temperature of LED's doesn't change with reducing current.)

The need for efficiency means the dimmer circuits must switch the supply on and off very abruptly; otherwise it would become very hot and would need a large heat sink. The resulting square wave signal is rich in harmonics that will often cause interference over a wide frequency spectrum.

Interestingly, switch mode power supplies use 'swinging' or 'flywheel' chokes to improve their efficiency. However, these cause high frequency oscillation (ringing) which only adds to the interference! :)

Otto
17th November 2016, 01:13 PM
The odd thing is that all these devices are CE marked which is supposed to mean that they don't cause interference, or at least keep it within acceptable levels!

Naughty Nigel
17th November 2016, 01:26 PM
The odd thing is that all these devices are CE marked which is supposed to mean that they don't cause interference, or at least keep it within acceptable levels!

This is true in theory, but in my experience, CE marking guarantees very little, even if you understand what it is supposed to mean!

I think in the case of a mobile phone tethered to a car, you have a charging cable connected to a SMPS which is radiating all sorts of nasties. This cable can also be used to carry the audio signal, so straight away there is a problem.

Even if the audio signal is carried by Bluetooth (in the digital domain) the phone and cable could be radiating noise generated by the power supply.

Ian
17th November 2016, 02:42 PM
The ferrite chokes were very inexpensive (in the end I bought 16 of them and they only cost about 6) and the idea is to minimise RF interference from the various gadgets installed in the car (dashcam, phone/tablet, etc.). There does seem to have been an improvement in the FM quality through the car's head unit but ultimately I'm planning to use a Nexus 7 tablet integrated into the dashboard for satnav and audio entertainment. This would be connected through the existing head unit, which remains in-situ, using the AUX input (there is no Bluetooth music/audio support in the car, only hands-free telephony). A ground loop isolator has fixed a nasty problem with the AUX input audio caused by connecting the phone/tablet to the car's electrical power via a 12V-USB adapter. So I think I'm all set now! :)

Ian

snaarman
17th November 2016, 03:09 PM
The odd thing is that all these devices are CE marked which is supposed to mean that they don't cause interference, or at least keep it within acceptable levels!

Ha!

Excuse my laughter, having spent hours getting gear though CE marking then looking at Chinese imports where no apparent filtering has been done..

Pete

Naughty Nigel
17th November 2016, 07:42 PM
Ha!

Excuse my laughter, having spent hours getting gear though CE marking then looking at Chinese imports where no apparent filtering has been done..

Pete

Exactly!

I suspect the Chinese put CE marks on their goods just because everyone else does. :rolleyes:

Steambuff
17th November 2016, 07:55 PM
Exactly!

I suspect the Chinese put CE marks on their goods just because everyone else does. :rolleyes:

Thats because CE stands for China Export

Dave

Wee man
19th November 2016, 09:09 AM
I thought it was just short for ChinesE?

Wee Man

Invicta
19th November 2016, 12:25 PM
The ferrite chokes were very inexpensive (in the end I bought 16 of them and they only cost about 6) and the idea is to minimise RF interference from the various gadgets installed in the car (dashcam, phone/tablet, etc.). There does seem to have been an improvement in the FM quality through the car's head unit but ultimately I'm planning to use a Nexus 7 tablet integrated into the dashboard for satnav and audio entertainment. This would be connected through the existing head unit, which remains in-situ, using the AUX input (there is no Bluetooth music/audio support in the car, only hands-free telephony). A ground loop isolator has fixed a nasty problem with the AUX input audio caused by connecting the phone/tablet to the car's electrical power via a 12V-USB adapter. So I think I'm all set now! :)

Ian

How many wraps did you use in the end Ian?

I notice the Olympus cable release has such a ferrite choke - I guess to stop interference firing a photo?

Ian
19th November 2016, 09:53 PM
How many wraps did you use in the end Ian?

I notice the Olympus cable release has such a ferrite choke - I guess to stop interference firing a photo?

I managed one loop with an audio cable but the USB cable was too thick for the chokes I bought to do a loop.

Ian