How to block GSM interference with car head unit

[hamz01]

My scenario is this: I have created a mount for my iPhone in my car which sits just above the head unit (pic attached). When the iPhone uses GSM (for calling or texting) I get unbearable interference in my sound system (painful high-pitched buzzing). If I move the iPhone a couple feet away, the interference goes away. I have helped the situation by placing a sheet of aluminum foil behind the iPhone, but it hasn't completely fixed the issue. Any ideas on how to solve this problem would be appreciated.

 

[BaCaRdi]

The issue is wide spread, a very well installed system will not have this issue. Shielded cables are used to the speakers, most likley your actually picking up AM waves in the gsm band. Using Aluminum will reduce your signal strength significantly. If you could shield the speakers, close as possible by wrapping they wires (On the Insulator please:/) with Aluminum foil and tying them to ground via a solder wire from the foil to ground. That should help allot, should silence it totally, I can't offer you and circuit solution because it is the GSM band itself and poorly installed factory systems.

Hope this helps, maybe someone else has a better idea though

-BaC

 

[hamz01]

The issue is wide spread, a very well installed system will not have this issue. Shielded cables are used to the speakers, most likley your actually picking up AM waves in the gsm band. Using Aluminum will reduce your signal strength significantly. If you could shield the speakers, close as possible by wrapping they wires (On the Insulator please:/) with Aluminum foil and tying them to ground via a solder wire from the foil to ground. That should help allot, should silence it totally, I can't offer you and circuit solution because it is the GSM band itself and poorly installed factory systems.

Hope this helps, maybe someone else has a better idea though

-BaC

Thanks for the reply.

It is not a factory system, though. Everything is aftermarket and installed by me: head unit, speakers, amps, all wiring.

I'm not saying my install is perfect but you really think the problem is in the wiring? It seems like the problem is an improperly shielded head unit because if I move the phone away from the head unit, the interference goes away, regardless of how close I move the phone to any of the speakers.

 

[hamz01]

Do you think putting ferrite beads around my RCAs would help?

 

[3iMaJ]

Using a better conductor might help as well. Additionally it might be possible to figure out what polarization your iphone has and then you can orient it correctly to maximize the scattering of the gsm signal by the conductor.

 

[RadioRon]

Here is a previous thread where we talked about this sort of thing:
https://www.electro-tech-online.com/threads/question-about-audio-and-rfi.37569/

Shielded wiring might be helpful, but then again the effectiveness of shielding at cellphone frequencies is very dependent on the details of how the shielding is terminated at each end of the cable. If not done properly it can be a waste of time. The ideal shield termination is the kind where the shield is brought completetly over the connection and directly to the metal housing of the box the cable is plugged into. Many connectors have this concept designed-in from the start, including coaxial connectors and also D-Sub types that are metallic.

We focus on cables as the first place where RF can get into your audio system because they are the best receiving antennas, since they are typically long relative to a wavelength. However, at cellphone frequencies, wavelengths are fairly short so RF coupling directly into the amplifier housing or onto circuit boards can also be a problem. There are dozens of points where the RF can cause trouble once it gets into the amplifier so giving specific advice on this forum is, at best, very hit and miss so to speak. To deal with RF on the circuit board, we often use surface mount ceramic capacitors, of low value like 27 pF for example, at specific spots to short the RF to ground. The better spots include at the amplifier outputs, or across amp IC inputs and so on. This is sort of a trial and error process that is much easier if you have a schematic for the amp.

 

[hamz01]

Here is a previous thread where we talked about this sort of thing:
https://www.electro-tech-online.com/threads/question-about-audio-and-rfi.37569/

Shielded wiring might be helpful, but then again the effectiveness of shielding at cellphone frequencies is very dependent on the details of how the shielding is terminated at each end of the cable. If not done properly it can be a waste of time. The ideal shield termination is the kind where the shield is brought completetly over the connection and directly to the metal housing of the box the cable is plugged into. Many connectors have this concept designed-in from the start, including coaxial connectors and also D-Sub types that are metallic.

We focus on cables as the first place where RF can get into your audio system because they are the best receiving antennas, since they are typically long relative to a wavelength. However, at cellphone frequencies, wavelengths are fairly short so RF coupling directly into the amplifier housing or onto circuit boards can also be a problem. There are dozens of points where the RF can cause trouble once it gets into the amplifier so giving specific advice on this forum is, at best, very hit and miss so to speak. To deal with RF on the circuit board, we often use surface mount ceramic capacitors, of low value like 27 pF for example, at specific spots to short the RF to ground. The better spots include at the amplifier outputs, or across amp IC inputs and so on. This is sort of a trial and error process that is much easier if you have a schematic for the amp.

Ok, that makes sense. I think my head unit is actually shielded pretty well, and that shielding is tied to ground. There is no speaker wire running near the phone, just line-level RCAs (from the phone and output to the amplifier in the trunk). I don't think it'd be realistically possible for me to shield the entire length of wire, from the head unit to the trunk, but I will experiment with shielding near where the phone sits.

Anyone know if ferrite beads help with this kind of thing?

 

[hamz01]

My head unit seems to be pretty well shielded and grounded. Also, there is no amplification happening at the head unit. I think it's a good possibility that the RCA connections are picking up the interference. It's going to be very hard to shield those RCAs, and I don't think I can shield them 100%, but I'll see what I can do.

Does anyone know if putting ferrite beads around those RCAs would help this problem?

 

[Nigel Goodwin]

I suspect the problem is you that have a fairly high power, ultra high frequency, transmitter in very close proximity - it's the distance that's killing you, probably too close to cure?.

 

[3iMaJ]

This is the sort of problem that is dealt with all the time in electromagnetic compatability. You might find it useful to reference a few books on EMC and see if there is anything applicable to you.

I have a similar problem of my television making annoying sounds everytime my cell phone pings a cell tower for an update.

The point that always bothered me about cd players/tvs picking up cell phone radiation is exactly what are they picking up? Almost certainly the interference that is being picked up by the head unit is higher than 800 MHZ, so exactly how is that signal getting down converted (or aliased) and amplified so that it can be passed through the speakers at an audible sound.

 

[Nigel Goodwin]

It's rectified and it's the modulation you hear.

 

[3iMaJ]

If it were that simple then blocking it is just as simple.

 

[Nigel Goodwin]

If it were that simple then blocking it is just as simple.

Why would that be simple? (because that's what you hear).

 

[3iMaJ]

If it is indeed the modulation that is heard, which by the time its heard is doubtful as it is likely to contain so many intermodulation distortion terms I doubt its that simple.

However, if we take what you say to be true, there are very few places that the interference can be picked up, "rectified" and amplified. Then finding where the interference is being picked up shouldn't be a particularly difficult problem, because at some point it has to be amplified. Even if the speaker wire is acting as a wire antenna, a cell phone doesn't radiate enough power to create the sound level thats coming out of the speakers. So its safe to assume that the interference is being amplified.

But in the end I highly doubt that this is as simple a problem as an AM demodulation circuit.

Shielding everything is a brute force approach to the problem, and my original question was I was curious to know exactly what was going on in the circuit, because solving the problem might be easier than shielding all possible receive antennas.

My contention is that chalking it up to rectification and amplification is a far over simplifcation of the problem, however should that be the case solving that problem isn't particularly difficult as there are probably only a few places the interference can be rectified and amplified.

 

[Diver300]

GSM interference is particularly difficult to get rid of because it is pulsed, and the pulsing is at 216 Hz, which is audible. The pulses start and stop very quickly so the harmonics of the 216 Hz will cover the whole audio range. In effect, the amplitude is modulated by 100% all of the time. That is not how the information is sent, but it is how the interference happens.

Often the GSM transmission causes voltages large enough to drive circuits into non-linear regions. The signal are large enough to do that to a lot of audio circuits. It is a form of rectification but it is very inefficient. It isn't saturating anything, but any rectification is a problem when you are 5 miles from what you want to recieve and 5cm from what you want to block.

The 900 / 1800 / 1900 MHz is high enough to jump from track to track and the layout techniques required for those frequencies are not usually applied to lower frequency stuff. A low pass circuit that, for instance, filters out the 44.1 kHz CD sampling frequency may use capacitors so large that they have a lot of inductance, and so they wouldn't be effective at GHz.

 

[hamz01]

I do believe it is my pre-amp RCAs that are acting as an antenna and picking up the interference. They then send the interference to the amplifier which boosts it, sends it to the speakers, and then it's audible. I don't think it's the head unit that's picking up the interference because the volume level of the GSM buzzing is the same no matter what volume position the head unit is at. And it's not the amplifier that's picking up the interference because it's out of range, all the way back in the trunk.

So I think I will focus on shielding the RCAs as much as possible and see if that has any effect of the interference.

 

[3iMaJ]

Make sure to shield with a metal with a conductivity as high as possible. The purpose of shielding is to eliminate the tangential electric field in the interferance signal. An perfect electric conductor has a conductivity of infinity, but physically realizable conductors have varying conductances.

Additionally, conductivity is likely to vary with angle of incidence, so you need to make sure that your conductor will work from any incidence angle.

 

[RadioRon]

If it were me trying to solve this, I would not bother with shielding wires as this is difficult and not entirely productive. I would start by doing some experiments to simplify the system in an attempt to isolate the mechanism by which the energy is coupling into the audio system. If the head unit is separate from the amp, one experiment might be to disconnect the head unit but leave the amp in place and then try another audio source with short wires directly to the amp or for that matter just leave the amp inputs disconnected at the amp and power the map up and see if the cellphone interference can still be heard in the speakers. You could also reverse this experiment by connecting the existing head unit to a separate amp. In each case you need to find a way to remove parts of the system or use alternate parts that are remote from the interfering cellphone. Sort of a "divide and conquer" approach.

If you find a line input, for example, that when removed makes a huge difference to the problem, then the next thing I would do would be to open up the amp or head unit and solder a small ceramic capacitor, (the smaller the better if leaded, or a chip type if possible), between the hot line and chassis or circuit board ground. To avoid corrupting the operation of the system, you have to keep the capacitance small, but it still has to be a large enough value to have an impedance of less than a couple of ohms at 850 MHz. Perhaps 27 pF.

Yes, ferrite beads may be a very good idea as I have used them before and they do indeed help. You have to use the right kind of ferrite beads since many of them are made to be most effective at lower frequencies.
This sort of thing:
https://www.fair-rite.com/cgibin/catalog.pgm?THEAPPL=Suppression+Components&THEWHERE=Cable+Component&THEPART=Round+Cable+Snap-Its#select:freq1
can work but you have to pick the type suited to the highest frequencies. You can also buy a kit of assorted shapes from this company and others too. Nice to have a kit for experimentation. If you have beads handy it is easier to try these before fooling around with soldering capacitors inside the boxes.

I have no doubt that your problem is rectification of the cellphone transmitter energy within the head unit or amp. GSM transmissions have sharp changes in amplitude at a rate of 220 Hz (approximately) and this (plus audio harmonics) is what you hear. The RF energy is a high enough frequency that it can couple very easily from one trace to another once it is inside the box so it tends to "flow" everywhere to varying degrees. Any PN junction that sees this RF voltage can rectify it and some are worse than others due to junction capacitance and lead inductance. There are many more PN junctions than you think inside equipment like this because every diode, every bipolar transistor, every JFET and many other semiconductors have PN junctions, and most ICs have protection diodes built in to their I/O pads that are also PN junctions. So there are lots of places for the rectification to occur.

If you can't stop the RF from coming in by treating cables and connections, the next thing to try is to apply bypass capacitors across the most likely PN junctions. Since a small value capacitor is very low impedance to such RF, putting one across a PN junction forces the voltage on both sides of the junction to be identical and with no potential difference across the junction there is no rectification. I mention this as a bit of background theory so that you can understand that the components used to solve the problem are cheap and simple, but finding where to put them is very difficult.

Fixes that are helpful are often accumulative. For example, a ferrite bead on one cable might not fix the problem and a ferrite bead on another cable might also not fix it, but both ferrite beads on both cables might indeed make a difference. So, as you go along, you might find it useful to leave some fixes in place as you try others.

 

[hamz01]

I thought the interference was being picked up by the RCAs, however when I disconnect them from the head unit, the interference goes away. Using ferrite beads from RadioShack on the RCAs did not help the problem. I also tried wrapping the first few inches of RCA with aluminum foil, which didn't help, but I didn't give that one much effort. If I wrapped them with aluminum, I would then have to ground the aluminum, right?

The angle of the cell phone has a direct effect on the level of interference. If i hold it parallel to the head unit, the interference goes away almost completely.

I also notices that as a move the iPhone closer and closer to the dock, the interference gets higher and higher. Then, as soon as the iPhone touches the metal on the connector in the dock, the interference skyrockets. And then, when I slide the aluminum foil behind the iPhone, the interference goes away.

 

[Nigel Goodwin]

However, if we take what you say to be true, there are very few places that the interference can be picked up, "rectified" and amplified.

Every semiconductor junction in the unit (thousands of them), even resistors and capacitors, and even worse, any less than 100% perfect connection.

I presume you know what a crystal radio is?, and cat's whiskers and galena crystals? - under a strong enough RF field almost any contact can act as a rectifier.

This is standard engineer and radio ham stuff - particularly radio ham, who deal with this on a daily basis.