Audio noise (ground loop?)

[2fast]

I have a electronic question. I use a Chatterbox X1 which has connected to it a Garmin iQue GPS and a radar detector audio output. Whenever I have the external cig lighter charger connected to the iQue, I get loud horrible screeching sounds in my headset. I read somewhere that this may be caused by "ground loop feedback" or something like that. Does anyone know what this means, and how I can eliminate it? There must be some kind of filter or something. Help if you have any ideas! Thanks

[e_sloan]

Not sure if it is the same as cars as I have never had many electronics on my bike but on a previous car I would get back feedback on the radio, also called alternator whine, went to my local audio store to pick up a little unit that cancels it out.

I would recommend trying the car version, I got mine for around $10

[randy]

2fast,

You most likely have the units grounded at seperate points on the frame. Try bringing theier ground leads to ONE common point. That should do it.

[DiverRay]

Randy has the answer. The power and ground for all the devices you are trying to use must be taken from the same point. Run a 14 gauge black wire for ground from the frame near the battery connection, or the battery negitive terminal, to the equipment. Then run a red 16 or 18 gauge wire from the same fused point on the positive side of the circut to both units.
The wires can be any length, and wind around whatever as long as they both terminate from the same source. One thing I do when I've more than one radio is put a second ground wire between both cases. You may also want to fuse the ground wire, as well as the "hot", to keep from frying the radio in case of an electrical "misshap".
Ray

[Deni]

Hi, 2fast. I have been a ham radio operator for over 25 years and have lots of experience with radio equipment, so hopefully some of this will help.

You didn't say if you had the chatterbox powered through the bike's electrical system or not. Hopefully not. Looking at your post again, now I'm really not sure what you have connected to what and how.

There are 2 kinds of "grounds," power grounds (DC in this case) and RF (radio frequency) grounds. A ground loop applies to the RF ground and is when RF that builds up on a chasis or wire cannot "see" ground because the path it has to take through multiple wires presents a high impedance, so it travels around the circuit looking for a ground, which it doesn't find. This gets in to transmitters and receivers, which I've babbled about below, maybe needlessly.

Would you please post again and say 1) exactly how you have everything connected when you get the problem and 2) how you have everything connected when you don't have the problem?

I just looked up the chatterbox and Garmin on the web to get an idea of what they are. The chatterbox is a FRS radio (like the little handhelds) which operates or FRS frequencies around 49 MHz, just below the 6 meter ham band, and operates using FM, which is much better than AM for not picking up alot of noise and interference. If it's the alternator you are picking up, then it should be audible in the background as a "whine" only when you are receiving a transmission (someone talking to you). You said it was "loud horrible screeching sounds," which doesn't sound like an alternator whine.

The Garmin GPS is actually a radio. It is a receiver of the satellite signals, of course. But, superheterodyne receivers are also very low power transmitters because they have oscillators. These are very low power and ususlly aren't picked up by a receiver, unless very close. Adding the cigarette lighter plug can actually act as a transmitting antenna. This is called an unintentional radiator, and there are FCC regulations limiting this kind of interference. But chances are, Garmin makes a minimal effort to limit unintentional radiation, and chatterboxes aren't very selective receivers. Combine the two, and it isn't hard to imagine that you are getting interference.

But, I actually don't think it's the receiver in the GPS that giving you grief, I think it's the microprocessor. The GPS is a computer, which has a microprocessor, which necessarily is a transmitter. Listen to your chatterbox next to your home computer and see if it makes a similar noise. Also put the GPS right next to the chatterbox antenna and see if it makes that screeching. If I set my handheld FRS radio next to my computer, then it screeches. Home computers are very bad for this because they have all those peripheral devices with cables that act as antennas. You've probably noticed the little "warts" or bumps along those cables, which are ferrite chokes designed to reduce unintentional radiation. Microprocessors/computers are notorious for wreaking havoc on receivers when in very close proximity.

So, what can you do? Chances are your cigarette lighter cord is 2 conductor without shielding. Adding shielding to that cord would probably cut down or eliminate your problem. To see if this is it, you could wrap aluminum foil all around the cord. Then use an alligator clip to connect from the foil on the cord to a place on the frame that you KNOW is well grounded. A short thick braided cable goes from the battery to the engine case (I think), so all the bolts holding the engine to the frame should make the entire frame very well grounded.

All of this does not apply if your noise was a "whine" instead of a "screech." If it is an alternator whine, then it will raise in pitch when you accelerate. If this doesn't happen, then it's not the alternator. Those filters that were suggested to eliminate alternator whine only work for the receiver side, which I'm pretty sure is not your problem, as it might be with car audio equipment. The filters filter out residual AC slipping past the rectifier and getting into the receiver audio. If it was alternator whine, then it shouldn't make any difference whether or not you had the GPS plugged into the bike or not. That is, unless you have the chatterbox powered through the bike's electrical system.

Hope this babbling wasn't a total waste of your time, and please repost your specific connections.
-Deni

[Deni]

2fast,
I also forgot to say that if your screeching happens when the bike is not running, then it obviously cannot be the alternator. If it only happens when the bike is running, then it probably is the alternator.
-Deni

[2fast]

Thanks for the thoughts. To try and clarify a bit, I will detail what is plugged into what.
The Chatterbox is powered by the bike battery through a small inline filter box (provided by Chatterbox) and direct wired to an always hot wire in the bike harness, or if unplugged, it has internal batteries which will charge when connected to the bike power. I usually leave it unplugged unless it needs charging. It does not seem to have any effect on noise either way.
The Garmin iQue is powered or charged by the bike via a cig lighter. No noise when not connected to the cig lighter power. The screeching noise happens with or with out the bike running if the iQue is connected to charge. I usually try to charge while parked so I can use it with batteries while riding and not have the noise.
So, I only hear the noise when the iQue is connected to the cig lighter to charge. The screech changes frequency as the amount of charge changes. In other words, the longer it is plugged in, the more it hertz (pun intended). Also, there is a type of pattern, kind of a pulsing to the sound.
I think the first thing I will try is changing all these power connections so they share the same hot and ground source. If that does not do it........not sure.
Appreciate all the help, though!

[Deni]

Great. So you know it's not alternator whine.
The best way to wire things is to have your positive leads go as short as possible to the fuse block (common positive point). The negative leads should go as short as possible to any good connection to the frame (common negative point; and fusing these leads as Ray said are always a good idea). I just checked and the negative battery cable goes directly and only directly to the frame. So, the frame IS the common ground (for everything). I measured the resistance from a rear shock to the front valve cover and measured 0.0 ohms, and this path is through the frame and engine case. Ground loops develop when RF can't get to ground as easily as it ought to be able to (e.g. a longer than necessary wire). A short wire to the frame is better than a long wire to anywhere for the negative side.

I'd do 2 simple things before you change the wiring (unless to that above). First, and you probably know this already, I'd see if it's frequency specific. If it is, then it's almost certainly microprocessor hash from the GPS getting radiated from the power cord getting picked up by the chatterbox antenna. This is the case with my FRS handheld near my computer. Since it happens when the chatterbox is on batteries and not just when wired, this supports it coming in through the antenna also. You have to stop the noise on the transmitter side (GPS).

Second, I would do the power cord shielding with the foil and ground to the chasis with a wire or alligator clip. This may be enough. If it is, you could buy a a hollow braid or use the foil. Then put shrink wrap or electrical tape around the shielding . A ferrite core on the cable as close to the GPS as possible may also work. Here is a useful site discussing ferrites:

http://www.amidoncorp.com/aai_ferritesforrfi.htm

I just looked at a picture of the chatterbox power cord. The small box is probably just a ferrite. The GPS cord definitely is unfiltered. I'll bet a a couple wraps of this cord around a ferrite as close to the GPS as possible would do the trick.

Please post whatever works in the end because I may end up with a similar arrangement.

[2fast]

Problem solved!

Thanks to everyone for the suggestions. Here is what I did, and what fixed the noise.

I rewired all audio associated items to a single power and ground, with a radio shack noise filter in the lines as well. No change. Then I bought a radio shack ground loop isolator and experimented with different connection points. It goes in the audio feed from point A to point B.

Long story short, using it between the GPS audio output and the Chatterbox audio input is what cured the noise. I am a happy camper! Now I can run with bike battery power to everything, and virtually no extraneous noise. FYI, the ground loop isolator is about $16.

[Deni]

2fast,
Great to hear that! It's probably acting as an RF choke and choking out the RF microprocessor hash from the GPS. It looks like it's an audio isolation transformer that lets the audio frequencies through, but chokes out RF. I guess you are using only one side? since it has 2 sides for stereo? Good deal.
-Deni

[2fast]

Deni,
Actually, I am using both sides. My GPS is a Garmin iQue, which has a stereo mini plug output. It is capable of playing mp3's for music (in stereo). Using this unit I get GPS with spoken turn by turn directions, music and all the standard PDA stuff.

Since I only got the noise when it was plugged into the bike power, do you think it was RF noise? I did not get any noise when just using the iQue internal rechargeable batts.

[Deni]

I don't know for sure, but I don't know what else it could be. Ground loops can only occur with AC, either 60Hz or RF. The Radio Shack ground loop isolator you put in I think was designed for 60Hz AC ground loops in audio systems like those discussed in:
http://www.epanorama.net/documents/groundloop/
If it happened to you only with the bike running and alternator going, then I would say that's it, and your isolator would work to get rid of that. But, it occurs with the bike off, so it can't be an AC ground loop. So, I guess when the iQue is on the charger, one possibility is that the power cord itself is generating a signal (it may have an active circuit in it). It looks just like a cord, but it may have a circuit that is generating AC or RF noise, which gets filtered out by the isolator. Or, the cord turns on a charging circuit in the iQue, which is generating the noise. Or, for some reason, plugging the iQue in to external power creates an RF ground loop and doesn't let the RF go wherever it was going besides to your chatterbox. In any case, the isolator is passing through the audio frequencies like it's supposed to, and since the screeching is gone, it wasn't just audio, which leaves RF. The Radio Shack isolator appears to be an audio isolation transformer. This would also act as an RF choke.