# Car Radio Noise Reduction?



## favguy (May 2, 2008)

OK,
So I've now done 100 EV miles, hoorah!  and been busy checking everything, fine tuning my driving technique and trying to track down squeaks that you just don't hear with an ICE engine running etc. All is pretty good, except...

My usually very nice sound system is completely unusable on FM with the car in motion, (with the exception of the very dire local town radio station which must have a very strong signal broadcasting close by) I just get RFI noise. So how do the rest of you get on? Can I filter the noise somehow, or do I just have to stick to CD's when on the move?!


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## dougingraham (Jul 26, 2011)

You really should not be hearing anything in the FM radio. Is there a tone that changes with motor speed? Does it get stronger at higher power settings. Does it go away when coasting? Are you running a DC-DC converter? Do you have a 12V battery to power the car systems? Is the 12V system isolated from the traction system? What controller and motor?


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## favguy (May 2, 2008)

Hi,

I've found the culprit, it's the Chinese volt & amp meters in the dash, they are above the radio and seem to be throwing noise out. When I unplug them I'm getting no noise at all, so it's not the motor, controller, or DC/DC. So that is sort of good news...

I like the meters, they work well and I don't want to change them at this point. I can't box in the actual face of the meters, but I might build a thin sheet metal box around the rear of the meters and ground it to the car chassis, do you think this will help reduce the noise?


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## frodus (Apr 12, 2008)

It might not be the meters at all.... but the source or what they're measuring that is connected to them.... the controller/battery circuit could be throwing noise into those wires. Open the wires, no path, connect them and you now have a path for that noise.


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## dougingraham (Jul 26, 2011)

I am guessing the meters are not isolated so when they are plugged in they tie the traction ground to the car ground which can be a dangerous thing.

With the meters disconnected you should check for a frame leak. Connect a DVM to the car frame and the other lead to the battery positive. Should be zero volts. Connect between car frame and the battery negative. Again should be zero volts. Hook up the chinese meters and repeat the test.

If this turns out to be the problem you can buy a device that will do the isolation and this would probably allow you to continue to use your meters. I think you can get these from the EVTV store.


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## favguy (May 2, 2008)

No, they are all isolated via small individual dc/dc's to each meter. I've also checked and don't have a frame leak. 

The meters seem to give off a lot of RFI, with the meters switched on but disconnected from the sense wires I'm still picking up noise on FM


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## kennybobby (Aug 10, 2012)

The dc/dc converters have an internal switching inverter--that is the likely culprit, not the meters.


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## favguy (May 2, 2008)

Ah! so will putting them in a metal box and grounding the box shut them up?


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## kennybobby (Aug 10, 2012)

*Re: Car Radio Noise Reduction*

It depends. 

Does the DC/DC converter have a metal housing already? Are there ventilation slots or holes in that housing?

Mechanical shielding (grounded metal box) might work for radiated electromagnetic interference, if that is the EMI source (slits and slots can act as antenna).

But if the RFI source is from conducted emissions then the box probably won't solve it, it will require filtering on the lines. Maybe adding some ferrite beads on the wires. If that doesn't work then some sort of L-C filter network.

Without knowing the source it depends. Start with the simple, cheap approach and work your way up.


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## dtbaker (Jan 5, 2008)

...jumping in since I am having a version of same problem with a little bit of FM static. Mine is not tied to meters at all I don't think. It's perfectly fine when no amps flowing (accel/controller off). Doesn't seem to get much different with low->high amps flowing... just a slight hiss audible with quiet music.

I did recently add a powered 'shorty' antenna to replace the stock one that had a stuck retract motor. Reception is better, but introduced this hiss when I'm under power. It's only with radio, not CD, so I suspect the 12vdc power/ground thru the antenna.

I added a cheapo filter right at the 12v antenna power and chassis ground.... no change. Should I add one in power to radio?

I have a Zilla controller, and a tiny battery in parallel w dc-dc, so I don't think I have any big voltage spikes/sags..... but might be getting high freq from the zilla feeding back all the way thru the dc-dc to the 12v?

WHERE would be the next most likely place to put a filter, and what kind?


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## kennybobby (Aug 10, 2012)

The solution would be determined by the source of the noise. 

The first approach would be to use a spectrum analyzer or oscilloscope and do an electromagnetic survey of the wires and areas around all the boxes and the radio input and output with an electric field probe, and also repeat with a magnetic field probe. This would characterize the issue and provide clues of where and what to do. 

The second approach would be trial and error, could you totally disconnect or turn off the dc/dc and make a short test drive to see if the problem goes away?


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