I have a problem. My car radio is picking up interference when ever I plug a device into the power outlet/ cig lighter. I know that the solution would be to put shielding around the antenna wire, but the shielding wont fit over the antenna plug. So I thought I might put the shielding around the power wire (source of the interference). Provided the shield is properly grounded, does it make a difference which one I shield?
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\$\begingroup\$ Is it anything plugged into the outlet, or just a specific thing or two? I had a bad usb charger that killed most radio signals, in multiple cars. Ditched it for a better/newer one (not shielded, same ic and general layout). Another thing, is the antenna crossing the outlet's power cable? Moving them around could help. \$\endgroup\$– PasserbyCommented Apr 12, 2014 at 2:39
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\$\begingroup\$ Is it the same on AM and FM alike? \$\endgroup\$– Andy akaCommented Apr 12, 2014 at 9:31
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\$\begingroup\$ Yeah it only happens when some device is drawing on the circuit.. ie cell phone charging. \$\endgroup\$– ChrisCommented Apr 12, 2014 at 19:00
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1 Answer
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You should always start by using devices that comply to decent EMC guidelines (which often rules out cheap garbage products you buy for 1 or 2 $). Then the next step is shielding the source of interference, but that only works when you actually have radiated interference. Conducted interference needs different approach eg. using power line filters or proper decoupling designed into the circuit.
Normally the antenna cabling in a car is already shielded / coaxial, so an extra shield there will not be very likely to work.
It is likely that your interference is conducted rather than radiated. This means that the device you plug in is doing bad things to the power supply lines, and the copper wire distribute this noise everywhere to all other devices in the car.
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\$\begingroup\$ I didn't think about conducted interference but the plug is 12 VDC and the plug is 5 VDC out... I think the load is resistive so there shouldn't be any harmonics produced.... of course I could be wrong. \$\endgroup\$– ChrisCommented Apr 12, 2014 at 19:03
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\$\begingroup\$ @Chris the circuitry in the plug is non-resistive. That can't be resistive because it would get too hot during operation. \$\endgroup\$– jippieCommented Apr 13, 2014 at 8:27
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\$\begingroup\$ Just figured the car's voltage was stepped down using voltage drop across a resistor in series with the load (phone). At 5 mA I wouldn't think the heat would be something you couldn't handle with ambient cooling. If this is not the case? How do car chargers produce harmonics? \$\endgroup\$– ChrisCommented Apr 14, 2014 at 11:52
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\$\begingroup\$ @Chris These are switching regulators. They quickly switch current on and off. A low pass filter is used to make the load see a continuous voltage, but the high voltage side sees steep slopes from switching. Steep sloped pulses have huge amounts of harmonics. \$\endgroup\$– jippieCommented Apr 14, 2014 at 14:27
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\$\begingroup\$ Do after market chargers put out more harmonics than OEM? Or are they about the same? \$\endgroup\$– ChrisCommented Apr 14, 2014 at 17:25