Looking for a EMI Filter

[fpacheco]

Hi!
I have some problems with a car alarm when it is installed in a car that has a CB radio. I found a 34khz(Aprox) sine wave bothering over the reset circuit (I´m using the MC34064 from Motorola)when I press the talk button. I tried to filter it out using a small bead and some ceramic capacitors (0.1, 0.01 & 0.001 uF) but nothing. Does anybody knows how can I get rid of it.
You may notice I know nothing of RF.

 

[melone]

34kHz is pretty low frequency for a .1u, try something bigger. Try looking around on the net for frequency response curves for capacitors (be careful, know the dielectric). Good luck.

 

[dpc]

In many cases, the car alarm wiring acts as an antenna, conducting RF voltage directly into your alarm unit. Noise can also be coupled through the 12V power supply wiring.

Some things you may have to do:

Try disconnecting all external wiring into the alarm unit, if possible. See if the interference goes away. If so, you might try adding ferrite toroids to all the wiring into the alarm. Wrap several turns of wire around the toroid - one turn won't enough.

If possible, power your CB using shielded wiring DIRECTLY to your battery (with fuses).

You might visit

http://www.arrl.org

and search on RFI. They have a lot of expertise on dealing with RFI in mobile installations.

Good luck.

 

[fpacheco]

Thank you for your help!
I will do try anythig you have suggested and see what happens.