How to remove AC signal on DC supply

[Cbrunner]

Ok, I'm a hydraulic guy - and although I do a lot of control work my level of electrical "enginering" is quite limited (so this problem is making me feel real dumb).

I'm working on a 12v DC system that is installed on a chassie that also has 208v 3phase 60hz power running "around" it. The frame/chassie of this vehicle (rubber tires) is normally grounded or bonded to ground. I read a voltage differential of around 40 to 50 volts AC between the chassie and earth ground before bonding. Now the chassie of this vehicle is used by the electricians as a bonding point for their ground systems (whenever they mount a box on the frame, the box is grounded - so the frame becomes grounded).

With a meter and scope I've been trying to trace some reception issue for our radio system that is installed on this vehicle. The manufacturer of the system was kind enough to bond the antenna grounding plane to DC negative power connection. Putting the scope on DC negative, I find 60hz frequency - which I'm assuming is being induced into my DC power supply lines during their travel from front to rear of the vehicle. Now, knowing about shield grounding and good wiring practice I've tried to make sure that the wiring is correect for this application. Now the antenna/radio system is opereating at around 433mhz - so anything I build needs to pass that frequency (I've been trying to pass 400-500mhz to leave a broad spectrum).

Short story is this, how do I design a filter to remove the AC voltage/ripple voltage from the circuit while leaving the DC alone? I have an external ground plane installed on the antenna, and if I can break the bonding between the ground plane between the antenna and DC negative inside the receiver radio module, it will all work great (I can't find the trace on the PCB that bonds the two - for some applications is good to have the bond - but not for this one).

I've tried placing a bridge rectifier between the DC supply and receiver module, but I still see ripple before the power enters the receiver. I installed a pair of capacitors to try and remove the ripple, but I could never calculate the proper size to determine the frequency.

I know this is a simple problem - and that's why it's driving me nuts! Can someone please tell me how to filter the 60hz signal off of my DC supply voltage? The answer is right in front of me - so I can't see it!

Thanks
Chris

 

[IRstuff]

Why do you think you need a filter?

Since you have AC on the negative, you'd also have AC on the positive, and everything is common-moded.

TTFN

FAQ731-376

 

[Cbrunner]

The AC voltage on the ground plane of the antenna makes the reception for the radio next to impossable. The AC voltage I'm finding is only on DC- because DC - is the same connection point as AC ground (which is bonded to AC neutral at some point in the circuit).

If I place a power supply next to the receiver, and use short cables (not connecting to anything other than the receiver) I have a few hundred feet of reception. (That's because the antenna ground plane is isolated - at least that's my understanding of it). When I use the same power supply over a long run of wire that travels through a steel frame that has been wrapped in copper AC carrying wire, I get very poor reception (ten feet or less).

My point through all of this being that I wanted to filter all AC off of the supply voltage to the receiver module.

Chris