I'm trying to block AC voltage from the shield on coaxial cable. I know that a capacitor will block DC on the center conductor but not AC. Any ideas?

 

Perhaps a capacitor between the signal an neutral, or an inductor in series with the load, or both, like in a LC filter. It depends on the requirements of your circuit. A LC filter might be the best option. There is also the Pi filter, having one first capacitor in paralel, then the inductor in series, then again another capacitor.

 

I'm trying to block AC voltage from the shield on coaxial cable. I know that a capacitor will block DC on the center conductor but not AC. Any ideas?

Before throwing components at the problem, it might be a good idea to explain to us the source of the AC voltage , and the coupling mechanism. In an RF application one end of the coax is already at ground. What more do you need?

 

The source of the AC voltage is from potential differences in the grounding. The shield is supposed to be already at ground but AC is showing up on there. Any ideas on how to neutralize the AC off of the shield.

 

Your analysis is slightly off. The AC voltage is not caused by ground differences between the two ends of the cable. If there is a potential difference then a DC current will flow. This usually takes several hundred meters of cable to show appreciable IR drops. If one of the grounds is actually moving with respect to the other one then you have a different problem.

Coupling mechanisms come in two flavors. Electric fields couple capacitatively, magnetic fields couple inductively. To reduce the effect of inductive coupling we twist the signal pairs to reduce the area of the loops. You can't twist coax, so it probably is not inductive coupling. To identify the source of the AC on the shield you need to start turning things off to see if you can identify the source.

Back to the two power supplies. If one supply is moving with respect to the other then you need to put both ends of the system in close proximity and run both units from the same supply. If the AC goes away now you have to figure out why the grounds are moving with respect to each other. If it doesnt go away then it is coupling capacitatively from an external source.

If it is fromn an external source then connect the shield to EARTH ground and the V-(GND) of one supply at one point only. You can choose either supply for the connection of V- and Earth ground. The other end can be connected to V-(GND) but not to Earth Ground This point is so important that I'm going to suppress my natural urge to avoid yelling. CONNECT THE SHIELD TO EARTH GROUND AND THE V- OF ONE SUPPLY AT A SINGLE POINT ONLY.