I very much doubt that an electronic device would keep a determined cat out of your yard. I have a similar problem and had only partial success - see below.
Cats are territorial animals, what you have to do is exclude your yard from their territory. Cats territories are marked chemically, by spraying. You could try some tiger piss and spray that on the fence :lol: , however, getting it from the source might be a trifle exiting.
Seriously, only a cat hating dog will keep your yard cat free but you are likely replacing one problem with another :wink:.
I did manage to stop cats having their late night love affairs under my house. It involved wiring the single entry point with a 15,000V HV supply (salvaged from an old computer cathode ray display monitor), like an electric fence. It only needed one contact and the cat took off at 100Mph, perhaps with singed wiskers. Never came back after this. I left the wires in place but switched the HV supply off, it produced nice corona's and crackling noises whenever it was a bit humid.
If its your vegie patch the cat is using you could try to rig a motion sensor to turn on the sprinkler, I think a wet bum might deter the cat doing its business there.
:twisted: