I'm not an electrical engineer, but I play one in the forums
Some timers are better than others. The timer on my first bait tank was a do-it-yourself kit that I got at Willy's. I had to solder all of the components on the board on that one. It wasn't very well suited for the task and it soon crapped out. Before it died, I noticed some interference on my FF. I got a couple of the timers from Anglers Yak Shack and they have been working well. The whole thing is potted, so you don't have to worry about corrosion. I've used an AYS timer with a couple of different fishfinders and I have never seen any sort of interference with the sonar's operation or display. The AYS timers are a little more expensive than others, but it's worth the money.
AYS 3-17 Timer
AYS 7-14 Timer