Plugged my guitar into one channel of the scope, as you do, and the sig gen into the other channel. Synced to the sig gen and adjusted the pitch of the strings so that the plucked note trace drifted neither to the right or left. What I noticed is that the frequency of the plucked note would initially be dead on, then after maybe half a second it would drift to the right because it's frequency had dropped slightly. Why would this happen? I would have expected it to stay constant.