@Kittles
90 mins is ok !
But it´s advisable to have it all on one ticket ! If so your transfer will be a total no-brainer because even if your inbound to FRA is late and you miss the FRA- Berlin (TXL) Lufthansa will rebook you to the next flight (LH runs a "shuttle-like" service between FRA and TXL with flights leaving in roughly hourly intervals)
If both flights are on separate tickets i´d say its still doable but without great margin...
MHG's post just reminded me of something. OP says he/she is travelling US Airways to FRA and then LH onwards. While your bags will be checked through to your final destination, you will need to pick up a new BP in FRA. Do NOT go to the transfer desk as we did two years ago and burned 70 minutes. Either go directly to your gate or as you are going through the terminal you will see service desks where you can get a BP. Good luck!