@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...