SR3500 = DSM1
SR3520 = DSM2
SR3520 is a little bigger but weights less according to Spektrum website
If your tranny's DSM1 you can't use DSM2 with it
If your tranny's DSM2 you can bind DSM1 to it. At least mine did.
If you have DSM2 tranny it should have this sticker on it
http://www.horizonhobby.com/Products...MR3140&Index=0
Correct if I'm wrong