yes you can vary the frequency of the output signal,but their's some consequence of doing that...first you must convert the square wave to sine wave.Second one is after you converted the square wave to sine wave,the operating frequency of this sine wave is greatly reduce,from 15khz to 200hz ,depending on the type of crystal clock you use.then convert the sine wave back to square wave.if you are interested on the circuit of square wave to sine wave converted,email me.