Rejection of Crystal filter

[foxgan]

Some guy told me, that a crystal will reject single tone signal of adjacent channel by 25dB, but the crystal can only reject cdma signal with the same distance from the center frequency by about 12dB.
I don't know the reason, can you teach me ?
Thanks

 

[G4BCH]

A cw signal takes up no bandwidth so will be the stated 25dB down the skirt of the filter.
A single crystal will not have steep sided response, and the cdma signal will occupy a given bandwidth this will cause some of the signal to be rejected by less than the 25dB at cdma channel centre.
To work it out accurately you will have to integrate the power of the cdma signal over the filter skirts and compare it with the cw rejection.