Hi vfone,
first of all thanks for your reply.
Yes, all what you say is clear for me.
But my problem is that one guy have told me, that for example if we take the Rx part of a duplexer. it rejects around 55 dB your own Tx signal. But if you take this part and used it as a "stand-alone" Rx filter ( without connection to Tx path) you only get around 35 dB rejection in Tx band. As stand-alone it means, you have separated Tx and Rx antennas, so no directly duplex function in filters. His explanation was that in a duplexer, part of the Tx signal goes back in the Tx path, so that the Rx filter part only needs to reject a lower Tx signal.
I do not answer immeditaly to this guy, but at my home I reflect this issue and it couldn´t understand it.
Regards,
zampa