In a communications receiver, a crystal filter would be located in the:

A crystal filter is a highly selective component used to pass the desired signal while rejecting adjacent, unwanted frequencies. Because it is designed to operate at a fixed frequency, it is not suitable for the tunable Radio Frequency (RF) stage. The Intermediate Frequency (IF) stage is the ideal location for a crystal filter because all incoming signals are converted to this single, fixed IF. This allows the receiver to utilize a highly optimized, narrow-bandwidth filter without needing to physically retune the filter as you change channels. Placing the filter in the detector or audio stages would be too late in the signal path, as interfering RF signals would have already mixed and caused interference that audio filtering cannot easily remove.